Chemical Reactions and Energy

Transcription

1 Topic 7 Chemical Reactions and Energy Part A Unit-based exercise Unit 29 Energy changes in chemical reactions Fill in the blanks 1 The amount o heat required to raise the temperature o 1 g o a substance by 1 K is the speciic heat capacity o the substance. 2 The amount o heat required to raise the temperature o a substance by 1 K is the heat capacity o the substance. 3 The internal energy o a system has two components: kinetic energy and potential energy. 4 The heat released or taken in by a system kept at constant pressure is called enthalpy change. 7 The standard enthalpy change o ormation o a substance is the enthalpy change when one mole o the substance is ormed rom its elements in their standard states. 8 The standard enthalpy change o combustion o a substance is the enthalpy change when one mole o the substance is completely burnt in oxygen under standard conditions. 9 The standard enthalpy change o neutralization is the enthalpy change when an acid reacts with an alkali to orm one mole o water under standard conditions. 10 The standard enthalpy change o solution o a substance is the enthalpy change when one mole o the substance dissolves in an ininite volume o solvent under standard conditions. Part A Unit 29 5 a) In an exothermic reaction, heat is released to the surroundings. b) The total enthalpy o the products is less than that o the reactants. c) The enthalpy change o the reaction is a negative quantity. 6 a) In an endothermic reaction, heat is taken in rom the surroundings. b) The total enthalpy o the products is greater than that o the reactants. c) The enthalpy change o the reaction is a positive quantity. True or alse Decide whether each o the ollowing statements is true or alse. 11 Energy is the capacity to provide heat or T to do work. 12 The universe is composed o the T surroundings and the system. 13 The heat released or taken in by a system F kept at constant volume is called enthalpy change. 1

2 14 In an exothermic reaction, the total T enthalpy o the products is less than that o the reactants. 15 In an endothermic reaction, heat is F transerred rom the system to the surroundings. 16 The sign or ΔH o an endothermic T reaction is positive J o heat were added to a kg sample o thallium at 27.0 C. What would be the inal temperature o the sample? (Speciic heat capacity o thallium = J g 1 K 1 ) A 12.4 C B 28.9 C C 33.2 C D 39.4 C D Part A Unit In an exothermic reaction, the amount o F energy released in the bond-orming step is less than the amount o energy used in the bond-breaking step. 18 The standard enthalpy change o F ormation o diamond is zero. 19 The standard enthalpy change o T combustion o hydrogen equals the standard enthalpy change o ormation o water. 20 The standard enthalpy change o F neutralization between hydrochloric acid and sodium hydroxide solution is the same as that between ethanoic acid and sodium hydroxide solution. 23 How much heat is required to heat 5.00 g o Pyrex glass rom 15.0 C to 65.0 C? (Speciic heat capacity o Pyrex glass = J g 1 K 1 ) A 62.8 kj B 209 J C 272 J D 299 J J o heat are required to raise the temperature o 93.8 g o zinc rom 28.5 C to 37.0 C. What is the speciic heat capacity o zinc? A J g 1 K 1 B J g 1 K 1 C J g 1 K 1 D J g 1 K 1 B D Multiple choice questions 21 A 525 g aluminium pizza pan cools rom 175 C to 27 C. How many joules o heat does it lose? (Speciic heat capacity o aluminium = J g 1 K 1 ) A 12.8 kj B 41.4 kj C 69.9 kj D 86.3 kj C Directions: Questions 25 and 26 reer to the ollowing experiment. In an experiment, g o alcohol X at 18.2 C were mixed with 60.0 g o water at 50.3 C. The inal temperature o the mixture was 36.5 C. (Speciic heat capacity o water = 4.18 J g 1 K 1 ) 25 The heat lost by the water was A 3.46 kj. B 8.61 kj. C 11.9 kj. D 26.2 kj. A 2

3 26 What is the speciic heat capacity o alcohol X? A J g 1 K 1 B 1.72 J g 1 K 1 C 3.55 J g 1 K 1 D 5.76 J g 1 K 1 27 A 177 g sample o gold at some temperature was added to 22.1 g o water. The initial water temperature was 25.0 C and the inal temperature was 27.5 C. What was the initial temperature o the gold sample? (Speciic heat capacities: gold = J g 1 K 1, water = 4.18 J g 1 K 1 ) A 17.5 C B 23.8 C C 30.5 C D 37.5 C 28 One beaker contains 156 g o water at 22.0 C. Another beaker contains 85.2 g o water at 95.0 C. The water in the two beakers is mixed. What is the inal water temperature? (Speciic heat capacity o water = 4.18 J g 1 K 1 ) A 47.8 C B 58.5 C C 69.2 C D 83.0 C 29 A 25.0 g sample o an alloy was heated to C and dropped into a beaker containing 90.0 g o water at 25.3 C. The temperature o the water rose to a inal value o 27.2 C. What is the speciic heat capacity o the alloy? (Speciic heat capacity o water = 4.18 J g 1 K 1 ) A J g 1 K 1 B J g 1 K 1 C J g 1 K 1 D J g 1 K 1 B D A D 30 A student perormed a calorimetry experiment and recorded the data below. Mass o aluminium calorimeter g Mass o calorimeter water g Initial temperature o calorimeter 23.0 C and water Mass o silicon 52.0 g Initial temperature o silicon 61.6 C Final temperature o calorimeter, 24.6 C water and silicon (Speciic heat capacities: water = 4.18 J g 1 K 1, aluminium = J g 1 K 1 ) What is the amount o heat lost by silicon in the experiment? A 670 J B 1.36 kj C 2.26 kj D 3.82 kj 31 Which o the ollowing is an endothermic process? A Photosynthesis B Cellular respiration C Formation o ethanol D Combustion o propane 32 Commercially available hot packs contain an inner pouch o a solid ionic compound within an outer pouch containing water. When the inner pouch is broken, the solid dissolves in the water o the outer pouch. B A Part A Unit 29 3

4 Part A Unit 29 A hot pack may contain A calcium chloride, which undergoes an exothermic dissolving process. B calcium chloride, which undergoes an endothermic dissolving process. C ammonium nitrate, which undergoes an exothermic dissolving process. D ammonium nitrate, which undergoes an endothermic dissolving process. A 33 A basketball player comes out o the shower, still damp, eeling cooler than he did when he entered the locker room. The water on the player s skin undergoes an A endothermic physical change. B endothermic chemical change. C exothermic physical change. D exothermic chemical change. 34 Cans o sel-heating coee were not available until recently. Inside the can, in separate compartments, are calcium oxide and water. When a button is pressed, the two substances react. The reaction is A endothermic, releases heat and has a positive ΔH value. B endothermic, takes in heat and has a negative ΔH value. C exothermic, releases heat and has a negative ΔH value. D exothermic, takes in heat and has a positive ΔH value. C 35 Hydrogen iodide (HI) is ormed rom the reaction o hydrogen and iodine. H 2 (g) + I 2 (g) 2HI(g) ΔH = +52 kj mol 1 When two moles o HI decompose, A 52 kj o heat are released. B 52 kj o heat are taken in. C 104 kj o heat are released. D 104 kj o heat are taken in. A A 36 Given the ollowing data: C 3 H 7 OH(g) O 2(g) 3CO 2 (g) + 4H 2 O(l) ΔH = kj mol 1 What is the ΔH value or the ollowing process? 6CO 2 (g) + 8H 2 O(l) A kj B +987 kj C 987 kj D kj 2C 3 H 7 OH(g) + 9O 2 (g) 37 Consider the ollowing thermochemical equation: N 2 (g) + 3F 2 (g) 2NF 3 (g) = 264 kj What is the enthalpy change that occurs when mole o NF 3 (g) is ormed rom N 2 (g) and F 2 (g) under standard conditions? A kj B kj C 33.8 kj D 67.6 kj Directions: Questions 38 and 39 reer to the ollowing inormation. Commercial drain cleansers typically contain sodium hydroxide and aluminium. When the solid cleanser is poured down the drain and water is added, the reaction that occurs is represented by the equation: 2NaOH(s) + 2Al(s) + 2H 2 O(l) 2NaAlO 2 (aq) + 3H 2 (g) = kj (Relative atomic masses: H = 1.0, O = 16.0, Na = 23.0) 38 In this reaction, the oxidation number o aluminium changes rom A 0 to +1. B 0 to +3. C +2 to +6. D +3 to What is the amount o heat released when 24.0 g o NaOH(s) have reacted? A 128 kj B 255 kj C 510 kj D kj A C B B 4

5 40 When 5.40 g o butanal (relative molecular mass = 72.0) were burnt, 201 kj o heat were released. What is the enthalpy change o combustion o butanal? A kj mol 1 B 6.70 kj mol 1 C kj mol 1 D kj mol 1 D 44 The table below shows the standard enthalpy changes o combustion o the our hydrocarbons: ethyne, propyne, propene and propane. Compound Name Relative molecular mass c (kj mol 1 ) HC CH ethyne HC CCH 3 propyne The combustion o octane can be represented by the ollowing thermochemical equation: 2C 8 H 18 (g) + 25O 2 (g) 16CO 2 (g) + 18H 2 O(g) ΔH = kj (Relative atomic masses: H = 1.0, C = 12.0) What is the amount o heat produced by the complete combustion o 45.0 kg o octane? A 2.16 x 10 3 kj B 4.32 x 10 3 kj C 2.16 x 10 6 kj D 4.32 x 10 6 kj 42 In an experiment, a student heated g o water rom 25.0 C to 91.0 C using mole o ethanol. What is the enthalpy change o combustion o ethanol under the experimental conditions? (Speciic heat capacity o water = 4.18 J g 1 K 1 ) A 684 kj mol 1 B 848 kj mol 1 C kj mol 1 D kj mol 1 C C H 2 C=CHCH 3 propene CH 3 CH 2 CH 3 propane Complete combustion o 2.00 g o one o the above hydrocarbons releases exactly 100 kj o heat. This hydrocarbon is A ethyne. B propyne. C propene. D propane. Directions: Questions 45 and 46 reer to the ollowing inormation o our uels. Fuel Formula Name Enthalpy change o combustion (kj mol 1 ) A Molar mass (g mol 1 ) A CH 3 OH methanol B C 2 H 5 OH ethanol C C 4 H 10 butane D C 8 H 18 octane Part A Unit The enthalpy change o combustion o methane is 890 kj mol 1. What is the minimum mass o methane that must be burnt to warm 4.00 dm 3 o water rom 22.4 to 94.8 C? (Relative atomic masses: H = 1.0, C = 12.0; density o water = 1.00 g dm 3 ; speciic heat capacity o water = 4.18 J g 1 K 1 ) A 1.36 g B 2.18 g C 13.6 g D 21.8 g D 45 Which uel, A, B, C or D, produces the greatest amount o energy per gram on complete combustion? A CH 3 OH B C 2 H 5 OH C C 4 H 10 D C 8 H 18 C 5

6 Part A Unit Scientists give governments advice on technical issues. What inormation would scientists use when advising governments on the choice o one o these uels, i the aim was to minimize carbon dioxide production? A Mass o carbon per gram o uel B Mass o carbon per kilojoule produced C Number o kilojoules produced per gram D Number o kilojoules produced per mole B 47 Given the ollowing data: C 2 H 6 (g) 2CO 2 (g) + 3H 2 O(l) 3H 2 (g) O 2(g) 2C(s) + 3H 2 (g) ΔH = +85 kj C 2 H 6 (g) O 2(g) ΔH = kj 3H 2 O(l) ΔH = 858 kj 2C(s) + 2O 2 (g) 2CO 2 (g) ΔH = 788 kj Which o the ollowing statements is correct? A 286 kj o heat are released when 1 mole o steam is ormed rom its elements. B The enthalpy change o combustion o hydrogen is more exothermic than that o carbon. C The enthalpy change o ormation o ethane rom its elements is endothermic. D The enthalpy change o combustion o ethane is more exothermic than its enthalpy change o ormation. D Directions: Questions reer to the ollowing experiment cm 3 o 1.00 mol dm 3 hydrochloric acid were added to 50.0 cm 3 o 1.00 mol dm 3 sodium hydroxide solution. A temperature rise o T C was observed. (Speciic heat capacity o water = 4.18 J g 1 K 1 ) 48 What is the enthalpy change o neutralization (in kj mol 1 )? A 50.0 x 4.18 x T B x 20.0 x T x 4.18 C x 20.0 x x T D x 20.0 x 4.18 x T D 49 Which o the ollowing pairs would give the same value o enthalpy change o neutralization? A 50.0 cm 3 o 1.00 mol dm 3 sulphuric acid and 50.0 cm 3 o 1.00 mol dm 3 aqueous ammonia B 50.0 cm 3 o 1.00 mol dm 3 nitric acid and 50.0 cm 3 o 1.00 mol dm 3 potassium hydroxide solution C 50.0 cm 3 o 1.00 mol dm 3 ethanoic acid and 50.0 cm 3 o 1.00 mol dm 3 sodium hydroxide solution D 50.0 cm 3 o 1.00 mol dm 3 ethanoic acid and 50.0 cm 3 o 1.00 mol dm 3 aqueous ammonia B 50 I cm 3 o 1 mol dm 3 hydrochloric acid were added to cm 3 o 1 mol dm 3 sodium hydroxide solution, the temperature rise would be 1 A 2 T C. B T C. 3 C 2 T C. D 2T C. 51 The reaction between hydrochloric acid and sodium hydroxide solution can be represented by the ollowing equation. HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) n = 57.1 kj mol cm 3 o 2.00 mol dm 3 HCl(aq), at 28.0 C, are mixed with 40.0 cm 3 o 2.00 mol dm 3 NaOH(aq), also at 28.0 C, in a wellinsulated calorimeter. The heat capacity o the calorimeter and contents is 420 J K 1. What is the inal temperature o the solution mixture? A 11.7 C B 17.1 C C 38.9 C D 44.3 C B C

7 52 The ollowing table shows some inormation on mixing acids with sodium hydroxide solution: Mixture 25 cm 3 o 1 mol dm 3 HCl(aq) + 25 cm 3 o 1 mol dm 3 NaOH(aq) 25 cm 3 o 1 mol dm 3 CH 3 COOH(aq) + 25 cm 3 o 1 mol dm 3 NaOH(aq) 50 cm 3 o 1 mol dm 3 HCl(aq) + 50 cm 3 o 1 mol dm 3 NaOH(aq) 50 cm 3 o 1 mol dm 3 CH 3 COOH(aq) + 50 cm 3 o 1 mol dm 3 NaOH(aq) Temperature rise ( C) Which o the ollowing concerning the values o temperature rise is correct? A w > x = y > z B w = y > x = z C w = x > y = z D w = y < x = z 53 The ollowing table shows some inormation on mixing sulphuric acid with sodium hydroxide solution: Mixture 25 cm 3 o 0.5 mol dm 3 H 2 SO 4 (aq) + 25 cm 3 o 1 mol dm 3 NaOH(aq) 50 cm 3 o 0.5 mol dm 3 H 2 SO 4 (aq) + 50 cm 3 o 1 mol dm 3 NaOH(aq) 25 cm 3 o 1 mol dm 3 H 2 SO 4 (aq) + 25 cm 3 o 2 mol dm 3 NaOH(aq) 50 cm 3 o 1 mol dm 3 H 2 SO 4 (aq) + 50 cm 3 o 2 mol dm 3 NaOH(aq) w x y z B Temperature rise ( C) Which o the ollowing concerning the values o temperature rise is correct? A w < x < y < z B w < x = y < z C w = y < x = z D w = x < y = z w x y z D 54 The enthalpy change o solution o NaOH(s) is 44.6 kj mol g o NaOH(s) are dissolved in water in a calorimeter. What is the amount o heat released? (Relative atomic masses: H = 1.0, O = 16.0, Na = 23.0) A 27.9 kj B 71.4 kj C 1.12 MJ D 1.78 MJ 55 In an experiment to ind the enthalpy change o solution o potassium hydroxide, 3.60 g o solid potassium hydroxide were added to water kj o heat were produced. What is the enthalpy change o solution o potassium hydroxide? (Relative atomic masses: H = 1.0, O = 16.0, K = 39.1) A kj mol 1 B kj mol 1 C 1.03 kj mol 1 D 57.8 kj mol g urea (NH 2 CONH 2 ) are dissolved in g o water in a simple calorimeter. A temperature all o 3.7 C is observed. What is the enthalpy change o solution o urea? (Relative atomic masses: H = 1.0, C = 12.0, N = 14.0, O = 16.0; speciic heat capacity o water = 4.18 J g 1 K 1 ) A 13.9 kj mol 1 B 38.7 kj mol 1 C kj mol 1 D kj mol x 10 4 J o heat are required to vaporize g o benzene (C 6 H 6 ). What is the enthalpy change o vaporization o benzene? (Relative atomic masses: H = 1.0, C = 12.0) A 3.39 x 10 4 kj mol 1 B 33.9 kj mol 1 C kj mol 1 D x 10 4 kj mol 1 A D C C Part A Unit 29 7

8 Part A Unit Which o the ollowing is / are endothermic process(es)? (1) Dilution o concentrated sulphuric acid (2) Evaporation o water (3) Formation o methane rom its elements A (1) only B (2) only C (1) and (3) only D (2) and (3) only 59 Which o the ollowing processes are exothermic? (1) Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) (2) 2C 4 H 10 (g) + 13O 2 (g) 8CO 2 (g) + 10H 2 O(l) (3) NH 4 NO 3 (s) + H 2 O(l) NH 4 NO 3 (aq) A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) 60 Consider the ollowing enthalpy level diagram: Enthalpy (kj mol 1 ) C 2 H 6 (g) O 2 (g) = kj mol 1 2CO 2 (g) + 3H 2 O(l) B A 61 For which o the ollowing reactions does the value o represent both a standard enthalpy change o combustion and a standard enthalpy change o ormation? (1) H 2 (g) O 2(g) H 2 O(l) (2) 2C(s) + O 2 (g) 2CO(g) (3) 2H 2 O(l) + O 2 (g) 2H 2 O 2 (aq) A (1) only B (2) only C (1) and (3) only D (2) and (3) only 62 The reaction between iron and oxygen can be represented as ollows: 2Fe(s) O 2(g) A Fe 2 O 3 (s) ΔH = 820 kj Which o the ollowing statements are correct? (1) Iron(III) oxide is not readily decomposed by heat. (2) The enthalpy change o the above reaction represents the enthalpy change o ormation o iron(iii) oxide. (3) 820 kj are released when 1 mole o iron(iii) oxide is decomposed into its elements. A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) A Which o the ollowing terms could be used or the enthalpy change shown above? (1) c (2) (3) r A (1) only B (2) only C (1) and (3) only D (2) and (3) only C 8

9 Directions : Each question (Questions 63 67) consists o two separate statements. Decide whether each o the two statements is true or alse; i both are true, then decide whether or not the second statement is a correct explanation o the irst statement. Then select one option rom A to D according to the ollowing table : A Both statements are true and the 2nd statement is a correct explanation o the 1st statement. B Both statements are true but the 2nd statement is NOT a correct explanation o the 1st statement. C The 1st statement is alse but the 2nd statement is true. D Both statements are alse. 1st statement 2nd statement 63 In an exothermic reaction, heat is taken in rom In an exothermic reaction, the total enthalpy D the surroundings. o the products is greater than that o the reactants. Part A Unit An endothermic reaction occurs when quicklime When quicklime is added to water, the C is added to water. temperature o the water rises. 65 The combustion o ethanol is an endothermic Heat is released when ethanol burns. C reaction. 66 The standard enthalpy change o combustion Both enthalpy changes reer to the enthalpy D o carbon (graphite) equals the standard change o the reaction between carbon enthalpy change o ormation o carbon (graphite) and oxygen to orm carbon monoxide. monoxide under standard conditions. 67 Iron powder can be used to make warm-packs When iron powder reacts with oxygen, an A or keeping users warm. exothermic reaction occurs. 9

10 Unit 30 Hess s Law and its applications Fill in the blanks 1 Hess s Law states that the enthalpy change o a reaction depends on the initial and inal states o the reaction. 2 The ollowing inormation is required to calculate the enthalpy change o ormation o magnesium oxide: 7 I a reaction can be written as the sum T o several steps, the enthalpy change or the overall reaction equals the sum o the enthalpy changes o the steps. 8 The enthalpy change o ormation o T magnesium carbonate can be obtained by applying Hess s Law. 9 The standard enthalpy change o ormation F o ethyne (C 2 H 2 ) is the sum o the standard enthalpy changes o ormation o carbon (graphite) and hydrogen. Part A Unit 30 a) enthalpy change or the reaction between magnesium and hydrochloric acid ; b) enthalpy change or the reaction between magnesium oxide and hydrochloric acid ; 10 The standard enthalpy change or the F decomposition o calcium carbonate is the sum o the standard enthalpy changes o ormation o calcium oxide and carbon dioxide. 10 c) enthalpy change o ormation o water. 3 Standard enthalpy change o ormation o a compound = sum o standard enthalpy changes o combustion o constituent elements standard enthalpy change o combustion o compound 4 Standard enthalpy change o a reaction = sum o standard enthalpy changes o ormation o products sum o standard enthalpy changes o ormation o reactants True or alse Decide whether each o the ollowing statements is true or alse. 5 The enthalpy change o ormation o F solid sodium hydroxide can be determined directly rom an experiment. 6 The enthalpy change o combustion o T butane can be determined directly rom an experiment. Multiple choice questions 11 Which o the ollowing values CANNOT be determined directly by an experiment? A Enthalpy change o ormation o water B Enthalpy change o combustion o butane C Enthalpy change o ormation o carbon monoxide D Enthalpy change o solution o ammonium nitrate C Directions: Questions 12 and 13 reer to the ollowing inormation. A + B C + D = 10.0 kj mol 1 C + D E = kj mol 1 12 What is the standard enthalpy change o the ollowing reaction? 2C + 2D A kj B kj C 10.0 kj D 20.0 kj 2A + 2B A

11 13 What is the standard enthalpy change o the ollowing reaction? A + B A kj B +5.0 kj C 5.0 kj D 25.0 kj 14 Given the ollowing data: E N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = +88 kj N 2 (g) + 2O 2 (g) N 2 O 4 (g) ΔH = +10 kj What is the enthalpy change o the ollowing reaction? 4NO 2 (g) A +196 kj B +156 kj C 156 kj D 196 kj 2N 2 O 4 (g) B C A 200 kj B 295 kj C 690 kj D 790 kj 17 The energy level diagram shown below relates to the ollowing two reactions: 2Cu(s) + O 2 (g) 2CuO(s) ΔH = 312 kj 2Cu(s) O 2(g) Enthalpy (kj) 2Cu(s) + O 2 (g) ΔH = 312 2CuO(s) A Cu 2 O(s) ΔH = 170 kj ΔH = 170 Cu 2 O(s) O 2 (g) Part A Unit The standard enthalpy changes o ormation o two wolram bromides are given below. W(s) + 2Br 2 (l) W(s) + 3Br 2 (l) WBr 4 (s) = 147 kj mol 1 WBr 6 (s) = 184 kj mol 1 What is the standard enthalpy change o the ollowing reaction? Br 2 (l) + WBr 4 (s) A 37 kj mol 1 B 331 kj mol 1 C +37 kj mol 1 D +331 kj mol 1 16 Given the ollowing data: WBr 6 (s) S(s) + O 2 (g) SO 2 (g) ΔH = 295 kj and S(s) O 2(g) SO 3 (g) ΔH = 395 kj What is the enthalpy change o the ollowing reaction under the same conditions? 2SO 2 (g) + O 2 (g) 2SO 3 (g) A What is the value o ΔH o the ollowing reaction? 4CuO(s) A 284 kj B 142 kj C +142 kj D +284 kj 2Cu 2 O(s) + O 2 (g) 18 Given the ollowing data: C(s) + O 2 (g) CO 2 (g) = 394 kj 2C(s) + O 2 (g) 2CO(g) = 222 kj How much heat is released in the complete combustion o 56.0 g o CO(g)? (Relative atomic masses: C = 12.0, O = 16.0) A 172 kj B 566 kj C 616 kj D kj D B 11

12 Part A Unit Many insects and small animals have unique deence systems. Bombardier beetles ight o predators with a hot chemical spray. This spray consists o solutions o hydroquinone (C 6 H 4 (OH) 2 (aq)), hydrogen peroxide (H 2 O 2 (aq)) and enzymes. The ollowing equations relate to the equation or spray ormation: I 2H 2 O(l) + O 2 (g) 2H 2 O 2 (aq) ΔH = +189 kj II H 2 O(l) H 2 (g) O 2(g) ΔH = +286 kj III C 6 H 4 (OH) 2 (aq) C 6 H 4 O 2 (aq) + H 2 (g) ΔH = +177 kj A chemical reaction that occurs in order to produce the hot chemical spray can be represented by the equation C 6 H 4 (OH) 2 (aq) + H 2 O 2 (aq) hydroquinone C 6 H 4 O 2 (aq) + 2H 2 O(l) quinone What is the enthalpy change o reaction or the production o the hot chemical spray? A +274 kj B 145 kj C 204 kj D 298 kj 20 Consider the enthalpy change cycle shown below. C 21 Consider the enthalpy changes o the ollowing reactions: N 2 O(g) N 2 (g) O 2(g) ΔH = p NO(g) O 2(g) NO 2 (g) ΔH = q N 2 (g) + O 2 (g) 2NO(g) ΔH = r N 2 O(g) + NO 2 (g) 3NO(g) ΔH = s What is the relationship between the enthalpy changes p, q, r and s? A s = p + q + r B s = p q r C s = p + q r D s = p q + r 22 The table below lists the standard enthalpy changes o combustion o three substances. Substance c (kj mol 1 ) C(graphite) 394 H 2 (g) 286 C 3 H 6 (g) cyclopropane What is the standard enthalpy change o ormation o cyclopropane? A kj mol 1 B 50 kj mol 1 C +50 kj mol 1 D kj mol 1 D C W ΔH = 210 kj mol 1 Z 23 The table below lists the standard enthalpy changes o combustion o three substances. ΔH = 70 kj mol 1 Substance c (kj mol 1 ) C(graphite) X ΔH = 98 kj mol 1 What is the enthalpy change o the reaction Z to Y? A +42 kj mol 1 B 42 kj mol 1 C +378 kj mol 1 D 378 kj mol 1 Y A H 2 (g) 286 CH 3 C CH(g) propyne What is the standard enthalpy change o ormation o propyne? A kj mol 1 B 184 kj mol 1 C +184 kj mol 1 D kj mol 1 C

13 24 The table below lists the standard enthalpy changes o combustion o three substances. Substance c (kj mol 1 ) C(graphite) 394 H 2 (g) 286 CH 3 COOH(l) 870 What is the standard enthalpy change o ormation o ethanoic acid? A 654 kj mol 1 B 490 kj mol 1 C +490 kj mol 1 D +654 kj mol 1 25 Given the ollowing data: Standard enthalpy change o ormation o methanoic acid = x kj mol 1 Standard enthalpy change o combustion o methanoic acid = y kj mol 1 Standard enthalpy change o combustion o hydrogen = z kj mol 1 What is the standard enthalpy change o ormation o carbon dioxide? A x + y z B z x y C x y + z D z x + y 26 The standard enthalpy changes o ormation o ethene, water and ethanol are kj mol 1, 286 kj mol 1 and 278 kj mol 1 respectively. What is the standard enthalpy change o the ollowing reaction? C 2 H 4 (g) + H 2 O(l) A kj mol 1 B 44.2 kj mol 1 C 60.2 kj mol 1 D 512 kj mol 1 C 2 H 5 OH(l) B B B 27 Given the ollowing data: [FeO(s)] = 270 kj mol 1 [Fe 2 O 3 (s)] = 820 kj mol 1 What is the standard enthalpy change o the ollowing reaction? 2FeO(s) O 2(g) A +140 kj B 140 kj C +280 kj D 280 kj Fe 2 O 3 (s) 28 The standard enthalpy changes o ormation o cyclopropane and propene are kj mol 1 and kj mol 1 respectively. What is the standard enthalpy change o the ollowing reaction? CH 2 CH 2 CH 2 (g) CH 3 CH=CH 2 (g) A kj mol 1 B kj mol 1 C 34.8 kj mol 1 D 75.6 kj mol 1 29 Consider the ollowing thermochemical equations: H 2 (g) O 2(g) D C H 2 O(l) = 286 kj mol 1 H 2 O(l) H 2 O(g) = +44 kj mol 1 What is the standard enthalpy change o the ollowing reaction? 2H 2 (g) + O 2 (g) A 484 kj B 242 kj C +242 kj D +484 kj 2H 2 O(g) A Part A Unit 30 13

14 Part A Unit Given the ollowing data: Compound (kj mol 1 ) HCOOH(l) 409 CO(g) 110 H 2 O(l) 286 What is the standard enthalpy change o the ollowing reaction? HCOOH(l) A +13 kj mol 1 B 585 kj mol 1 C 695 kj mol 1 D 805 kj mol 1 CO(g) + H 2 O(l) Directions: Questions 31 and 32 reer to the ollowing inormation. Glucose is a biological uel used by cells to satisy the energy needs o plants and animals. The overall reaction or the metabolism o glucose is represented by the ollowing equation: C 6 H 12 O 6 (s) + 6O 2 (g) Given the ollowing data: Compound 6CO 2 (g) + 6H 2 O(l) (kj mol 1 ) C 6 H 12 O 6 (s) glucose CO 2 (g) 394 H 2 O(l) What is the standard enthalpy change o the reaction or the metabolism o glucose? A kj mol 1 B kj mol 1 C kj mol 1 D kj mol 1 A C 32 I solid glucose is completely burnt in the lame o a Bunsen burner, the enthalpy change is A greater than it is during cellular respiration because the production o H 2 O(g) releases more energy than does the production o H 2 O(l). B less than it is during cellular respiration because the production o H 2 O(g) releases less energy than does the production o H 2 O(l). C the same as it is in the body because the enthalpy change is independent o the state o the products. D the same as it is in cellular respiration because they are identical processes. B 33 Ammonium nitrate decomposes according to the ollowing equation: NH 4 NO 3 (s) N 2 O(g) + 2H 2 O(g) The standard enthalpy changes o ormation o NH 4 NO 3 (s), N 2 O(g) and H 2 O(g) are 365 kj mol 1, +80 kj mol 1 and 242 kj mol 1 respectively. What is the standard enthalpy change o the decomposition o ammonium nitrate? A 39 kj mol 1 B 41 kj mol 1 C +43 kj mol 1 D +203 kj mol 1 34 Given the ollowing data: Substance (kj mol 1 ) NH 3 (g) 46 F 2 (g) 0 NF 3 (g) 114 NH 4 F(s) 467 What is the standard enthalpy change o the ollowing reaction? 4NH 3 (g) + 3F 2 (g) A 5.35 x 10 2 kj mol 1 B 1.10 x 10 3 kj mol 1 C 1.33 x 10 3 kj mol 1 D 1.87 x 10 3 kj mol 1 NF 3 (g) + 3NH 4 F(s) A C 14

15 35 H 2 S(g) burns in oxygen to make H 2 O(g) and SO 2 (g). Given the ollowing standard enthalpy changes o ormation: Given the ollowing data: Compound (kj mol 1 ) H 2 (g) + S(s) H 2 (g) O 2(g) S(s) + O 2 (g) H 2 S(g) = 40 kj mol 1 H 2 O(g) = 242 kj mol 1 SO 2 (g) = 297 kj mol 1 What is the standard enthalpy change o combustion o H 2 S(g)? A 579 kj mol 1 B 499 kj mol 1 C 95 kj mol 1 D 55 kj mol 1 36 The Thermite Process involves the reaction between aluminium and iron(iii) oxide to produce iron and aluminium oxide. Given the ollowing data: Compound (kj mol 1 ) Al 2 O 3 (s) Fe 2 O 3 (s) 825 What is the standard enthalpy change o the ollowing reaction? 2Al(s) + Fe 2 O 3 (s) A kj mol 1 B +851 kj mol 1 C 851 kj mol 1 D kj mol 1 Al 2 O 3 (s) + 2Fe(s) 37 The uranium-235 isotope is used as a uel in some nuclear power plants. This isotope is used to enrich natural uranium ore. Prior to the enrichment process, the uranium ore, UO 2 (s), is converted to UF 6 (s). This conversion is represented by the ollowing equations. Equation I UO 2 (s) + 4HF(g) Equation II UF 4 (s) + F 2 (g) UF 4 (s) + 2H 2 O(g) UF 6 (s) B C UO 2 (s) UF 4 (s) UF 6 (s) HF(g) 271 H 2 O(g) 242 What is the amount o heat involved in producing 2.00 x 10 6 g o UF 6 (s) rom natural uranium ore, UO 2 (s)? (Relative atomic masses: F = 19.0, U = 238.0) A 2.18 x 10 6 kj B 2.76 x 10 6 kj C 5.46 x 10 6 kj D 7.65 x 10 6 kj 38 The standard enthalpy changes o ormation o some vanadium compounds are listed below. V(s) + Cl 2 (g) V(s) Cl 2(g) V(s) + 2Cl 2 (g) VCl 2 (s) = 452 kj mol 1 A VCl 3 (s) = 581 kj mol 1 VCl 4 (l) = 569 kj mol 1 What is the amount o heat involved when mole o VCl 4 (l) decomposes to orm VCl 2 (g) and Cl 2 (g)? A +176 kj B kj C 46.8 kj D 176 kj 39 Given the ollowing data: C 3 H 6 (g) O 2(g) 3CO 2 (g) + 3H 2 O(l) = kj C(s) + O 2 (g) CO 2 (g) = 394 kj 2H 2 (g) + O 2 (g) 2H 2 O(l) = 572 kj B Part A Unit 30 15

16 What is the standard enthalpy change o ormation o cyclopropane (C 3 H 6 )? A +993 kj mol 1 B +81 kj mol 1 C 81 kj mol 1 D 993 kj mol 1 C 42 What is the standard enthalpy change o combustion o octane? A 421 kj mol 1 B kj mol 1 C +421 kj mol 1 D kj mol 1 B 40 Methyl tert-butyl ether (MTBE) is prepared by the reaction o methanol with 2-methylpropene according to the ollowing equation: 43 Given the ollowing data: Compound (kj mol 1 ) (CH 3 ) 2 C=CH 2 + CH 3 OH CO 2 (g) 394 CH 3 H 2 O(l) 286 Part A Unit 30 CH 3 C O CH 3 CH 3 MTBE = 57.8 kj mol 1 Standard enthalpy changes o ormation o MTBE and methanol are 314 kj mol 1 and 239 kj mol 1 respectively. What is the standard enthalpy change o ormation o 2-methylpropene? A 133 kj mol 1 B 17.2 kj mol 1 C kj mol 1 D +133 kj mol 1 41 The standard enthalpy changes o combustion o graphite and carbon monoxide are 394 kj mol 1 and 283 kj mol 1 respectively. What is the standard enthalpy change o ormation o carbon monoxide? A +677 kj mol 1 B +111 kj mol 1 C 111 kj mol 1 D 677 kj mol 1 Directions: Questions 42 and 43 reer to the combustion o octane (C 8 H 18 ). Complete combustion o 1.00 g o octane liberates 48.0 kj at 298 K under atmospheric pressure. (Relative atomic masses: H = 1.0, C = 12.0) B C What is the standard enthalpy change o ormation o octane? A 256 kj mol 1 B 479 kj mol 1 C +256 kj mol 1 D +479 kj mol 1 Directions: Questions reer to the ollowing inormation. The enthalpy change o ormation o MgO(s) can be determined by Hess s Law. Mg(s) O 2(g) MgO(s) Mg(s) and MgO(s) were added to separate 50.0 cm 3 portions o HCl(aq). The acid was in excess in both cases. The ollowing results were obtained with MgO(s): Mass o MgO(s) used = 1.10 g Temperature change = C 44 Which o the ollowing statements concerning the reaction o MgO(s) with HCl(aq) is correct? (Density o HCl(aq) = 1.00 g cm 3 ; speciic heat capacity o HCl(aq) = 4.18 J g 1 K 1 ) A An exothermic process releasing J o heat B An exothermic process absorbing J o heat C An endothermic process releasing J o heat D An endothermic process absorbing J o heat A A 16

17 45 What is the enthalpy change when 1 mole o MgO(s) reacts completely with HCl(aq)? (Molar mass o MgO = 40.3 g mol 1 ) A kj mol 1 B +126 kj mol 1 C 94.2 kj mol 1 D 126 kj mol 1 46 The enthalpy change when 1 mole o Mg(s) reacts completely with HCl(aq) is 452 kj mol 1 and the enthalpy change o ormation o water is 286 kj mol 1. What is the enthalpy change o ormation o MgO(s)? A kj mol 1 B +612 kj mol 1 C 612 kj mol 1 D kj mol 1 47 Which o the ollowing values can be determined directly by an experiment? (1) Enthalpy change o ormation o propane (2) Enthalpy change o combustion o hydrogen (3) Enthalpy change o solution o calcium chloride A (1) only B (2) only C (1) and (3) only D (2) and (3) only 48 What inormation are needed to determine the standard enthalpy change o ormation o methane? (1) Enthalpy change o combustion o methane (2) Enthalpy change o combustion o carbon (graphite) (3) Enthalpy change o combustion o hydrogen A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) D C D D 49 A decomposition o ammonium dichromate is shown by the ollowing equation: (NH 4 ) 2 Cr 2 O 7 (s) N 2 (g) + 4H 2 O(l) + Cr 2 O 3 (s) What inormation are needed to calculate the enthalpy change o the decomposition o ammonium dichromate? (1) [(NH 4 ) 2 Cr 2 O 7 (s)] (2) c [H 2 (g)] (3) [Cr 2 O 3 (s)] A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) 50 Calcium hydroxide decomposes on strong heating to orm calcium oxide and water. Ca(OH) 2 (s) CaO(s) + H 2 O(l) What inormation are needed to calculate the enthalpy change o the decomposition o Ca(OH) 2 (s)? (1) E n t h a l p y c hange o the reaction between calcium hydroxide and dilute hydrochloric acid (2) Enthalpy change o the reaction between calcium oxide and dilute hydrochloric acid (3) Enthalpy change o ormation o water A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) D A Part A Unit 30 17

18 Directions : Each question (Questions 51 55) consists o two separate statements. Decide whether each o the two statements is true or alse; i both are true, then decide whether or not the second statement is a correct explanation o the irst statement. Then select one option rom A to D according to the ollowing table : A Both statements are true and the 2nd statement is a correct explanation o the 1st statement. B Both statements are true but the 2nd statement is NOT a correct explanation o the 1st statement. C The 1st statement is alse but the 2nd statement is true. D Both statements are alse. 1st statement 2nd statement Part A Unit The standard enthalpy change o combustion The complete combustion o methane gives D o methane is the sum o the standard enthalpy carbon monoxide and water. change o combustion o carbon and that o hydrogen. 52 The standard enthalpy change o ormation o Carbon and hydrogen would give ethanol D ethanol can be determined directly rom an when burnt in oxygen. experiment. 53 The standard enthalpy change o thermal Potassium hydrogencarbonate decomposes to B decomposition o potassium hydrogencarbonate give potassium carbonate, carbon dioxide and can be obtained by applying Hess s Law. water when heated. 54 The enthalpy change o hydration o CuSO 4 (s) Both CuSO 4 (s) and CuSO 4 5H 2 O(s) are C can be determined directly rom an experiment. soluble in water. 55 The enthalpy change o the reaction between The standard enthalpy change o ormation B calcium and water can be determined directly o calcium is zero. rom an experiment. 18

19 Topic-based exercise Multiple choice questions 1 The decomposition o hydrogen peroxide can be represented by the thermochemical equation: H 2 O 2 (l) H 2 O(l) O 2(g) ΔH = 98.2 kj mol 1 From this equation, it can be concluded that the ormation o two moles o hydrogen peroxide rom water and oxygen is an A exothermic process releasing 98.2 kj o heat. B exothermic process releasing 196 kj o heat. C endothermic process absorbing 98.2 kj o heat. D endothermic process absorbing 196 kj o heat. D 2 The combustion o propane and cellular respiration are similar processes. The reactions that occur in both processes are A exothermic, and carbon is reduced. B exothermic, and carbon is oxidized. C endothermic, and carbon is reduced. D endothermic, and carbon is oxidized. 3 In an experiment, liquid sulphur is cooled and converted to solid state as represented by the ollowing equation: S(l) S(s) This conversion is A endothermic, releases heat and has a positive ΔH value. B exothermic, releases heat and has a negative ΔH value. C exothermic, takes in heat and has a negative ΔH value. D endothermic, takes in heat and has a positive ΔH value. B B 4 A 77.5 g sample o brass is heated to 98.7 C in a boiling water bath. The brass is quickly transerred to a calorimeter containing 103 g o water at 18.5 C. The inal temperature o the water and the brass is 23.5 C. What is the speciic heat capacity o the brass? (Speciic heat capacity o water = 4.18 J g 1 K 1 ) A J g 1 K 1 B J g 1 K 1 C J g 1 K 1 D 1.32 J g 1 K 1 5 For a system, ΔE is measured at constant volume while ΔH is measured at constant pressure. Consider the ollowing reaction: 2C(s) + O 2 (g) 2CO(g) ΔH < 0 How do ΔE and ΔH compare or the system during this reaction? A ΔE < ΔH B ΔE > ΔH C ΔE = ΔH D Impossible to tell rom this inormation 6 C 2 H 2 (g) undergoes complete combustion as represented by the ollowing equation: 2C 2 H 2 (g) + 5O 2 (g) B A 4CO 2 (g) + 2H 2 O(l) ΔH = kj What is the amount o heat released by the complete combustion o g o C 2 H 2 (g)? (Relative atomic masses: H = 1.0, C = 12.0) A 2.50 x 10 3 kj B 5.00 x 10 3 kj C 7.50 x 10 3 kj D 10.0 x 10 3 kj B 19

20 7 When 2.42 g o ethanal (CH 3 CHO(l)) are burnt in a calorimeter to produce H 2 O(l) and CO 2 (g), 65.5 kj o heat are produced. What is the enthalpy change o combustion o ethanal under the experimental conditions? (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0) A kj mol 1 B 76.6 kj mol 1 C 165 kj mol 1 D kj mol 1 8 A student uses an aluminium calorimeter to determine the enthalpy change o solution o ammonium nitrate. The student assumes that the heat capacity o the calorimeter is negligible. The data were collected and recorded in the ollowing table. Mass o aluminium calorimeter Mass o aluminium calorimeter and contents Mass o ammonium nitrate Initial temperature o calorimeter and contents Final temperature o calorimeter and contents g g 1.68 g 22.3 C 21.0 C (Molar mass o NH 4 NO 3 = 80.0 g mol 1 ; density o inal solution = 1.00 g cm 3 ; speciic heat capacity o inal solution = 4.18 J g 1 K 1 ) What is the enthalpy change o solution o ammonium nitrate? A kj mol 1 B kj mol 1 C 38.8 kj mol 1 D 81.5 kj mol 1 D B 9 A laboratory technician adds 43.1 cm 3 o 11.6 mol dm 3 hydrochloric acid to water to orm cm 3 o dilute solution. A temperature rise o 2.60 C is observed. What is the enthalpy change o dilution o hydrochloric acid? (Density o dilute solution = 1.00 g cm 3 ; speciic heat capacity o dilute solution = 4.18 J g 1 K 1 ) A 5.45 kj mol 1 B 10.9 kj mol 1 C kj mol 1 D kj mol 1 10 Given the ollowing data: 2CH 3 OH(l) + 3O 2 (g) B 2CO 2 (g) + 4H 2 O(g) = kj What is the amount o methanol that must be burnt to raise the temperature o g o water rom 20.0 C to 35.0 C? (Speciic heat capacity o water = 4.18 J g 1 K 1 ) A 6.12 x 10 3 mol B 1.23 x 10 2 mol C 2.46 x 10 2 mol D 2.46 x 10 1 mol cm 3 o 2.00 mol dm 3 hydrochloric acid were mixed with 50.0 cm 3 o 1.00 mol dm 3 sodium hydroxide solution. Both solutions were initially at 18.0 C. Ater mixing, the temperature o the inal solution was 26.5 C. What is the enthalpy change o neutralization between hydrochloric acid and sodium hydroxide solution? (Density o inal solution = 1.00 g cm 3 ; speciic heat capacity o inal solution = 4.18 J g 1 K 1 ) A kj mol 1 B kj mol 1 C 35.6 kj mol 1 D 53.2 kj mol 1 C D 20

21 cm 3 o 1.50 mol dm 3 sodium hydroxide solution were put in a polystyrene cup cm 3 o dilute sulphuric acid in 5.0 cm 3 portions were added to the alkali. The temperature o the reaction mixture was taken ater each addition. 14 In an experiment, 17.2 g o ammonium sulphate were dissolved in cm 3 o water in a simple calorimeter. A temperature change rom 25.2 C to 22.3 C was observed. What is the enthalpy change o solution o ammonium sulphate under the experimental conditions? (Relative atomic masses: H = 1.0, N = 14.0, O = 16.0, S = 32.1; speciic heat capacity o water = 4.18 J g 1 K 1 ) A 13.1 kj mol 1 B 1.22 kj mol 1 C kj mol 1 D kj mol 1 D What is the molarity o the sulphuric acid? A mol dm 3 B mol dm 3 C 1.07 mol dm 3 D 2.14 mol dm 3 13 The ollowing experimental results were obtained when dilute hydrochloric acid reacted with dilute sodium hydroxide solution. Expt. Acid Alkali cm 3 o 1 mol dm 3 HCl(aq) 50 cm 3 o 2 mol dm 3 HCl(aq) 50 cm 3 o 1 mol dm 3 NaOH(aq) 50 cm 3 o 2 mol dm 3 NaOH(aq) Heat change (kj) q 1 q 2 C Enthalpy change o neutralization (kj mol 1 ) ΔH n1 ΔH n2 Which o the ollowing combinations is correct? Heat change Enthalpy change o neutralization A q 1 < q 2 ΔH n1 < ΔH n2 B q 1 = q 2 ΔH n1 = ΔH n2 C q 1 < q 2 ΔH n1 = ΔH n2 D q 1 = q 2 ΔH n1 < ΔH n2 C 15 Consider the enthalpy changes o the ollowing reactions: 2NO 2 (g) + 2H 2 O(g) 3O 2 (g) + N 2 H 4 (g) ΔH = p 1 2 N 2(g) + O 2 (g) NO 2 (g) ΔH = q H 2 (g) O 2(g) H 2 O(g) ΔH = r N 2 (g) + 2H 2 (g) N 2 H 4 (g) ΔH = s What is the relationship between the enthalpy changes p, q, r and s? A p = 2q + 2r s B p = q + r 2s C p = s q + r D p = s 2q 2r 16 Given the ollowing data: C(s) + H 2 O(g) CO 2 (g) + H 2 (g) CO(g) + H 2 (g) = +131 kj mol 1 CO(g) + H 2 O(g) = +41 kj mol 1 What is the standard enthalpy change o the ollowing reaction? 2CO(g) A +172 kj B +90 kj C 90 kj D 172 kj C(s) + CO 2 (g) D D 21

22 17 Given the ollowing data: 1 2 I 2(s) Cl 2(g) ICl(g) = +18 kj mol 1 I 2 (s) I 2 (g) = +62 kj mol 1 What is the standard enthalpy change o the ollowing reaction? I 2 (g) + Cl 2 (g) A 80 kj B 26 kj C +26 kj D +80 kj 2ICl(g) 18 Given the ollowing data: B 20 Given the ollowing standard enthalpy changes o reaction: C(s) + O 2 (g) CO 2 (g) = 394 kj H 2 (g) O 2(g) H 2 O(l) = 286 kj CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) = 890 kj What is the standard enthalpy change o ormation o CH 4 (g)? A kj mol 1 B +782 kj mol 1 C 76 kj mol 1 D kj mol 1 C N 2 (g) + 2O 2 (g) 2NO 2 (g) = kj N 2 (g) + 2O 2 (g) N 2 O 4 (g) = kj What is the standard enthalpy change o dimerization o NO 2 (g)? A 57.9 kj B 77.3 kj C kj D kj 19 The table below lists the standard enthalpy change o ormation o our compounds: Compound (kj mol 1 ) A Directions: Questions 21 and 22 reer to the ollowing inormation. One component o acid rain can be ormed in the atmosphere by the ollowing reaction: SO 3 (g) + H 2 O(l) 21 As SO 3 (g) dissolves, acid rain H 2 SO 4 (aq) = kj mol 1 A increases in ph and decreases in temperature. B increases in ph and increases in temperature. C decreases in ph and increases in temperature. D decreases in ph and decreases in temperature. C H 2 O(l) 286 HCl(g) 92 SiO 2 (s) 910 SiCl 4 (l) 640 What is the standard enthalpy change o the hydrolysis o SiCl 4 (l)? SiCl 4 (l) + 2H 2 O(l) A 76 kj mol 1 B 66 kj mol 1 C +66 kj mol 1 D +76 kj mol 1 SiO 2 (s) + 4HCl(g) B 22 The table below lists the standard enthalpy changes o ormation o two compounds. Compound (kj mol 1 ) SO 3 (g) H 2 O(l) 286 What is the standard enthalpy change o ormation o H 2 SO 4 (aq) in the atmosphere? A 454 kj mol 1 B 587 kj mol 1 C 814 kj mol 1 D 910 kj mol 1 D 22

23 23 The table below lists the standard enthalpy changes o combustion o three substances. Substance c (kj mol 1 ) C(graphite) Ethanoic acid is prepared by the reaction o ethanol with oxygen. CH 3 CH 2 OH(l) + O 2 (g) Given the ollowing data: CH 3 COOH(l) + H 2 O(l) H 2 (g) 286 Compound (kj mol 1 ) CH 3 OH(l) 726 CH 3 CH 2 OH(l) 278 What is the standard enthalpy change o ormation o methanol? A 548 kj mol 1 B 240 kj mol 1 C +240 kj mol 1 D +548 kj mol 1 24 Given the ollowing data: Substance c (kj mol 1 ) C(graphite) 394 H 2 (g) 286 C 4 H 6 (g) buta-1,3-diene What is the enthalpy change o ormation o buta-1,3-diene? A kj mol 1 B +108 kj mol 1 C 108 kj mol 1 D kj mol 1 B B CH 3 COOH(l) 485 H 2 O(l) 286 What is the standard enthalpy change o the above reaction o ethanol with oxygen? A 79 kj mol 1 B 477 kj mol 1 C 493 kj mol 1 D +79 kj mol 1 Directions: Questions 27 and 28 reer to the ollowing inormation. Disposable lighters contain butane gas which undergoes combustion, as represented by the ollowing equation: 2C 4 H 10 (g) + 13O 2 (g) 8CO 2 (g) + 10H 2 O(g) The table below lists the standard enthalpy changes o ormation o three substances. Substance (kj mol 1 ) H 2 O(g) 242 C 25 Given that the enthalpy changes o combustion o carbon, hydrogen and propane are x, y and z kj mol 1 respectively, then the enthalpy change o ormation o propane is given by A 3x 4y + z B 3x + 4y z C 3x + 4y + z D 3x 8y z A CO 2 (g) 394 C 4 H 10 (g) What is the enthalpy change or the combustion o 1 mole o butane gas, as represented by the above equation? A kj B kj C kj D kj A 23

24 28 How much heat is produced when 1.00 g o butane gas in a disposable lighter is completely burnt to orm carbon dioxide and water vapour? (Relative atomic masses: H = 1.0, C = 12.0) A 25.0 kj B 45.9 kj C 91.5 kj D 100 kj B Directions: Questions 31 and 32 reer to the ollowing experiment. When calcium is placed in water, calcium hydroxide and hydrogen are ormed. In an experiment, 1.00 g o calcium was placed in g o water and the temperature o water increased by 12.2 C ater the reaction completed. (Relative atomic mass: Ca = 40.1; speciic heat capacity o water = 4.18 J g 1 K 1 ) 29 Given the ollowing data: Compound (kj mol 1 ) NH 3 (g) 46 H 2 O(l) 286 What is the standard enthalpy change o the ollowing reaction o ammonia? 4NH 3 (g) + 3O 2 (g) A kj B 868 kj C +868 kj D kj 30 Consider the ollowing reaction: 2N 2 (g) + 6H 2 O(l) X + 3Y 2Z + 2W = kj Given the ollowing data: Substance (kj mol 1 ) X 22.5 Y Z 54.8 What is the standard enthalpy change o ormation o substance W? A +442 kj mol 1 B 120 kj mol 1 C 240 kj mol 1 D 451 kj mol 1 A B 31 What is the enthalpy change o the reaction between calcium and water? A +409 kj mol 1 B +102 kj mol 1 C 102 kj mol 1 D 409 kj mol 1 32 [Ca(OH) 2 (s)] reers to the enthalpy change o the ollowing process: Ca(s) + O 2 (g) + H 2 (g) Ca(OH) 2 (s) The standard enthalpy change o ormation o water is 286 kj mol 1. What is the enthalpy change o ormation o calcium hydroxide? A 981 kj mol 1 B 695 kj mol 1 C +695 kj mol 1 D +981 kj mol 1 33 Which o the ollowing reactions are endothermic? (1) C 9 H 20 (l) C 2 H 6 (g) + C 3 H 6 (g) + C 4 H 8 (g) (2) C(graphite) + O 2 (g) CO 2 (g) (3) CaCO 3 (s) CaO(s) + CO 2 (g) A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) D A B 24

25 34 Consider the ollowing calorimetry experimental data: (1) Mass change o ethanol (2) Mass o aluminium calorimeter (3) Mass o aluminium calorimeter and water (4) Initial temperature o aluminium calorimeter (5) Maximum temperature change o ethanol (6) Maximum temperature change o aluminium calorimeter and water Which o the calorimetry experimental data are required to determine the enthalpy change o combustion o ethanol? A (1), (3) and (4) only B (3), (4) and (5) only C (1), (2), (3) and (6) only D (2), (4), (5) and (6) only 35 For which o the ollowing reactions does the value o represent both the standard enthalpy change o combustion o an element and the standard enthalpy change o ormation o a compound? (1) C(s) O 2(g) CO(g) (2) Ca(s) O 2(g) CaO(s) (3) H 2 (g) + O 2 (g) H 2 O 2 (l) A (1) only B (2) only C (1) and (3) only D (2) and (3) only C B 37 Sodium hydrogencarbonate decomposes according to the ollowing equation: 2NaHCO 3 (s) Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) = +128 kj What other inormation is required to calculate the standard enthalpy change o ormation o Na 2 CO 3 (s)? (1) (2) (3) [NaHCO 3 (s)] [H 2 O(g)] [CO 2 (g)] A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) 38 The enthalpy change o ormation o Mg(OH) 2 (s) reers to the enthalpy change o the ollowing process: Mg(s) + O 2 (g) + H 2 (g) Mg(OH) 2 (s) What inormation is needed to estimate the enthalpy change o ormation o Mg(OH) 2 (s)? (1) Enthalpy change o the reaction between magnesium and dilute hydrochloric acid (2) E n t h a l p y c hange o the reaction between magnesium hydroxide and dilute hydrochloric acid (3) Enthalpy change o ormation o water A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) D D 36 The enthalpy changes o which o the ollowing reactions can ONLY be obtained by applying Hess s Law? (1) The conversion o hydrated magnesium sulphate to anhydrous magnesium sulphate (2) The ormation o ethanol rom its elements (3) The combustion o sucrose A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3) A 25

26 Directions : Each question (Questions 39 44) consists o two separate statements. Decide whether each o the two statements is true or alse; i both are true, then decide whether or not the second statement is a correct explanation o the irst statement. Then select one option rom A to D according to the ollowing table : A Both statements are true and the 2nd statement is a correct explanation o the 1st statement. B Both statements are true but the 2nd statement is NOT a correct explanation o the 1st statement. C The 1st statement is alse but the 2nd statement is true. D Both statements are alse. 1st statement 2nd statement 39 The enthalpy change o an endothermic reaction In an endothermic reaction, heat is transerred C is a negative value. rom the surroundings to the system. O 40 ΔH r values or reactions involving carbon should The enthalpy o diamond is lower than that D relate to diamond rather than graphite. o graphite. 41 The combustion o methanol is an exothermic During the combustion o methanol, the A reaction. amount o energy released in the bond-orming step is greater than that used in the bond-breaking step. 42 The standard enthalpy change o neutralization During the neutralization between a strong acid C between hydrochloric acid and sodium hydroxide and a strong alkali, the only chemical change is solution is the same as that between hydrochloric the combination o H + (aq) ions and OH (aq) acid and aqueous ammonia. ions to orm water. 43 The standard enthalpy change o ormation o Magnesium reacts with oxygen to give C magnesium oxide can be determined directly magnesium oxide. rom an experiment. 44 The standard enthalpy change o solution o Hess s Law states the enthalpy change o a D ammonium nitrate can only be determined by process depends on the route by which the applying Hess s Law. process occurs. 26

27 Short questions 45 Classiy each o the ollowing changes as exothermic or endothermic. a) Discharge o a lashlight (1 mark) exothermic (1) b) Melting o ice (1 mark) endothermic (1) c) Evaporation o rubbing alcohol (1 mark) endothermic (1) d) Reaction o sodium with water (1 mark) exothermic (1) e) A lightning discharge (1 mark) exothermic (1) 46 How much heat is required to heat 168 g o zinc rom 12.2 C to C? (1 mark) (Speciic heat capacity o zinc = J g 1 K 1 ) Amount o heat required = 168 g x J g 1 K 1 x ( ) K = J (1) 47 A 44.9 g sample o copper at 99.8 C is dropped into a beaker containing 152 g o water at 18.5 C. What is the inal temperature o the copper? (3 marks) (Speciic heat capacities: copper = J g 1 K 1, water = 4.18 J g 1 K 1 ) Let T C be the inal temperature. Amount o heat released by copper = 44.9 g x J g 1 K 1 x (99.8 T ) K (1) Amount o heat taken in by water = 152 g x 4.18 J g 1 K 1 x (T 18.5) K (1) 44.9 g x J g 1 K 1 x (99.8 T ) K = 152 g x 4.18 J g 1 K 1 x (T 18.5) K the inal temperature is 20.7 C. T = 20.7 C (1) 27

28 48 How much heat is released or taken in the reaction o 2.51 g o Fe 2 O 3 (s) with enough carbon monoxide to produce iron metal? Fe 2 O 3 (s) + 3CO(g) 2Fe(s) + 3CO 2 (g) r = 24.8 kj (2 marks) (Relative atomic masses: O = 16.0, Fe = 55.8) 2.51 g Number o moles o Fe 2 O 3 reacted = g mol 1 Amount o heat released = 24.8 kj x kj o heat is released. = mol (1) = kj (1) 49 State a term which can describe the standard enthalpy change o each o the ollowing reactions: a) H 2 (g) + O 2 (g) H 2 O 2 (l) = 188 kj mol 1 (1 mark) standard enthalpy change o ormation o H 2 O 2 (l) (1) b) C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) = kj mol 1 (1 mark) standard enthalpy change o combustion o C 2 H 5 OH(l) (1) c) HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) = 57.1 kj mol 1 (1 mark) standard enthalpy change o neutralization o HCl(aq) and NaOH(aq) (1) d) CuSO 4 (s) + 5H 2 O(l) CuSO 4 5H 2 O(s) = 79.0 kj mol 1 (1 mark) standard enthalpy change o hydration o CuSO 4 (s) (1) 28

29 Structured questions 50 The standard enthalpy changes o combustion ( c ) o cyclohexa-1,3-diene (C 6 H 8 ), cyclohexane (C 6 H 12 ) and hydrogen are as ollows: c [C 6 H 8 (l)] = kj mol 1 c [C 6 H 12 (l)] = kj mol 1 c [H 2 (g)] = 286 kj mol 1 a) In c, what are the conditions indicated by the symbol O? (3 marks) At a temperature o 25 C (298 K). (1) At a pressure o 1 atmosphere (1 atm). (1) All substances involved are in their standard states, i.e. each in its most stable physical state at 25 C and 1 atm. (1) b) Write a chemical equation to represent the complete hydrogenation o cyclohexa-1,3-diene. (1 mark) C 6 H 8 (l) + 2H 2 (g) C 6 H 12 (l) (1) c) Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the standard enthalpy change o hydrogenation o cyclohexa-1,3-diene. (4 marks) C 6 H 8 (l) + 2H 2 (g) + 9O 2 (g) C 6 H 12 (l) + 9O 2 (g) c [C 6 H 8 (l)] + 2 x c [H 2 (g)] c [C 6 H 12 (l)] 6CO 2 (s) + 6H 2 O(l) (1) + c [C 6 H 12 (l)] = c [C 6 H 8 (l)] + 2 x c [H 2 (g)] = c [C 6 H 8 (l)] + 2 x c [H 2 (g)] c [C 6 H 12 (l)] (1) = [( 3 584) + 2( 286) ( 3 924)] kj mol 1 (1) = 232 kj mol 1 (1) the standard enthalpy change o hydrogenation o cyclohexa-1,3-diene is 232 kj mol 1. 29

30 51 A student used the set-up shown below to determine the enthalpy change o combustion o propan-1-ol. Results : Mass o water in beaker = g Weighings Spirit lamp + propan-1-ol beore combustion = g Spirit lamp + propan-1-ol ater combustion = g Temperatures Water beore heating = 28.0 C Water ater heating = 64.1 C Observations: When the spirit lamp was being weighed, its mass was continually alling. A black substance ormed on the bottom o the beaker as the propan-1-ol burnt. a) Calculate the enthalpy change o combustion o propan-1-ol rom the experimental results. (2 marks) (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0; speciic heat capacity o water = 4.18 J g 1 K 1 ; you may assume the container has negligible heat capacity.) Amount o heat released during combustion = g x 4.18 J g 1 K 1 x ( ) K = J Number o moles o propan-1-ol burnt = = 30.2 kj (1) ( ) g 60.0 g mol 1 = mol Enthalpy change o combustion o propan-1-ol = 30.2 kj mol = kj mol 1 (1) the enthalpy change o combustion o propan-1-ol is kj mol 1. 30

31 b) i) Give a reason why the mass o the spirit lamp ell as it was being weighed. (1 mark) Evaporation o propan-1-ol (1) ii) Suggest the identity o the black substance that orms on the beaker. State the eect on the value o the enthalpy o combustion obtained. (2 marks) Carbon (1) Less exothermic (1) c) Use the ollowing data to construct an enthalpy change cycle and use it to calculate the standard enthalpy change o combustion o propan-1-ol. (4 marks) Substance (kj mol 1 ) C 3 H 7 OH(l) 303 H 2 O(l) 286 CO 2 (g) 394 C 3 H 7 OH(l) O 2 (g) c [C 3 H 7 OH(l)] 3CO 2 (g) + 4H 2 O(l) [C 3 H 7 OH(l)] 3 x [CO 2 (g)] + 4 x [H 2 O(l)] 3C(graphite) + 4H 2 (g) + 5O 2 (g) (1) c [C 3 H 7 OH(l)] + c [C 3 H 7 OH(l)] = 3 x [C 3 H 7 OH(l)] = 3 x [CO 2 (g)] + 4 x [H 2 O(l)] [CO 2 (g)] + 4 x [H 2 O(l)] [C 3 H 7 OH(l)] (1) = [3( 394) + 4( 286) ( 303)] kj mol 1 (1) = kj mol 1 (1) the standard enthalpy change o combustion o propan-1-ol is kj mol 1. 31

32 52 Complete combustion o 3.60 g o glucose (C 6 H 12 O 6 ) liberates 56.0 kj o heat at 298 K under atmospheric pressure. a) i) Write a chemical equation or the complete combustion o glucose. (1 mark) C 6 H 12 O 6 (s) + 6O 2 (g) 6CO 2 (g) + 6H 2 O(l) (1) ii) What name is given to the above energy release process when it occurs in a living species? (1 mark) Respiration (1) b) Calculate the standard enthalpy change o combustion o glucose. (2 marks) (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0) 3.60 g Number o moles o glucose burnt = g mol 1 Standard enthalpy change o combustion o glucose = = mol (1) 56.0 kj mol the standard enthalpy change o combustion o glucose is kj mol 1. = kj mol 1 (1) c) The table below lists two standard enthalpy changes o ormation. Compound (kj mol 1 ) CO 2 (g) 394 H 2 O(l) 286 Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the standard enthalpy change o ormation o glucose. (4 marks) [C 6 H 12 O 6 (s)] reers to the standard enthalpy change o the ollowing process: 6C(graphite) + 6H 2 (g) + 3O 2 (g) C 6 H 12 O 6 (s) 6C(graphite) + 6H 2 (g) + 3O 2 (g) + 6O 2 (g) [C 6 H 12 O 6 (s)] C 6 H 12 O 6 (s) + 6O 2 (g) 6 x [CO 2 (g)] + 6 x [H 2 O(l)] c [C 6 H 12 O 6 (s)] 6CO 2 (g) + 6H 2 O(l) (1) 32

33 [C 6 H 12 O 6 (s)] + c [C 6 H 12 O 6 (s)] = 6 x [CO 2 (g)] + 6 x [H 2 O(l)] [C 6 H 12 O 6 (s)] = 6 x [CO 2 (g)] + 6 x [H 2 O(l)] c [C 6 H 12 O 6 (s)] (1) [C 6 H 12 O 6 (s)] = [6( 394) + 6( 286) ( 2 800)] kj mol 1 (1) = kj mol 1 (1) the standard enthalpy change o ormation o glucose is kj mol 1. d) Draw an enthalpy level diagram to relate the enthalpy changes depicted in the enthalpy change cycle in (c). (2 marks) Enthalpy (kj) 6C(graphite) + 6H 2 (g) + 9O 2 (g) [6( 394) + 6( 286)] kj kj C 6 H 12 O 6 (s) + 6O 2 (g) kj 6CO 2 (g) + 6H 2 O(l) (1 mark or correct enthalpy levels; 1 mark or correct labels) (2) 33

34 53 a) In an experiment to determine the enthalpy change o combustion o pentan-1-ol, a calorimeter containing g o water was used. Burning 1.32 g o pentan-1-ol caused the temperature o the water in the calorimeter to rise by 40.5 C. i) Draw a labelled diagram o the set-up used in the experiment. (3 marks) (1 mark or basic set-up; 2 marks or correct labels; award 0 mark i the set-up is not workable) (3) ii) Assuming that the heat capacity o the calorimeter is negligible, calculate the enthalpy change o combustion o pentan-1-ol under the conditions o the experiment. (2 marks) (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0; speciic heat capacity o water = 4.18 J g 1 K 1 ) Amount o heat released = g x 4.18 J g 1 K 1 x 40.5 K = J Number o moles o pentan-1-ol = = 42.3 kj (1) 1.32 g 88.0 g mol 1 = mol Enthalpy change o combustion o pentan-1-ol = 42.3 kj mol = kj mol 1 (1) the enthalpy change o combustion o pentan-1-ol is kj mol 1. b) Enthalpy changes o ormation can be used, with Hess s Law, to calculate the enthalpy change o combustion o pentan-1-ol. i) State Hess s Law. (2 marks) The enthalpy change o a reaction depends on the initial and inal states o the reaction (1) and is independent o the route by which the reaction may occur. (1) 34

35 ii) Calculate the standard enthalpy change o combustion o pentan-1-ol. The ollowing table gives some standard enthalpy changes o ormation. (3 marks) C 5 H 11 OH(l) O 2 (g) CO 2 (g) H 2 O(l) (kj mol 1 ) c [C 5 H 11 OH(l)] reers to the standard enthalpy change o the ollowing process: C 5 H 11 OH(l) O 2(g) 5CO 2 (g) + 6H 2 O(l) c = c = 5 x [products] [reactants] [CO 2 (g)] + 6 x [H 2 O(l)] [C 5 H 11 OH(l)] (1) = [5( 394) + 6( 286) ( 354)] kj mol 1 (1) = kj mol 1 (1) the standard enthalpy change o combustion o pentan-1-ol is kj mol 1. iii) State how you would expect the value obtained in part (ii) to dier i steam, rather than liquid water, is ormed. (1 mark) Less negative (1) c) Suggest why the experimental value o the enthalpy change o combustion obtained in (a) is less reliable than the value obtained in (b). (1 mark) Any one o the ollowing: Heat loss occurs in (a) / no heat loss in (b) (1) Incomplete combustion o pentan-1-ol in (a) (1) 54 A rocket propellant consists o a uel and an oxidizer. Rockets can be launched using liquid oxygen as the oxidizer and kerosene as uel. a) Based on the standard enthalpy changes o ormation given below, calculate the standard enthalpy change o combustion o decane (C 10 H 22 ). (3 marks) Compound (kj mol 1 ) C 10 H 22 (l) 301 CO 2 (g) 394 H 2 O(l) 286 The standard enthalpy change o combustion o decane reers to the enthalpy change o the ollowing process: C 10 H 22 (l) O 2(g) 10CO 2 (g) + 11H 2 O(l) 35

36 c = 10 x [CO 2 (g)] + 11 x [H 2 O(l)] [C 10 H 22 (l)] (1) = [10( 394) + 11( 286) ( 301)] kj mol 1 (1) = kj mol 1 (1) b) An important property o a uel is its energy density. This is the energy produced per kilogram o uel. Assuming that the kerosene used in the rockets contains only decane, calculate the energy density o the uel. (1 mark) (Relative molecular mass: C 10 H 22 = 142.0) x Energy density o uel = kj kg 1 = kj kg 1 (1) c) In the rockets, kj o energy are produced or every kilogram o liquid oxygen used to oxidize the decane. Calculate the mass o decane that would produce kj o energy kj Mass o decane = kj kg 1 (1 mark) = 2.26 kg (1) d) The theoretical mass o decane that could be burnt or every kilogram o liquid oxygen used is 3.48 kg. Suggest ONE reason why the rockets are designed to use less than the theoretical mass o decane per kilogram o oxygen. (1 mark) To ensure complete combustion. (1) 55 a) The table below lists the standard enthalpy changes o ormation o three substances. Substance (kj mol 1 ) Water, H 2 O(l) 286 Carbon dioxide, CO 2 (g) 394 Octane, C 8 H 18 (l) 250 i) With the help o a chemical equation, state the meaning o the term standard enthalpy change o combustion o octane. (2 marks) The standard enthalpy change o combustion o octane is the enthalpy change when one mole o octane is completely burnt in oxygen under standard conditions. (1) C 8 H 18 (l) O 2(g) 8CO 2 (g) + 9H 2 O(l) (1) 36

37 ii) Calculate the standard enthalpy change o combustion o octane. (3 marks) c [C 8 H 18 (l)] = = 8 x [products] [reactants] [CO 2 (g)] + 9 x [H 2 O(l)] [C 8 H 18 (l)] (1) = [8( 394) + 9( 286) ( 250)] kj mol 1 (1) = kj mol 1 (1) the standard enthalpy change o combustion o octane is kj mol 1. b) To travel 100 km, a small petrol car requires 3.60 x 10 4 kj. Assume that petrol contains only octane. I the engine eiciency is 20.0%, what is the volume o petrol used or this journey? (4 marks) (Relative molecular mass o C 8 H 18 = 114.0; density o C 8 H 18 = g cm 3 ) 3.60 x 10 4 kj At 100.0% eiciency, number o moles o C 8 H 18 used = kj mol 1 = 6.57 mol (1) Since engine eiciency is 20.0% number o moles o C 8 H 18 used = 6.57 mol x = 32.9 mol (1) Mass o C 8 H 18 used = 32.9 mol x g mol 1 = g (1) g Volume o C 8 H 18 used = g cm 3 = cm 3 = 5.68 dm 3 (1) 5.68 dm 3 o petrol are used or the journey. 37

38 c) The standard enthalpy change o combustion o ethanol is kj mol 1. Compare the enthalpy changes o combustion, in kj g 1, o petrol and an alternative uel containing petrol and 10% ethanol by mass. (4 marks) (Relative molecular masses: C 8 H 18 = 114.0, C 2 H 5 OH = 46.0) kj mol 1 Enthalpy change o combustion o C 8 H 18 = g mol 1 = 48.1 kj g 1 (1) Enthalpy change o combustion o C 2 H 5 OH = kj mol g mol 1 = 29.7 kj g 1 (1) Enthalpy change o combustion o alternative uel = [0.9( 48.1) + 0.1( 29.7)] kj g 1 = 46.3 kj g 1 (1) The alternative uel has a lower enthalpy change o combustion. (1) d) Besides cutting petroleum consumption, suggest ONE additional advantage o using the alternative uel over using gasoline. (1 mark) Any one o the ollowing: Ethanol is an oxygen-containing compound. This makes it easier or the alternative uel to undergo complete combustion / less CO is produced / less particulates are ormed. (1) Ethanol is a renewable energy source. It can be obtained rom crops. (1) The cost or the production o ethanol is low in agricultural countries. (1) 56 Lauric acid, which is ound in some animal ats, has the structure shown below. CH 3 (CH 2 ) 10 COOH Complete combustion o 1.00 g o lauric acid liberates 36.9 kj o heat at 298 K under atmospheric pressure. a) Write a chemical equation or the complete combustion o lauric acid. (1 mark) CH 3 (CH 2 ) 10 COOH(s) + 17O 2 (g) 12CO 2 (g) + 12H 2 O(l) (1) b) Calculate the standard enthalpy change o combustion o lauric acid. (2 marks) (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0) 1.00 g Number o moles o lauric acid burnt = g mol 1 = 5.00 x 10 3 mol (1) 38

39 Standard enthalpy change o combustion o lauric acid = 36.9 kj 5.00 x 10 3 mol the standard enthalpy change o combustion o lauric acid is kj mol 1. = kj mol 1 (1) c) Deine the term standard enthalpy change o ormation. (2 marks) The standard enthalpy change o ormation o a substance is the enthalpy change when one mole o the substance (1) is ormed rom its elements in their standard states. (1) d) Use the relevant enthalpy changes to construct an enthalpy change cycle and use it to calculate the standard enthalpy change o ormation o lauric acid. (4 marks) (Standard enthalpy changes o ormation o CO 2 (g) and H 2 O(l) are 394 kj mol 1 and 286 kj mol 1 respectively.) [CH 3 (CH 2 ) 10 COOH(s)] reers to the standard enthalpy change o the ollowing process: 12C(graphite) + 12H 2 (g) + O 2 (g) CH 3 (CH 2 ) 10 COOH(s) 12C(graphite) + 12H 2 (g) + O 2 (g) + 17O 2 (g) [CH 3 (CH 2 ) 10 COOH(s)] CH 3 (CH 2 ) 10 COOH(s) + 17O 2 (g) 12 x [CO 2 (g)] + 12 x [H 2 O(l)] c [CH 3 (CH 2 ) 10 COOH(s)] 12CO 2 (g) + 12H 2 O(l) (1) [CH 3 (CH 2 ) 10 COOH(s)] + [CH 3 (CH 2 ) 10 COOH(s)] = [12 x c [CH 3 (CH 2 ) 10 COOH(s)] = 12 x [CO 2 (g)] + 12 x [H 2 O(l)] [CO 2 (g)] + 12 x [H 2 O(l)] c [CH 3 (CH 2 ) 10 COOH(s)] (1) [CH 3 (CH 2 ) 10 COOH(s)] = [12( 394) + 12( 286) ( 7 380)] kj mol 1 (1) = 780 kj mol 1 (1) the standard enthalpy change o ormation o lauric acid is 780 kj mol 1. 39

40 57 Ethanol is a common uel burnt in some lightweight, compact stoves suitable or use when hiking and camping. A diagram o such a stove is given below. Consider the ollowing inormation: The standard enthalpy change o combustion o ethanol is kj mol 1. The cooking pot is made rom aluminium and has a mass o g. The speciic heat capacity o aluminium is J g 1 K 1. The speciic heat capacity o water is 4.18 J g 1 K 1. (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0) a) Calculate the amount o heat required to heat g o water and the pot rom 28.5 C to C. (2 marks) Amount o heat required = g x 4.18 J g 1 K 1 x ( ) K g x J g 1 K 1 x ( ) K (1) = J = 234 kj (1) b) Calculate the mass o ethanol needed to be completely burnt to provide this heat. (2 marks) 234 kj Number o moles o C 2 H 5 OH needed = kj mol 1 Mass o C 2 H 5 OH needed = mol x 46.0 g mol 1 = mol (1) = 7.87 g (1) c) Only 38.0% o the heat released by the combustion o ethanol is transerred to the cooking pot and contents. Calculate the mass o ethanol that needs to be burnt in practice to heat the water and the pot rom 28.5 C to C. (1 mark) Mass o C 2 H 5 OH needed in practice = 7.87 g 38.0% = 20.7 g (1) 40

41 d) Other alcohols could potentially be used as uels. The ollowing graph shows the relationship between the number o carbon atoms in straight-chain alcohols and their standard enthalpy changes o combustion. Describe and explain the pattern shown by the graph. (4 marks) The greater the number o carbon atoms in a molecule o a straight-chain alcohol, the greater the standard enthalpy change o combustion is. (1) The standard enthalpy change o combustion o each successive alcohol diers by the same amount. (1) This is because the structure o each successive alcohol diers by a CH 2 unit. (1) As shown in the equations below, there is a constant dierence in the number o bonds broken (2 C H bonds, 1 C C bond and 3 2 O=O bonds) and bonds ormed (2 C=O bonds and 2 O H bonds) involved in the combustion o the alcohols. (1) CH 3 OH(l) O 2(g) CO 2 (g) + 2H 2 O(l) CH 3 CH 2 OH(l) + 3O 2 (g) CH 3 CH 2 CH 2 OH(l) O 2(g) 2CO 2 (g) + 3H 2 O(l) 3CO 2 (g) + 4H 2 O(l) 41

42 58 Consider the ollowing reaction: 4CO(g) + 2NO 2 (g) 4CO 2 (g) + N 2 (g) Using the ollowing data, calculate the standard enthalpy change o the above reaction. (4 marks) [NO(g)] = kj mol 1 [CO 2 (g)] = 394 kj mol 1 2NO(g) + O 2 (g) 2NO 2 (g) = 114 kj 2CO(g) + O 2 (g) 2CO 2 (g) = 566 kj The ollowing data is given: 1 (1) 2 N 2(g) O 2(g) NO(g) [NO(g)] = kj mol 1 (2) C(s) + O 2 (g) CO 2 (g) [CO 2 (g)] = 394 kj mol 1 (3) 2NO(g) + O 2 (g) 2NO 2 (g) = 114 kj (4) 2CO(g) + O 2 (g) 2CO 2 (g) = 566 kj Looking at the target equation, we need 4 moles o CO(g) as the reactant. So, multiply equation (4) by 2, giving equation (4). (4) 4CO(g) + 2O 2 (g) 4CO 2 (g) = kj We need 2 moles o NO 2 (g) as the reactant. Equation (3) has 2 moles o NO 2 (g), but it is on the product side. So, reverse the equation, giving equation (3). (3) 2NO 2 (g) 2NO(g) + O 2 (g) = +114 kj We need 1 mole o N 2 (g) as the product. Equation (1) has 1 2 mole o N 2(g), but it is on the reactant side. So, reverse the equation and multiply it by 2, giving equation (1). (1) 2NO(g) N 2 (g) + O 2 (g) = kj By combining the three equations, ollowed by collecting like terms, we can obtain the target equation. (1) 2NO(g) N 2 (g) + O 2 (g) = kj (3) 2NO 2 (g) 2NO(g) + O 2 (g) = +114 kj (4) 4CO(g) + 2O 2 (g) 4CO 2 (g) = kj (2) 4CO(g) + 2NO 2 (g) 4CO 2 (g) + N 2 (g) r r = [( 180.4) + (+114) + ( 1 132)] kj (1) = kj (1) the standard enthalpy change o the reaction is kj. 42

43 59 The table below lists the enthalpy changes o our reactions. Reaction ΔH (kj mol 1 ) (1) MgCO 3 (s) + 2HCl(aq) MgCl 2 (aq) + CO 2 (g) + H 2 O(l) 89.9 (2) Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) 512 (3) C(graphite) + O 2 (g) CO 2 (g) 394 (4) H 2 (g) O 2(g) H 2 O(l) 286 a) Outline the experimental procedure to determine the enthalpy change o reaction (1) in a school laboratory. (3 marks) Place excess hydrochloric acid in a polystyrene cup. Record the initial temperature o the acid. (1) Add a known mass o solid magnesium carbonate to the acid. (1) Record the maximum temperature o the reaction mixture. (1) b) Using the given data, construct an enthalpy change cycle and use it to calculate the enthalpy change o ormation o magnesium carbonate. (5 marks) ΔH [MgCO 3 (s)] reers to the enthalpy change o the ollowing process: Mg(s) + C(graphite) O 2(g) MgCO 3 (s) Mg(s) + C(graphite) O 2 (g) + 2HCl(aq) ΔH [MgCO 3 (s)] MgCO 3 (s) + 2HCl(aq) ΔH 2 + ΔH 3 + ΔH 4 ΔH 1 MgCl 2 (aq) + CO 2 (g) + H 2 O(l) (2) ΔH [MgCO 3 (s)] + ΔH 1 = ΔH 2 + ΔH 3 + ΔH 4 ΔH [MgCO 3 (s)] = ΔH 2 + ΔH 3 + ΔH 4 ΔH 1 (1) = [( 512) + ( 394) + ( 286) ( 89.9)] kj mol 1 (1) = kj mol 1 (1) the enthalpy change o ormation o magnesium carbonate is kj mol 1. 43

44 c) Draw an enthalpy level diagram to relate the enthalpy changes depicted in the enthalpy change cycle in (b). (2 marks) Mg(s) + C(graphite) O 2 (g) + 2HCl(aq) Enthalpy (kj mol 1 ) [( 512) + ( 394) + ( 286)] kj mol 1 MgCl 2 (aq) + CO 2 (g) + H 2 O(l) kj mol 1 MgCO 3 (s) + 2HCl(aq) 89.9 kj mol 1 (1 mark or orre t enthalpy le els; 1 mark or orre t la els) (2) 60 Silane (SiH 4 (g)) is orme in the rea tion o sili on an hy rogen. The ta le elo lists the stan ar enthalpy hanges o ormation o three ompoun s: Compound (kj mol 1 ) H 2 O(l) 286 SiO 2 (s) 910 SiH 4 (g) +34 a) Write a hemi al equation hi h orrespon s to the enthalpy hange o ormation o silane. (1 mark) Si(s) + 2H 2 (g) SiH 4 (g) (1) ) The om ustion o silane gi es sili on ioxi e an ater. i) Write a hemi al equation or the om ustion o silane. (1 mark) SiH 4 (g) + 2O 2 (g) SiO 2 (s) + 2H 2 O(l) (1) ii) Using the a o e ata, al ulate the stan ar enthalpy hange o om ustion o silane. State ONE assumption ma e in your al ulation. (4 marks) [SiH 4 (g)] = [SiO 2 (s)] + 2 x [H 2 O(l)] [SiH 4 (g)] (1) = [( 910) + 2( 286) (+34)] kj mol 1 (1) = kj mol 1 (1) Assumption Hess s La is ollo e. (1) 44

45 61 Hess s Law can be veriied using the reactions shown below. KOH(s) ΔH 1 HCl(aq) KCl(aq) ΔH 2 H 2 O(l) HCl(aq) ΔH 3 KOH(aq) a) Complete the ollowing list o required measurements in order to determine ΔH 3. (4 marks) (1) Volume o KOH(aq) (2) Initial temperature o KOH(aq) (1) (3) Initial temperature o HCl(aq) (1) (4) Volume o HCl(aq) (1) (5) Final temperature o KCl(aq) (1) b) Hess s Law can be used to obtain enthalpy changes o reactions that cannot be measured directly. The table below lists the standard enthalpy changes o ormation o three compounds. Compound (kj mol 1 ) KClO 3 (s) 391 KCl(s) 437 MgO(s) 602 Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the standard enthalpy change o the ollowing reaction: KClO 3 (s) + 3Mg(s) KCl(s) + 3MgO(s) (4 marks) KClO 3 (s) + 3Mg(s) KCl(s) + 3MgO(s) [KClO 3 (s)] [KCl(s)] + 3 x [MgO(s)] 1 3 K(s) + Cl 2 (g) + O 2 (g) + 3Mg(s) 2 2 (1) 45

46 [KClO 3 (s)] + = = [KCl(s)] + 3 x [MgO(s)] [KCl(s)] + 3 x [MgO(s)] [KClO 3 (s)] (1) = [( 437) + 3( 602) ( 391)] kj (1) = kj (1) the standard enthalpy change o the reaction is kj. 62 An experiment was carried out to determine the enthalpy change o hydration o BaCl 2 (s), i.e. the enthalpy change o the ollowing process: BaCl 2 (s) + 2H 2 O(l) BaCl 2 2H 2 O(s) mole o BaCl 2 (s) and mole o BaCl 2 2H 2 O(s) were dissolved separately in 50.0 cm 3 o water in a polystyrene cup. The maximum change in temperature o each mixture was determined. The table below lists the results obtained: The orm o barium chloride used Maximum change in temperature ( C) BaCl 2 (s) BaCl 2 2H 2 O(s) 3.00 a) Calculate, under the conditions o the experiment, the enthalpy change o solution o each o the ollowing compounds. (You may assume that the barium chloride solution ormed has a speciic heat capacity o 4.18 J g 1 K 1 and a density o 1.00 g cm 3, and that the heat capacity o the polystyrene cup is negligible.) i) BaCl 2 (s) (2 marks) Heat released when BaCl 2 (s) dissolved = 50.0 g x 4.18 J g 1 K 1 x 1.20 K = 251 J (1) 251 J Enthalpy change o solution o BaCl 2 (s) = mol = J mol 1 = 8.37 kj mol 1 (1) 46

47 ii) BaCl 2 2H 2 O(s) (2 marks) Heat taken in when BaCl 2 2H 2 O(s) dissolved = 50.0 g x 4.18 J g 1 K 1 x 3.00 K = 627 J (1) +627 J Enthalpy change o solution o BaCl 2 2H 2 O(s) = mol = J mol 1 = kj mol 1 (1) b) From your results in (a), calculate the enthalpy change o hydration o BaCl 2 (s). (2 marks) water BaCl 2 (s) BaCl 2 (aq) ΔH 1 = 8.37 kj mol 1 water BaCl 2 2H 2 O(s) BaCl 2 (aq) ΔH 2 = kj mol 1 The enthalpy change o the ollowing process is required: BaCl 2 (s) + 2H 2 O(l) BaCl 2 2H 2 O(s) ΔH Looking at the target equation, we need 1 mole o BaCl 2 (s) as the reactant. So, keep the irst equation as it is. We need 1 mole o BaCl 2 2H 2 O(s) as the product. The second equation has 1 mole o BaCl 2 2H 2 O(s), but it is on the reactant side. So, reverse the second equation. ΔH = ΔH 1 ΔH 2 = [( 8.37) (+20.9)] kj mol 1 (1) = 29.3 kj mol 1 (1) the enthalpy change o hydration o BaCl 2 (s) is 29.3 kj mol 1. c) Suggest why the enthalpy change o hydration o BaCl 2 (s) CANNOT be determined directly. (1 mark) It is diicult to measure the temperature o a solid. (1) 47

48 63 Solutions o carbonates and hydrogencarbonates react with acids as shown below: CO 3 2 (aq) + 2H + (aq) H 2 O(l) + CO 2 (g) Reaction 1 HCO 3 (aq) + H + (aq) H 2 O(l) + CO 2 (g) Reaction 2 a) The standard enthalpy change o Reaction 2 was determined experimentally by mixing known volumes o 2.00 mol dm 3 NaHCO 3 (aq) and 2.20 mol dm 3 HCl(aq). The ollowing results were obtained. Volume o NaHCO 3 (aq) used = 40.0 cm 3 Volume o HCl(aq) used = 40.0 cm 3 Change in temperature = 1.0 C Use the above data to calculate the standard enthalpy change o Reaction 2. (Assume that the heat capacity o all solutions = 4.18 J cm 3 K 1 ) (3 marks) Total volume o solution = ( ) cm 3 = 80.0 cm 3 Amount o heat taken in = 80.0 cm 3 x 4.18 J cm 3 K 1 x 1.0 K = 334 J (1) Number o moles o NaHCO 3 used = 2.00 mol dm 3 x dm3 = mol Number o moles o HCl used = 2.20 mol dm 3 x dm3 = mol According to the equation o Reaction 2, 1 mole o HCO 3 (aq) would react with 1 mole o H + (aq). i.e mole o HCO 3 (aq) would react with mole o H + (aq). Hence H + (aq) was in excess. HCO 3 (aq) was the limiting reactant. (1) Standard enthalpy change o Reaction 2 = +334 J mol = J mol 1 = kj mol 1 (1) 48

49 b) i) Use the data below to calculate another value or the standard enthalpy change o Reaction 2. (3 marks) Species (kj mol 1 ) H 2 O(l) 286 CO 2 (g) 394 HCO 3 (aq) 692 H + (aq) 0.0 r = = [products] [reactants] [H 2 O(l)] + [CO 2 (g)] [HCO 3 (aq)] [H + (aq)] (1) = [( 286) + ( 394) ( 692)] kj mol 1 (1) = +12 kj mol 1 (1) ii) Compare your answers in (a) and (b)(i). Suggest one possible reason or any dierence. (1 mark) Heat was taken in rom the surroundings. Otherwise, the temperature drop could be more. (1) c) i) The standard enthalpy change o Reaction 1 is 2.3 kj mol 1. Calculate the o CO 3 2 (aq). (3 marks) r = r = [products] [reactants] [H 2 O(l)] + [CO 2 (g)] [CO 2 3 (aq)] 2 x [H + (aq)] (1) 2.3 kj mol 1 = [( 286) + ( 394)] kj mol 1 [CO 2 3 (aq)] [CO 3 2 (aq)] = [( 286) + ( 394) ( 2.3)] kj mol 1 (1) = 678 kj mol 1 (1) ii) Use your answer to (c)(i) to calculate the standard enthalpy change o the ollowing reaction: 2HCO 3 (aq) CO 3 2 (aq) + H 2 O(l) + CO 2 (g) (3 marks) r = = [products] [reactants] [CO 3 2 (aq)] + [H 2 O(l)] + [CO 2 (g)] 2 x [HCO 3 (aq)] (1) = [( 678) + ( 286) + ( 394) 2( 692)] kj (1) = +26 kj (1) 49

50 64 Zinc reacts with copper(ii) sulphate solution in an exothermic reaction. Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s) In an experiment to ind the enthalpy change or the reaction, a student weighed out 3.17 g o zinc powder. The student then placed 50.0 cm 3 o 1.00 mol dm 3 copper(ii) sulphate solution into a polystyrene cup. The temperature o the solution in the cup was measured every minute or 10.0 minutes, with the zinc being added at 3.5 minutes. The solution was continuously stirred during the experiment. The temperature readings obtained are shown in the table below. Time (min) Temperature ( C) a) i) Plot a graph o temperature ( C) against time (min). (1 mark) (1 mark or correctly plotted points) (1) ii) Find, rom the graph, the maximum temperature rise o the mixture. (You should show your working on the graph.) (2 marks) Working on the graph to show temperatures at 3.5 minutes. (1) Calculated temperature change must be in the range 43.5 C 44.5 C. (1) iii) Suggest ONE reason why a series o temperature readings was taken rather than just the initial and inal temperatures. (1 mark) To take into account the heat loss to the surroundings. (1) 50

51 b) i) Explain why a polystyrene cup, rather than a glass beaker, was used in the experiment. (1 mark) Low heat capacity / good insulator o heat / low mass / absorbs less heat (1) ii) Explain why the solution was continuously stirred in the experiment. (1 mark) To ensure uniorm temperature. (1) iii) Suggest a piece o apparatus suitable or measuring the 50.0 cm 3 the experiment. o copper(ii) sulphate solution in (1 mark) Burette / pipette / volumetric lask (1) c) Show, by calculations, which o the two reactants is in excess. (2 marks) (Relative atomic mass: Zn = 65.4) 3.17 g Number o moles o Zn = 65.4 g mol 1 Number o moles o CuSO 4 = 1.00 mol dm 3 x = mol (0.5) dm3 = mol (0.5) According to the equation, 1 mole o Zn reacts with 1 mole o CuSO 4. During the reaction, mole o Zn reacts with mol o CuSO 4. CuSO 4 is in excess. (1) d) Assuming that the speciic heat capacity and the density o the mixture are 4.18 J g 1 K 1 and 1.00 g cm 3 respectively, calculate the enthalpy change o this reaction, in kj mol 1. (2 marks) Amount o heat released during the reaction = 50.0 g x 4.18 J g 1 K 1 x 43.8 K = J Enthalpy change o reaction = 9.15 kj mol 1 = 9.15 kj (1) = 189 kj mol 1 (1) the enthalpy change o the reaction is 189 kj mol 1. 51

52 e) Suggest TWO improvements to the procedure that may give more accurate results. (2 marks) Any two o the ollowing: Put a lid on the polystyrene cup. (1) Put the polystyrene cup in another polystyrene cup / in a beaker. (1) Determine the heat capacities o the polystyrene cup and the thermometer. Take these values into account in the calculations. (1) 65 Cans o sel-heating coee were not available until recently. Inside the can, in separate compartments, are calcium oxide and water. When a button is pressed, these react together to give enough heat to warm up the coee. a) The reaction between calcium oxide and excess water orms calcium hydroxide solution. Write a balanced chemical equation or the reaction. Include state symbols. (1 mark) CaO(s) + H 2 O(l) Ca(OH) 2 (aq) (1) b) A group o students determined the enthalpy change o this reaction by placing a known mass o calcium oxide into cm 3 o water in an insulated lask and measuring the temperature rise. The group o students recorded the measurements shown in the table below. Mass o calcium oxide used 7.40 g Volume o water used cm 3 Initial temperature o water 20.0 C Final temperature o water 57.5 C Calculate the amount o heat transerred to the water by the reaction o 1.00 mole o CaO(s). (2 marks) (Relative atomic masses: O = 16.0, Ca = 40.1; speciic heat capacity o water = 4.18 J g 1 o water = 1.00 g cm 3 ) K 1 ; density Amount o heat released by 7.40 g o CaO(s) = g x 4.18 J g 1 K 1 x 37.5 K = J Number o moles o CaO(s) used = 7.40 g 56.1 g mol 1 = 39.2 kj (1) = mol Amount o heat transerred to water by the reaction o 1.00 mole o CaO(s) = 39.2 kj mol 297 kj o heat are transerred to the water by the reaction o 1.00 mole o CaO(s). = 297 kj mol 1 (1) 52

53 c) The reaction will produce solid calcium hydroxide i the exact molar ratio o water to calcium oxide is used. i) Suggest ONE reason why it is very diicult to measure this enthalpy change directly. (1 mark) It is hard to prevent the calcium hydroxide ormed rom dissolving. (1) ii) This enthalpy change (ΔH) can be measured indirectly by using common apparatus, solid calcium oxide, solid calcium hydroxide and dilute hydrochloric acid. Outline the data you need to obtain and the data treatment involved. (5 marks) Allow a known mass o solid calcium oxide to react with excess hydrochloric acid. Record the maximum temperature rise. (1) Calculate the enthalpy change or the reaction between solid calcium oxide and hydrochloric acid, in kj mol 1. (1) (1) CaO(s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(l) ΔH 1 Allow a known mass o solid calcium hydroxide to react with excess hydrochloric acid. Record the maximum temperature rise. (1) Calculate the enthalpy change or the reaction between solid calcium hydroxide and hydrochloric acid, in kj mol 1. (1) (2) Ca(OH) 2 (s) + 2HCl(aq) CaCl 2 (aq) + 2H 2 O(l) ΔH 2 Combine equation (1) and the reverse orm o equation (2) (i.e. equation (2) ). (1) CaO(s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(l) ΔH 1 (2) CaCl 2 (aq) + 2H 2 O(l) Ca(OH) 2 (s) + 2HCl(aq) ΔH 2 CaO(s) + H 2 O(l) Ca(OH) 2 (s) ΔH = ΔH 1 ΔH 2 (1) 53

54 66 a) The table below lists the enthalpy changes o ormation o three compounds under standard conditions. Compound (kj mol 1 ) H 2 O(l) 286 CO 2 (g) 394 C 4 H 10 (g) 125 i) State the meaning o the term standard conditions. (3 marks) At a temperature o 25 C (298 K). (1) At a pressure o 1 atmosphere (atm). (1) All the substances involved are in their standard states, i.e. each in its most stable physical state at 25 C and 1 atm. (1) ii) Write a chemical equation or the complete combustion o butane (C 4 H 10 (g)). C 4 H 10 (g) O 2(g) 4CO 2 (g) + 5H 2 O(l) iii) Calculate the standard enthalpy change o combustion o C 4 H 10 (g). (1 mark) (1) (3 marks) c [C 4 H 10 (g)] = [products] [reactants] = 4 x [CO 2 (g)] + 5 x [H 2 O(l)] [C 4 H 10 (g)] (1) = [4( 394) + 5( 286) ( 125)] kj mol 1 (1) = kj mol 1 (1) iv) Draw an enthalpy level diagram or the standard enthalpy change o combustion o butane. Label your diagram ully. (1 mark) C 4 H 10 (g) O 2 (g) Enthalpy (kj mol 1 ) c [C 4 H 10 (g)] = kj mol 1 4CO 2 (g) + 5H 2 O(l) (1) 54

55 b) The enthalpy changes o combustion o some compounds in kj g 1 and kj cm 3 are given below: Compound ΔH c (kj g 1 ) ΔH c (kj cm 3 ) Butane, C 4 H 10 (g)? 0.12 Ethanol, C 2 H 5 OH(l) ,2,4-trimethylpentane, C 8 H 18 (l) i) Use your answer to (a)(iii) to calculate the enthalpy change o combustion o butane in kj g 1. (Relative atomic masses: H = 1.0, C = 12.0) kj mol 1 Enthalpy change o combustion o butane = 58.0 g mol 1 (1 mark) = 49.7 kj g 1 (1) ii) Use the inormation in the table to compare the advantages and disadvantages o using these three compounds as uels or a motor vehicle. (3 marks) A comparison o any two or three uels by ΔH c in kj g 1 (1) e.g. butane gives out the greatest amount o heat per g. A comparison o any two or three uels by ΔH c in kj cm 3 (1) e.g. C 8 H 18 gives out the greatest amount o heat per cm 3. A comparison o states, and consequence o state on use as uel in motor vehicles (1) e.g. C 4 H 10 is a gas; it needs a big uel tank to be stored at high pressure. or C 2 H 5 OH / C 8 H 18 is a liquid; it needs a smaller uel tank. 55

56 67 Some students were measuring the enthalpy changes or the neutralization o 1 mole o dierent acids with dierent alkalis. a) In the irst experiment, a student investigated the enthalpy change o neutralization o hydrochloric acid with potassium hydroxide solution in the ollowing way: 50.0 cm 3 o 2.00 mol dm 3 hydrochloric acid were placed in a polystyrene cup and 50.0 cm 3 o 2.00 mol dm 3 potassium hydroxide solution were transerred to a beaker. Both solutions were allowed to stand or ive minutes beore their temperatures were noted. The potassium hydroxide solution was then added to the acid, the solution mixture was stirred thoroughly and the highest temperature was noted. A temperature change o C was noted. i) Why was it necessary to allow the two solutions to stand beore mixing? (1 mark) To obtain steady temperatures. (1) ii) Calculate the enthalpy change o neutralization, in kj mol 1. (3 marks) (Density o solution mixture = 1.00 g cm 3 ; speciic heat capacity o solution mixture = 4.18 J g 1 K 1 ) Total volume o the solution mixture = 50.0 cm cm 3 Mass o the solution mixture = g = cm 3 Amount o heat released during neutralization = g x 4.18 J g 1 K 1 x 13.5 K = J Number o moles o HCl reacted = number o moles o KOH reacted = 2.00 mol dm 3 x = 5.64 kj (1) dm3 = mol (1) mole o HCl reacted with mole o KOH to produce mole o H 2 O kj Enthalpy change o neutralization = mol = 56.4 kj mol 1 (1) the enthalpy change o neutralization or the reaction is 56.4 kj mol 1. 56

57 iii) State ONE assumption made when calculating this enthalpy change, other than those stated in (a) (ii). (1 mark) Any one o the ollowing: No heat was lost to the surroundings. (1) The polystyrene cup and thermometer had negligible heat capacity. (1) All the alkali was transerred rom the beaker to the polystyrene cup. (1) b) The enthalpy changes o neutralization o three acids are given below. Reaction 1 HNO 3 (aq) with NaOH(aq) ΔH = 57.1 kj mol 1 Reaction 2 HCl(aq) with NaOH(aq) ΔH = 57.1 kj mol 1 Reaction 3 CH 3 COOH(aq) with NaOH(aq) ΔH = 55.2 kj mol 1 Explain why the enthalpy change is the same or Reactions 1 and 2 but dierent or Reaction 3. (3 marks) Both Reactions 1 and 2 involve the neutralization between a strong monobasic acid and sodium hydroxide solution. The same chemical change occurs in both cases. (1) H + (aq) + OH (aq) H 2 O(l) Hence Reactions 1 and 2 have the same enthalpy change o neutralization. The enthalpy change or Reaction 3 is less because ethanoic acid is a weak acid. (1) Some energy is consumed when the acid dissociates to give hydrogen ions beore neutralization. (1) 68 Lactic acid has the structure shown below. H H O H C C C OH H OH a) Name the unctional groups in lactic acid. (2 marks) Hydroxyl group (1) Carboxyl group (1) 57

58 b) In an experiment, the concentration o a solution o lactic acid was determined by measuring the temperature change in its reaction with sodium hydrogencarbonate cm 3 o the lactic acid solution were placed in a polystyrene cup with negligible heat capacity, and the temperature o the solution was recorded every hal minute or 2.5 minutes. At precisely 3.0 minutes, excess sodium hydrogencarbonate was added to the cup. The mixture was stirred and its temperature was recorded or an additional 6 minutes. The graph below shows the plot o temperature against time. i) Find, rom the graph, the maximum temperature rise o the mixture. (You should show your working on the graph.) (2 marks) Working on the graph to show temperatures at 3.0 minutes. (1) Maximum temperature rise o the mixture = 13.0 C (1) ii) Assuming that the speciic heat capacity and the density o the solution are 4.18 J g 1 K 1 and 1.00 g cm 3 respectively, calculate the enthalpy change occurring during this experiment. (1 mark) Enthalpy change = g x 4.18 J g 1 K 1 x 13.0 K = kj (1) 58

59 iii) A data book gives the enthalpy change o this reaction as kj mol 1. Use this inormation and your answer to (b) to calculate the concentration o the lactic acid solution. (2 marks) kj Number o moles o lactic acid in the cup = kj mol 1 = mol (1) mol Concentration o the lactic acid solution = 25.0 ( ) dm3 = 1.20 mol dm 3 (1) c) As a check on the results rom the experiment involving the measurement o temperature change, the concentration o the lactic acid solution was also determined by titration with a standard sodium hydroxide solution. The equation or this reaction is CH 3 CHOHCOOH(aq) + NaOH(aq) CH 3 CHOHCOONa(aq) + H 2 O(l) 25.0 cm 3 o the lactic acid solution required 20.0 cm 3 o 1.60 mol dm 3 sodium hydroxide solution or neutralization. Calculate the concentration o the lactic acid solution. Number o moles o NaOH = 1.60 mol dm x dm3 (2 marks) = mol (1) According to the above equation, 1 mole o CH 3 CHOHCOOH requires 1 mole o NaOH or neutralization. i.e. number o moles o CH 3 CHOHCOOH = mol mol Concentration o the lactic acid solution = 25.0 ( ) dm3 = 1.28 mol dm 3 (1) d) Suggest TWO reasons why the value or the concentration o a lactic acid solution obtained by measuring the temperature change is usually lower than that determined by titration. (2 marks) Any two o the ollowing: The speciic heat capacity o the solution is greater than 4.18 J g 1 K 1. (1) The density o the solution is greater than 1.00 g cm 3. (1) The heat capacity o the apparatus is not taken into account. (1) 59

60 69 A student conducts the ollowing experiment to determine the enthalpy change o solution o ammonium nitrate. Add 50.0 cm 3 o distilled water to a polystyrene cup. Measure the steady temperature o the water. Add a sample o ammonium nitrate to the polystyrene cup, with stirring. Measure the inal temperature o the water. Data: Heat capacity o polystyrene cup and water 228 J K 1 Initial temperature o water 28.6 C Final temperature o water 25.3 C Mass o ammonium nitrate 2.55 g a) Deine the term standard enthalpy change o solution. (2 marks) The standard enthalpy change o solution o a substance is the enthalpy change when one mole o the substance dissolves (1) in an ininite volume o solvent (or enough solvent so that urther dilution has no additional eect) under standard conditions. (1) b) Calculate the enthalpy change o solution o ammonium nitrate. (2 marks) (Relative atomic masses: H = 1.0, N = 14.0, O = 16.0) Amount o heat taken in = 228 J K 1 x ( ) K Number o moles o NH 4 NO 3 = = 752 J (1) 2.55 g 80.0 g mol 1 = mol Enthalpy change o solution o NH 4 NO 3 = +752 J mol = J mol 1 the enthalpy change o solution o ammonium nitrate is kj mol 1. = kj mol 1 (1) 60

61 c) The dissolving process o this type may be slow and heat may be lost while the process is still going on. This can make it diicult to get an accurate measurement o the temperature change in the method above. Suggest how, using a clock, the method can be modiied to ind the maximum temperature change more accurately. (4 marks) Record the temperature o the water at intervals; then add the solid and stir; continue recording the temperature. (1) Plot the temperature o the water against time. (1) Join the points beore solid addition using a straight line and extrapolate to the time at which the solid is added. (1) Join the points ater solid addition using a straight line and extrapolate back to the time at which the solid is added. (1) The separation o the lines at the time o solid addition corresponds to the maximum temperature change or the process. 70 The heat capacity o a calorimeter and its contents can be determined by perorming in the calorimeter a reaction which produces a known amount o heat, and measuring the rise in temperature. In one experiment, 50.0 cm 3 o mol dm 3 lead(ii) nitrate solution were added to 50.0 cm 3 o 1.14 mol dm 3 potassium iodide solution in a calorimeter encased in an insulating jacket. The mixture was stirred continuously and the maximum temperature rise was 3.10 C as the ollowing reaction occurred. Pb(NO 3 ) 2 (aq) + 2KI(aq) PbI 2 (s) + 2KNO 3 (aq) ΔH = 49.0 kj mol 1 a) Deine the term heat capacity. (1 mark) The heat capacity o a substance is the amount o heat required to raise the temperature o the substance by 1 K (or 1 C) (1) b) Calculate the heat capacity o the calorimeter and its contents. (4 marks) Number o moles o Pb(NO 3 ) 2 = mol dm x dm3 Number o moles o KI = 1.14 mol dm 3 x = mol (0.5) dm3 = mol (0.5) According to the equation, 1 mole o Pb(NO 3 ) 2 reacts with 2 moles o KI. During the reaction, mole o Pb(NO 3 ) 2 reacted with mole o KI. 61

62 Pb(NO 3 ) 2 was in excess. (1) Amount o heat released by the reaction = J mol 1 x mol 2 = J (1) Heat capacity o calorimeter and its contents = J 3.10 K the heat capacity o the calorimeter and its contents is 452 J K 1. = 452 J K 1 (1) c) Two errors that might have occurred during this experiment are listed below. i) For each error, indicate the likely eect on the calculated heat capacity by placing a tick in the appropriate box. (2 marks) Error (1) The concentration o the lead(ii) nitrate solution had been incorrectly recorded and was actually mol dm 3. (2) The calorimeter was not placed in its insulating jacket. Calculated heat capacity too low No eect on calculated heat capacity Calculated heat capacity too high ii) Give an explanation or your answer in each case. (2 marks) Error (1) Pb(NO 3 ) 2 was already in excess. (1) Error (2) A lower temperature change leads to a higher heat capacity. (1) 71 Volumes o nickel(ii) chloride solution and 1.00 mol dm 3 sodium carbonate solution were mixed, with the total volume always being cm 3. A precipitate ormed in each case and the change in temperature was determined. The results are shown below. Volume o nickel(ii) chloride solution (cm 3 ) Change in temperature ( C)

63 a) Plot a graph o change in temperature ( C) against volume o nickel(ii) chloride solution (cm 3 ). (1 mark) (1 mark or correctly plotted points) (1) b) Find, rom the graph, the volumes o the two solutions that would produce the maximum temperature all when mixed. (You should show your working on the graph.) (2 marks) Mark two straight lines on the graph, one through the irst three points and (0,0) while the other through the last three points and (100.0,0). (1) The two straight lines intersect at 67.0 cm 3, i.e cm 3 o nickel(ii) chloride solution and 33.0 cm 3 o sodium carbonate solution would produce the maximum temperature all when mixed. (1) c) i) Write an ionic equation or the reaction between nickel(ii) chloride solution and sodium carbonate solution. (1 mark) Ni 2+ (aq) + CO 3 2 (aq) NiCO 3 (s) (1) ii) Calculate the concentration o the nickel(ii) chloride solution. Number o moles o Na 2 CO 3 = 1.00 mol dm x dm3 (1 mark) = mol According to the equation, 1 mole o Na 2 CO 3 reacts with 1 mole o NiCl 2. i.e. number o moles o NiCl 2 = mol mol Concentration o NiCl 2 (aq) = 67.0 ( ) dm3 = mol dm 3 (1) 63

64 the concentration o the nickel(ii) chloride solution is mol dm 3. d) Calculate the enthalpy change o the reaction between nickel(ii) chloride solution and sodium carbonate solution. (2 marks) (Density o solution mixtures = 1.00 g cm 3, speciic heat capacity o solution mixtures = 4.18 J g 1 K 1 ) Amount o heat taken in = g x 4.18 J g 1 K 1 x 4.50 K = J = 1.88 kj (1) kj Enthalpy change o reaction = mol = kj mol 1 (1) the enthalpy change o the reaction is kj mol Read the ollowing passage and answer the questions that ollow. Propellants or space shuttles NASA and commercial launch space shuttles use our types o propellants: petroleum; cryogenic propellant; hypergolic propellant; and solid propellant. The petroleum used as a rocket uel is a type o kerosene similar to the sort burnt in heaters and lamps. However, the rocket petroleum is highly reined. It is burnt with liquid oxygen (the oxidizer) to provide thrust. A cryogenic propellant is a mixture o liquid hydrogen (the uel) and liquid oxygen (the oxidizer). For a hypergolic propellant, the uel is CH 3 NHNH 2 (l) and the oxidizer is N 2 O 4 (l). The uel and oxidizer react on mixing and need no ignition source. CO 2 (g), H 2 O(g) and N 2 (g) are produced. A solid propellant is the oldest and simplest, dating back to the ancient Chinese. It is simply a casing illed with a mixture o Al(s) and NH 4 ClO 4 (s) which burns at a high rate. Al 2 O 3 (s), AlCl 3 (s), NO(g) and H 2 O(g) are produced. 64