Summer revision calculations for AS chemists September 2016

Transcription

1 Summer revision calculations for AS chemists September 206 Q.Dilute nitric acid reacts with potassium hydroxide solution. The equation for the reaction is: HNO 3 + KOH KNO 3 + H 2O A student investigated the temperature change in this reaction. This is the method the student used. Step Step 2 Step 3 Step 4 Step 5 Put 25 cm 3 of dilute nitric acid in a polystyrene cup. Use a thermometer to measure the temperature of the dilute nitric acid. Use a burette to add 4 cm 3 of potassium hydroxide solution to the dilute nitric acid and stir the mixture. Use a thermometer to measure the highest temperature of the mixture. Repeat steps 3 and 4 until 40 cm 3 of potassium hydroxide solution have been added. The dilute nitric acid and the potassium hydroxide solution were both at room temperature. (a) Figure shows part of the thermometer after some potassium hydroxide solution had been added to the dilute nitric acid. What is the temperature shown on the thermometer? The temperature shown is... C () (b) Errors are possible in this experiment. (i) Suggest two causes of random error in the experiment. Page

2 (2) Another student used a glass beaker instead of a polystyrene cup. This caused a systematic error. Why does using a glass beaker instead of a polystyrene cup cause a systematic error? () (c) The results of the student using the polystyrene cup are shown in Figure 2. (i) How do the results in Figure 2 show that the reaction between dilute nitric acid and potassium hydroxide solution is exothermic? () Page 2

3 Explain why the temperature readings decrease between 28 cm 3 and 40 cm 3 of potassium hydroxide solution added. (2) (iii) It is difficult to use the data in Figure 2 to find the exact volume of potassium hydroxide solution that would give the maximum temperature. Suggest further experimental work that the student should do to make it easier to find the exact volume of potassium hydroxide solution that would give the maximum temperature (2) (d) The student did further experimental work and found that 3.0 cm 3 of potassium hydroxide solution neutralised 25.0 cm 3 of dilute nitric acid. The concentration of the dilute nitric acid was 2.0 moles per dm 3. HNO 3 + KOH KNO 3 + H 2O Calculate the concentration of the potassium hydroxide solution in moles per dm Concentration =... moles per dm 3 (3) Page 3

4 (e) The student repeated the original experiment using 25 cm 3 of dilute nitric acid in a polystyrene cup and potassium hydroxide solution that was twice the original concentration. She found that: a smaller volume of potassium hydroxide solution was required to reach the maximum temperature the maximum temperature recorded was higher. Explain why the maximum temperature recorded was higher (2) (Total 4 marks) Q2.Methane (CH 4) is used as a fuel. (a) The displayed structure of methane is: Draw a ring around a part of the displayed structure that represents a covalent bond. () (b) Why is methane a compound? Tick ( ) one box. Methane contains atoms of two elements, combined chemically. Methane is not in the periodic table. Page 4

5 Methane is a mixture of two different elements. () (c) Methane burns in oxygen. (i) The diagram below shows the energy level diagram for the complete combustion of methane. Draw and label arrows on the diagram to show: the activation energy the enthalpy change, ΔH. (2) Complete and balance the symbol equation for the complete combustion of methane. CH CO (2) (iii) Explain why the incomplete combustion of methane is dangerous. (2) Page 5

6 (iv) Explain why, in terms of the energy involved in bond breaking and bond making, the combustion of methane is exothermic. (3) (d) Methane reacts with chlorine in the presence of sunlight. The equation for this reaction is: Some bond dissociation energies are given in the table. Bond Bond dissociation energy in kj per mole C H 43 C Cl 327 Cl Cl 243 H Cl 432 (i) Show that the enthalpy change, ΔH, for this reaction is 03 kj per mole. Page 6

7 The John Fisher School (3) Methane also reacts with bromine in the presence of sunlight. This reaction is less exothermic than the reaction between methane and chlorine. The enthalpy change, ΔH, is 45 kj per mole. What is a possible reason for this? Tick ( ) one box. CH 3Br has a lower boiling point than CH 3Cl The C Br bond is weaker than the C Cl bond. The H Cl bond is weaker than the H Br bond. Chlorine is more reactive than bromine. () (Total 5 marks) Q3.A student investigated the rate of reaction of magnesium and hydrochloric acid. Mg(s) + 2HCl(aq) MgCl 2(aq) + H 2(g) The student studied the effect of changing the concentration of the hydrochloric acid. She measured the time for the magnesium to stop reacting. Page 7

8 Concentration of hydrochloric acid in moles per dm (a) The student changed the concentration of the hydrochloric acid. Give two variables that the student should control (2) (b) (i) The rate of reaction increased as the concentration of hydrochloric acid increased. Explain why. (2) Explain why increasing the temperature would increase the rate of reaction. Page 8

9 The John Fisher School (3) (c) (i) The student had a solution of sodium hydroxide with a concentration of 0.00 moles per dm 3. She wanted to check the concentration of a solution of hydrochloric acid. She used a pipette to transfer 5.00 cm 3 of the hydrochloric acid into a conical flask. She filled a burette with the 0.00 moles per dm 3 sodium hydroxide solution. Describe how she should use titration to obtain accurate results. (4) Sodium hydroxide neutralises hydrochloric acid as shown in the equation: NaOH(aq) + HCl(aq) NaCl(aq) + H 2O(l) The student found that cm 3 of 0.00 moles per dm 3 sodium hydroxide neutralised 5.00 cm 3 of hydrochloric acid. Calculate the concentration of the hydrochloric acid in moles per dm 3. Give your answer to three significant figures. Page 9

10 Concentration of hydrochloric acid =... moles per dm 3 (3) (Total 4 marks) Q4.A spacecraft landed on Mars in Stocktrek Images (a) Probes on the spacecraft analysed some soil. The probes found a compound in the soil that scientists named compound X. (i) Compound X had the following percentage composition by mass: 0.8% magnesium, 3.8% chlorine and 57.4% oxygen. Relative atomic masses: Mg = 24; Cl = 35.5; O = 6 Calculate the empirical formula of compound X. You must show all of your working to get full marks. Page 0

11 Empirical formula =... (4) Compound X was shown to be ionic. Compound X had a high melting point. Use your knowledge of the structure and bonding of ionic compounds to explain why compound X had a high melting point. (3) (b) Hydrazine and hydrogen were both carried to be used as fuels on the spacecraft. Hydrazine is stored as a liquid but hydrogen is stored as a gas. Suggest three advantages of using a liquid fuel rather than a gaseous fuel (3) (Total 0 marks) Q5. Chlorine, bromine and iodine are all halogens. They belong to Group 7 of the periodic Page

12 table. (a) A teacher demonstrated the reactivity of the halogens to some students. The John Fisher School Halogen vapour was passed over heated iron wool in a fume cupboard. The teacher s observations are shown in the table below. Observations During the reaction After the reaction Bromine The iron wool glowed A red brown solid had been produced Chlorine The iron wool glowed brightly A dark brown solid had been produced Iodine The iron wool did not glow A black solid had been produced (i) What is the order of reactivity of these three halogens? Order of reactivity: most reactive halogen least reactive halogen 3... () Explain how you used the teacher s observations to decide your order of reactivity. Page 2

13 The John Fisher School (2) (iii) The students were asked to comment on the teacher s observations. One student said that the observations were not valid because not all the halogens are vapours chlorine is a gas, bromine is a liquid and iodine is a solid. Are the student s comments justified? Give reasons for your answer. (2) (b) Chlorine reacts with hot sodium hydroxide solution. A sample of a compound formed in the reaction was found to contain 0.8 g of sodium atoms, 6.7 g of chlorine atoms and 22.5 g of oxygen atoms. Calculate the empirical formula of the compound formed (4) (Total 9 marks) Q6.Scientists found that a compound contained: 22.8% sodium; 2.8% boron; and 55.4% oxygen. Page 3

14 Use the percentages to calculate the empirical formula of the compound. The John Fisher School Relative atomic masses (A r): B = ; O = 6; Na = 23 To gain full marks you must show all your working Empirical formula =... (Total 5 marks) Q7.(a) A student had a colourless solution. The student thought the solution was dilute hydrochloric acid. (i) The student added universal indicator to this solution. What colour would the universal indicator change to if the solution is hydrochloric acid? () Describe how the student could show that there are chloride ions in this solution. Page 4

15 The John Fisher School (2) (b) The results of a titration can be used to find the concentration of an acid. Describe how to use the apparatus to do a titration using 25 cm 3 of dilute hydrochloric acid. In your answer you should include: how you will determine the end point of the titration how you will make sure the result obtained is accurate Page 5

16 ... The John Fisher School... (4) (c) Hydrochloric acid is a strong acid. Ethanoic acid is a weak acid. What is meant by the term weak acid? () (d) The displayed formula of ethanoic acid is: (i) On the formula, draw a circle around the functional group in ethanoic acid. () Ethanoic acid and ethanol react together to make the ester ethyl ethanoate. Draw the displayed formula of ethyl ethanoate. (2) (Total marks) Page 6

17 Q8.(a) Ethanol (C 2H 5OH) is an alcohol. (i) Draw the displayed structure of ethanol. () Complete combustion of ethanol produces carbon dioxide and water. Complete the balanced symbol equation for this reaction. C 2H 5OH +... (3) (iii) Explain, in terms of bond breaking and bond forming, why the combustion of ethanol is exothermic. (3) (b) A group of students investigated the amount of energy given out when different alcohols are burned. The students used the apparatus shown in the figure below. Page 7

18 In one experiment the temperature of 50 g of water increased from 22.5 C to 38.3 C. The mass of alcohol burned was 0.85 g. (i) The energy used to heat the water, Q, can be found using the equation: Q = m c ΔT Calculate the heat energy, in joules, given out by burning 0.85 g of the alcohol. Assume that all of the heat energy given out by burning the alcohol is used to heat the water (specific heat capacity of water = 4.2 J / g / C). Heat energy =... J (2) Use your answer to part (b)(i)and the fact that 0.85 g of alcohol were burned to calculate the heat energy that would be given out by g of alcohol. If you could not answer part (b)(i), use 3000 J as the heat energy given out by burning 0.85 g of the alcohol. This is not the correct answer to part (b)(i). Heat energy =... J () (c) Another group of students investigated the amount of heat energy given out when different alcohols are burned. They used a different, better, set of apparatus than the first group of students. They used the results from their investigation to calculate the heat energy given out by burning g of each alcohol. They used a data book to find the theoretical amount of heat energy g of each Page 8

19 alcohol should have given out when burned completely. The John Fisher School The students recorded their experimental results and the theoretical values in the table below. Name of alcohol Number of carbon atoms in one molecule of alcohol Experimental amount of heat energy given out when g is burned in kj Theoretical amount of heat energy given out when g is burned completely in kj Methanol Ethanol Propanol Butanol Pentanol Hexanol (i) What is the relationship between the number of carbon atoms in one molecule of the alcohol and the heat energy given out when the alcohol is burned? () Suggest one reason why the students experimental results are lower than the theoretical values. () (iii) The students observed that as the number of carbon atoms in one molecule of the alcohol increased: the flame was more orange more carbon was left on the bottom of the calorimeter. Suggest why. Page 9

20 The John Fisher School () (iv) The heat energy given out when g of ethanol (C 2H 5OH) is burned is 29.7 kj. Calculate the heat energy, in kilojoules, that will be given out when mole of ethanol is burned. Give your answer to three significant figures. Relative atomic masses: H = ; C = 2; O = 6 Heat energy =... kj (3) (Total 6 marks) Q9.Printed pictures can be made using etchings. Page 20

21 Eduardo Jose Bernardino/iStock An etching can be made when a sheet of brass reacts with iron chloride solution. (a) Brass is a mixture of two metals, copper and zinc. (i) A mixture of two metals is called.... () Draw a ring around the correct answer to complete the sentence. Copper and zinc atoms are different sizes. harder This makes brass more flexible than the pure metals. softer () (b) Iron chloride has the formula FeCl 3 Relative atomic masses (A r): Cl = 35.5; Fe = 56. (i) Calculate the relative formula mass (M r) of iron chloride (FeCl 3). Page 2

22 The John Fisher School Relative formula mass (M r) of iron chloride =... (2) Calculate the percentage of iron in iron chloride (FeCl 3). Percentage of iron in iron chloride =...% (2) (Total 6 marks) Page 22

23 M.(a) 3 (b) (i) any two from: incorrect reading of thermometer / temperature incorrect measurement of volume of acid incorrect measurement of volume of alkali (burette). 2 glass is a (heat) conductor or polystyrene is a (heat) insulator answer needs to convey idea that heat lost using glass or not lost using polystyrene accept answers based on greater thermal capacity of glass (such as glass absorbs more heat than polystyrene ) (c) (i) temperature increases (iii) no reaction takes place or all acid used up or potassium hydroxide in excess cool / colder potassium hydroxide absorbs energy or lowers temperature ignore idea of heat energy being lost to surroundings take more readings ignore just repeat around the turning point or between 20 cm 3 and 32 cm 3 accept smaller ranges as long as no lower than 20 cm 3 and no higher than 32 cm 3 (d).6 or.6(2903) correct answer with or without working scores 3 if answer incorrect, allow a maximum of two from: moles nitric acid = (2 25 / 000) = 0.05 for mark moles KOH = (moles nitric acid) = 0.05 for mark concentration KOH = 0.05 / 0.03 answer must be correctly rounded (.62 is incorrect) 3 Page 23

24 (e) same amount of energy given out which is used to heat a smaller total volume or mixture has lower thermal capacity or number of moles reacting is the same but the total volume / thermal capacity is less if no other marks awarded award mark for idea of reacting faster [4] M2.(a) circle round any one (or more) of the covalent bonds any correct indication of the bond the line between letters (b) Methane contains atoms of two elements, combined chemically (c) (i) activation energy labelled from level of reagents to highest point of curve ignore arrowheads enthalpy change labelled from reagents to products arrowhead must go from reagents to products only 2 O 2 2 H 2O if not fully correct, award mark for all formulae correct. ignore state symbols Page 24

25 (iii) carbon monoxide is made The John Fisher School (iv) this combines with the blood / haemoglobin or prevents oxygen being carried in the blood / round body or kills you or is toxic or poisonous dependent on first marking point energy is taken in / required to break bonds accept bond breaking is endothermic energy is given out when bonds are made accept bond making is exothermic the energy given out is greater than the energy taken in this mark only awarded if both of previous marks awarded (d) (i) energy to break bonds = 895 calculation with no explanation max = 2 energy from making bonds = (= 03) or energy to break bonds = 656 energy from making bonds = (= 03) allow: bonds broken bonds made = = -03 for 3 marks. The C Br bond is weaker than the C Cl bond [5] M3.(a) any two from: temperature (of the HCl) mass or length of the magnesium surface area of the magnesium volume of HCl 2 Page 25

26 (b) (i) (a greater concentration has) more particles per unit volume allow particles are closer together The John Fisher School therefore more collisions per unit time or more frequent collisions. particles move faster allow particles have more (kinetic) energy therefore more collisions per unit time or more frequent collisions collisions more energetic (therefore more collisions have energy greater than the activation energy) or more productive collisions (c) (i) add (a few drops) of indicator to the acid in the conical flask allow any named indicator add NaOH (from the burette) until the indicator changes colour or add the NaOH dropwise candidate does not have to state a colour change but penalise an incorrect colour change. repeat the titration calculate the average volume of NaOH or repeat until concordant results are obtained moles of NaOH Page 26

27 = moles correct answer with or without working gains 3 marks The John Fisher School Concentration of HCl / = allow ecf from mp to mp2 correct number of significant figures [4] M4.(a) (i) 0.8 / / / 6 division by atomic mass proportion : 2 : 8 ratio MgCl 2O 8 formula if the candidate has just written down the correct answer with no working allow 3 marks. If working is shown award 4 marks. allow 2 marks if the candidate has the initial fractions upside down and gets the answer Mg 8Cl 4O as long as working has been shown allow symbols in any order but do not allow incorrect capitals or subscript and superscript numbers, eg mgcl2o8 (final mark) allow ecf after step 2 provided a sensible attempt has been made in step Page 27

28 because it has electrostatic forces of attraction or attraction between oppositely charged ions any mention of covalent bonds or molecules or intermolecular forces max. 2 marks giant structure / lattice or many bonds accept attraction between positive and negative ions do not accept atoms that need lots of energy to overcome / break (b) any three from: more can be stored in a given volume or takes up less space could be answered in terms of comparing liquid with gas, but not simply the disadvantages of storing a gaseous fuel (liquid has) greater energy density (liquid) does not require high pressure containers (liquid is) less likely to leak allow easier to detect a leak 3 [0] M5. (a) (i) chlorine > bromine > iodine accept symbols in place of names (Cl 2 or Cl, etc) any two suitable comparisons about the extent to which the iron wool glowed eg chlorine is more reactive than bromine because iron glowed morebrightly with chlorine than bromine eg bromine is more reactive than iodine because iron glowed withbromine but not with iodine eg iodine is the least reactive because it is the only one that did not glow Page 28

29 eg chlorine is the most reactive as it was the only one that glowed brightlyor glowed most brightly The John Fisher School 2 max. (iii) NO (no marks) accept YES (no marks) because the observations were made using the halogen vapours because the observations were made using the halogen vapours so the fact that the three elements are naturally solid, liquid or gas isnot relevant so the iodine (and bromine) must have been heated (b) division of masses by A r values if division inverted (A r / mass) then award zero marks correct answers of division if one slip in A r values used, do not award first mark, but award remainder consequentially simplification by division by smallest correct formula (NaClO 3) accept elements in any order (such as ClNaO 3) correct answer alone with or without working = 4 marks [9] M6.Divide by A r: Page 29

30 Na 2B 4O 7 Empirical formula () The John Fisher School Na = 22.8 / 23 B = 2.8 / O = 55.4 / 6 if student has calculated moles upside down they can score mp 3 mp 4 and mp 5 as follows: Na 23 / 22.8 B / 2.8 O 6 / 55.4 Values Divide by the smallest : 2 : 3.5 Divide by the smallest () 3.5 :.75 : Whole number ratio 2 : 4 : 7 Whole number ratio () 4 : 7 : 4 Empirical formula Na 4B 7O 4 if no working shown allow 4 marks for Na 2B 4O 7 [5] Page 30

31 M7.(a) (i) red ignore pink add silver nitrate (solution) white precipitate dependent on addition of silver nitrate ignore addition of another acid if hydrochloric acid added max mark (b) suitable named alkali / sodium hydroxide solution in burette add alkali solution until (indicator) becomes pink / red if acid to acid titration described, first two marking points not available any two from: wash / rinse equipment add dropwise or slowly (near end point) swirl / mix read (meniscus) at eye level white background read start and final burette levels / calculate the volume needed repeat 2 (c) does not ionise / dissociate completely allow for acids of the same concentration, weak acids have a higher ph or fewer hydrogen ions Page 3

32 (d) (i) ring round COOH if not fully correct, allow mark for correct ester group minimum 2 [] M8.(a) (i) allow OH (C 2H 5OH +) 3O 2 2CO 2 + 3H 2O formulae all 3 formulae gain 2 marks; 2 formulae gain mark Page 32

33 2 balanced dependent on formulae accept multiples (iii) energy is required to break bonds energy is released when bonds are formed exothermic because more energy is released than taken in (b) (i) 338 or 3320 (J) correct answer with or without working gains 2 marks if answer incorrect, allow mark for correct working, eg: temperature change = ( = ) 5.8 or Q = (50 x 4.2 x 5.8 =) allow ecf from temp change 2 if answer from part (i) used: (338 / 0.85) = 3904 allow ecf from part (i) if value of 3000 used: 3000 / 0.85 = 3529 accept 2 sig figs up to calculator value (c) (i) the more carbon atoms in one molecule, the more heat energy is given out do not accept proportional Page 33

34 heat loss (to surroundings or to the can or to the air) accept not all energy used to heat water allow incomplete combustion allow not all burned ignore thermometer or balance errors (iii) incomplete combustion (occurs when more C atoms in molecule) (iv) 370 correct answer with or without working gains 3 marks (M r ethanol =) 46 gains mark 46 x 29.7 gains 2 marks max 2 marks if incorrect sig figs (366.2, 366, 400) final answer ecf on incorrect M r max 2 marks 3 [6] M9.(a) (i) an alloy harder (b) (i) 62.5 correct answer with or without working gains 2 marks if no answer or incorrect answer then evidence of correct working [56 + (3x35.5)] gains mark accept rounding from correct answer with or without working gains 2 marks accept ecf from (b)(i) correctly calculated for 2 marks if no answer or incorrect answer then evidence of 56 / 62.5 Page 34

35 or 56 / answer to (b)(i) gains mark The John Fisher School 2 [6] Page 35