QUESTIONSHEETS ENERGETICS I ENTHALPY OF DISPLACEMENT BOND DISSOCIATION ENTHALPY HESS S LAW AND ENTHALPY DIAGRAMS HESS S LAW WITH CALORIMETRY

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1 CHEMISTRY QUESTIONSHEETS AS Level AS TOPIC 5 ENERGETICS I Questionsheet 1 Questionsheet 2 Questionsheet 3 Questionsheet 4 Questionsheet 5 Questionsheet 6 Questionsheet 7 Questionsheet 8 Questionsheet 9 Questionsheet 10 Questionsheet 11 ENTHALPY OF DISPLACEMENT THERMOMETRIC TITRATION ENTHALPY OF FORMATION ENTHALPY OF COMBUSTION BOND DISSOCIATION ENTHALPY HESS S LAW AND ENTHALPY DIAGRAMS HESS S LAW CALCULATIONS HESS S LAW WITH CALORIMETRY HESS S LAW WITH BOND DISSOCIATION ENTHALPY TEST QUESTION I TEST QUESTION II 16 marks 15 marks 12 marks 12 marks 16 marks 20 marks 15 marks 18 marks 15 marks 19 marks 19 marks Authors Trevor Birt Donald E Caddy Andrew Jones Adrian Bond Editors John Brockington Stuart Barker John Brockington Kevin Frobisher Andy Shepherd Stuart Barker Curriculum Press Licence Agreement: Paper copies of the A-Level Chemistry Questionsheets may be copied free of charge by teaching staff or students for use within their school, provided the Photocopy Masters have been purchased by their school. No part of these Questionsheets may be reproduced or transmitted, in any other form or by any other means, without the prior permission of the publisher. All rights are reserved. This license agreement is covered by the laws of England and Wales Curriculum Press March Curriculum P ress Curriculum Press Bank House 105 King Street Wellington Shropshire TF1 1NU

2 TOPIC 5 Questionsheet 1 ENTHALPY OF DISPLACEMENT An experiment was carried out to find the enthalpy change for the reaction: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) The temperature of 20.0 cm 3 of copper sulfate solution (1.0 mol dm -3 ) in a plastic cup was measured every minute for 3 minutes. Exactly on the third minute 1.95g of zinc powder was added rapidly and the mixture stirred. The temperature every half-minute was noted for a further 3 minutes. a) Calculate: (i) the mol copper ion used (ii) the mol zinc used b) Calculate and explain the mol used in the calculation c) The following graph was drawn : Temperature ( o C) T 3 Time (min) (i) Why was the zinc added after 3 minutes? (ii) Explain how the value of the temperature change?t was derived from the graph (iii) Explain why the method in (ii) was necessary TOTAL (Continued..) / 16

3 TOPIC TOPIC 5 Questionsheet 5 Questionsheet 1 Continued ENTHALPY OF DISPLACEMENT d) The value of T was derived as C. (i) Calculate the joules of energy exchange with the surroundings (assuming that 1 cm 3 solution requires 4.2 J to increase the temperature by 1 0 C). (ii) Calculate a value for the enthalpy of reaction... [4] e) If the experiment was repeated using magnesium instead of zinc, how and why would you expect the enthalpy of reaction to compare with your value in d)(ii)? TOTAL / 16

4 TOPIC 5 Questionsheet 2 THERMOMETRIC TITRATION a) The following results were obtained from an experiment to determine the heat of neutralisation of an acid with an alkali: Hydrochloric acid added to 50 cm 3 of 2.0 M sodium hydroxide (i) Volume of acid added / cm Temperature of mixture / 0 C Plot a graph of temperature against volume of acid added and use the graph to determine the volume of acid required to neutralise the 50 cm 3 of 2.0 M sodium hydroxide. Volume of acid required =...[5] TOTAL (Continued..) / 15

5 TOPIC TOPIC 5 Questionsheet 5 Questionsheet 2 Continued THERMOMETRIC TITRATION (ii) Calculate the molar concentration of the acid. (iii) Calculate the enthalpy of neutralisation of the acid by the alkali. (Take the specific heat capacity of dilute solutions to be 4.18 J g -1 K -1 ) b) To confirm this result, 40 cm 3 of 2 M hydrochloric acid were put into a Thermos flask and its temperature recorded.then 60 cm 3 of 2 M sodium hydroxide were added and the temperature rose by C. The flask was rinsed out and 100 cm 3 of water put in together with a small electrical heating coil. The coil supplied 30 Joules every second and after 145 seconds the same temperature rise was achieved. Use the electrical data to calculate the enthalpy of neutralisation of the acid by the alkali.... [4] TOTAL / 15

6 TOPIC 5 Questionsheet 3 ENTHALPY OF FORMATION a) Define standard enthalpy of formation of a compound. b) Calculate the standard enthalpy change for the oxidation of ammonia according to the equation: 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) Standard enthalpies of formation are as follows: NH 3 (g) = -46 kj mol -1 NO(g) = +90 kj mol -1 H 2 O(g) = -242 kj mol -1 c) Ethyne burns in oxygen according to the equation: 2C 2 H 2 (g) + 5O 2 (g) 4CO 2 (g) + 2H 2 O(l) Calculate the standard enthalpy of combustion of this gas given the following standard enthalpies of formation: C 2 H 2 (g) = kj mol -1 CO 2 (g) = -394 kj mol -1 H 2 O(l) = -286 kj mol -1 d) Calculate the standard enthalpy of formation of magnesium carbonate, using the following information. The standard enthalpy of combustion of magnesium is 602 kj mol -1 and that of carbon is 394 kj mol -1. The standard enthalpy change for the decomposition of magnesium carbonate in the reaction: MgCO 3 (s) MgO(s) + CO 2 (g) is +100 kj mol -1. TOTAL / 12

7 TOPIC 5 Questionsheet 4 ENTHALPY OF COMBUSTION a) Define standard enthalpy of combustion of an element or compound. b) Given the following data: 2C(s) + O 2 (g) 2CO(g); H θ = kj mol -1 2CO(g) + O 2 (g) 2CO 2 (g); H θ = kj mol -1 what is the standard enthalpy of combustion of carbon? c) The standard enthalpy changes for the combustion of decane, octane and ethene are 6778, and 1411 kj mol -1 respectively. Use these data to calculate the standard enthalpy change for the cracking of decane: C 10 H 22 (l) C 8 H 18 (l) + C 2 H 4 (g) d) In the presence of a nickel catalyst, but-1-ene can be hydrogenated to give butane: CH 2 =CHCH 2 CH 3 (g) + H 2 (g) CH 3 CH 2 CH 2 CH 3 (g) (i) Calculate the molar enthalpy of hydrogenation of but-1-ene, given the following standard enthalpies of combustion: H θ /kj mol-1 c C 4 H 8 (g) H 2 (g) C 4 H 10 (g) (ii) State, giving a reason, how you would expect the enthalpy of hydrogenation to change if platinum were to be used as a catalyst instead of nickel. Expected change... [1] Reason [1] TOTAL / 12

8 TOPIC 5 Questionsheet 5 BOND DISSOCIATION ENTHALPY a) Define the bond dissociation enthalpy of the H H bond. b) Distinguish between the bond dissociation enthalpy of the O H bond in water and the mean bond dissociation enthalpy of the O H bond in water.... [3] c) Arrange the following in increasing order of mean bond dissociation enthalpy: (i) C C, C=C, C C (ii) C Cl, C Br, C I d) What is the relationship between bond dissociation enthalpy and (i) bond strength, and (ii) bond length? Bond strength... [1] Bond length... [1] e) The combustion of propanone is given by the equation: CH 3 CH 3 C=O(g) + 4O 2 (g) 3CO 2 (g) + 3H 2 O(g) Use the following mean bond dissociation enthalpies (all in kj mol -1 ) to calculate the enthalpy change for this reaction: C C 346 C H 414 C=O 804 O=O 498 O H 463 [7] TOTAL / 16

9 a) State Hess s law. TOPIC 5 Questionsheet 6 HESS S LAW & ENTHALPY DIAGRAMS... [4] b) (i) Hess s law is in accordance with the first law of thermodynamics. State this law. (ii) Explain why, if Hess s law were not obeyed, the first law of thermodynamics would be violated. c) (i) Write an equation, with state symbols, for the combustion of propane. (ii) Sketch an enthalpy diagram for the formation and combustion of propane. (Hint The formation of propane is exothermic.) Clearly label start and finish on your diagram, and show two routes from start to finish. (iii) Apply Hess s law to your diagram in c) (ii) and hence calculate the standard enthalpy of formation of propane, given the following standard enthalpies of combustion (all in kj mol -1 ): C(s) = -394 H 2 (g) = -286 C 3 H 8 (g) = [6] TOTAL / 20

10 TOPIC 5 Questionsheet 7 HESS S LAW CALCULATIONS In this question you will need the following data. Standard enthalpy of formation /kj mol -1 H 2 O(l) 286 H 2 S(g) 20.2 Standard enthalpy of combustion/kj mol -1 S (rhombic) S (monoclinic) Molar enthalpy of vaporisation of water = 41.1 kj mol -1 a) (i) There are two solid allotropes (i.e. crystalline forms) of sulfur, described as rhombic sulfur and monoclinic sulfur. Construct an enthalpy diagram or cycle, and apply Hess s law to it so as to calculate the standard enthalpy change for the conversion of rhombic sulfur to monoclinic sulfur. [4] (ii) From your answer to a) (i) state, with reason, which allotrope of sulfur is the more stable under standard conditions. More stable form... [1] Reason... [1] TOTAL (Continued...) / 15

11 TOPIC 5 Questionsheet 7 Continued HESS S LAW CALCULATIONS b) Monoclinic sulfur is formed in volcanic regions by reaction between sulfur dioxide and hydrogen sulfide according to the equation: SO 2 (g) + 2H 2 S(g) 2H 2 O(g) + 3S(monoclinic). Draw an enthalpy diagram or cycle and calculate the standard enthalpy change for this reaction. c) For the oxidation of sulfur dioxide to sulfur trioxide according to the equation: [5] 2SO 2 (g) + O 2 (g) 2SO 3 (g); H θ reaction = kj mol -1. From an enthalpy diagram or cycle calculate the standard enthalpy of formation of sulfur trioxide from monoclinic sulfur. TOTAL / [4] 15

12 TOPIC 5 Questionsheet 8 HESS S LAW WITH CALORIMETRY A student poured exactly 50 cm 3 of 2 M hydrochloric acid into a polystyrene cup and recorded its temperature each minute for three minutes. At 3½ minutes, 0.24 g of magnesium turnings was added. The mixture was stirred and further temperature readings taken until a total time of nine minutes had elapsed. The recorded temperatures were as follows. Time / minutes Temperature / 0 C a) Plot a graph of temperature against time using these results and hence determine the maximum temperature change accompanying the reaction. Maximum temperature change...[5] TOTAL (Continued...) / 16

13 TOPIC TOPIC 5 Questionsheet 5 Questionsheet 8 Continued HESS S LAW WITH CALORIMETRY b) Use the value from a) and the data given in the question to calculate the enthalpy change for the reaction. (Assume the density of all solutions to be 1 g cm -3 and their specific heat capacity to be 4.2 J g -1 K -1 ) c) In a similar experiment, 0.01 mol of magnesium carbonate was added to exactly 50 cm 3 of 2 M hydrochloric acid in a polystyrene cup. The maximum temperature rise was C. Calculate the enthalpy change for the reaction. d) Write a balanced chemical equation to represent the standard enthalpy of formation for magnesium carbonate. e) Given the additional data for the standard enthalpies of formation of H 2 O(l) and CO 2 (g) as 286 and 394 kj mol -1 respectively, together with the reactions above, construct an enthalpy cycle or diagram to show how the standard enthalpy of formation for magnesium carbonate can be calculated. f) Apply Hess s law to the cycle or diagram you have devised and the results from b) and c) to calculate the standard enthalpy of formation for magnesium carbonate. [4] TOTAL / 18

14 TOPIC 5 Questionsheet 9 HESS S LAW WITH BOND DISSOCIATION ENTHALPY Ethanol production in the UK is mainly from the hydration of ethene. In this process, ethene and steam are reacted together at C and 70 atm pressure in the presence of phosphoric acid, which acts as a catalyst: CH 2 =CH 2 (g) + H 2 O(g) C 2 H 5 OH(l) a) Apply Hess s law to an enthalpy cycle or diagram to calculate the enthalpy change for this reaction, given the following data: Standard enthalpy of formation of ethanol = -278 kj mol -1 C(s) C(g); H = kj mol -1 Mean bond dissociation enthalpies / kj mol -1 : H H = 436 O O = 496 C C = 612 O H = 464 C H = 413 b) Calculate the enthalpy change for the same reaction, using the following standard enthalpies of combustion: CH 2 =CH 2 = 1411 kj mol -1 CH 3 CH 2 OH = 1368 kj mol -1 c) Why is the value calculated from bond dissociation enthalpies in a) different from that obtained in b)? d) For the formation of gaseous ethanol, CH 2 =CH 2 (g) + H 2 O(g) C 2 H 5 OH(g) would you expect the enthalpy of reaction to be more negative or less negative than the value for the formation of liquid ethanol? Give your reason. More or less negative... [1] Reason... [8] TOTAL / 15

15 TOPIC 5 Questionsheet 10 TEST QUESTION I NOTE In this Questionsheet, assume that dilute aqueous solutions have a specific heat capacity of 4.18 J g -1 K -1 and a density of 1.00 g cm -3. a) (i) Define the term molar enthalpy of solution. (ii) When 5.35 g of ammonium chloride was dissolved in 100 g of water, the temperature fell from C to C. Calculate the molar enthalpy of solution of ammonium chloride. (iii) What is the significance of the sign of the value calculated in (ii)? b) 50.0 cm 3 of 0.5 mol dm -3 NH 3 (aq) at o C. was mixed with 50.0 cm mol dm -3 HCl(aq), also at o C. The temperature of the mixture rose to o C. Calculate the molar enthalpy of neutralisation of HCl(aq) by NH 3 (aq). TOTAL (Continued..) / 17

16 TOPIC 5 Questionsheet10 Continued TEST QUESTION I c) (i) Write an equation to represent the formation of 1 mole of solid ammonium chloride from its elements in their standard states. (ii) Use the following data, together with your answers from a) and b), to calculate the molar enthalpy of solution of ammonia. Your answer should include an enthalpy cycle or diagram to which Hess s law is clearly applied. H θ f / kj mol-1 : NH 3 (g) = HCl(g) = NH 4 Cl(s) = -315 Molar enthalpy of solution of HCl(g) = -187 kj mol -1 Enthalpy cycle or diagram [5] Application of Hess s law... [1] Molar enthalpy of solution of ammonia [2] TOTAL / 19

17 TOPIC 5 Questionsheet 11 TEST QUESTION II A spirit burner containing ethanol was weighed. 80 cm 3 of water was measured into a metal calorimeter and the temperature measured. The burner was ignited under the calorimeter and extinguished after 5 minutes. The temperature of the water and the mass of the burner were then immediately measured. The following results were obtained: Mass of spirit burner at start (g) Mass of spirit burner after combustion (g) Temperature of water at start ( o C) 16.4 Temperature of water after combustion ( o C) 33.1 a) Calculate (i) the mass of ethanol burned (ii) the temperature rise of the water TOTAL (Continued...) / 17

18 TOPIC 5 Questionsheet11 Continued TEST QUESTION II b) Assuming 1cm 3 of water rises 1 o C for 4.2 J energy, calculate (i) the energy exchange with the surroundings (in joules) (ii) the energy yield for ethanol in joules per g (iii) the enthalpy of combustion for ethanol... [4] (iv) Explain why it is not possible to state that the answer to (iii) is the standard enthalpy of combustion. (v) Give an assumption used when calculating an answer to (i) c) If the standard enthalpy of combustion is kj mol -1, suggest and explain two reasons for the difference. Reason 1... [1] Explanation... Reason 2... [1] Explanation... TOTAL / 19