15.3 Past Paper Questions

1. Which combination of ionic charge and ionic radius give the largest lattice enthalpy for an ionic compound?

2. The lattice enthalpy values for Lithium Fluoride and Calcium Fluoride are shown below.

LiF_(s) ∆H^ο = +1022 kJ mol^–1

CaF_2(s) ∆H^ο = +2602 kJ mol^–1

Which of the following statements help(s) to explain why the value for Lithium Fluoride is less than that for Calcium Fluoride?

I. The ionic radius of Lithium is less than that of Calcium.

II. The ionic charge of Lithium is less than that of Calcium.

A. I only

B. II only

C. I and II

D. Neither I nor II

3. Which reaction has the most negative ∆H^ο value?

A. LiF_(s)→ Li⁺_(g) + F^–_(g)

B. Li⁺_(g) + F^–_(g)→ LiF_(s)

C. NaCl_(s)→ Na⁺_(g) + Cl^–_(g)

D. Na⁺_(g)+ Cl^–_(g)→ NaCl_(s)

4. Which type of reaction is referred to in the definition of standard enthalpy change of formation?

A. the formation of a compound from its elements

B. the formation of a crystal from its ions

C. the formation of a molecule from its atoms

D. the formation of a compound from other compounds

5. What is the correct order of decreasing entropy for a pure substance?

A. gas > liquid > solid

B. solid > liquid > gas

C. solid > gas > liquid

D. liquid > solid > gas

6. Which reaction has the largest positive value of ∆S^Ө?

A. CO_2(g)+ 3H_2(g) → CH₃OH_(g) + H₂O_(g)

B. 2Al_(s) + 3S_(s) → Al₂S_3(s)

C. CH_4(g)+ H₂O_(g) → 3H_2(g) + CO_(g)

D. 2S_(s) + 3O_2(g)→ 2SO_3(g)

7. Which is a correct equation to represent the lattice enthalpy of Magnesium Sulphide?

A. MgS_(s) → Mg_(s)+ S_(s)

B. MgS_(s) → Mg_(g)+ S_(g)

C. MgS_(s) → Mg⁺_(g) + S^–_(g)

D. MgS_(s) → Mg²⁺_(g) + S^2–_(g)

8. Which equation represents a change with a negative value for DS?

A. 2H_2(g) + O_2(g)→ 2H₂O_(g)

B. H₂O_(s) → H₂O_(g)

C. H_2(g) + Cl_2(g) → 2HCl_(g)

D. 2NH_3(g) → N_2(g) + 3H_2(g)

9. The expression for the standard free energy change of a reaction is given by

∆G^Ө = ∆H^Ө – ∆DS^Ө

What are the signs for ∆H^Ө and ∆S^Ө for a reaction that is spontaneous at all temperatures?

10. Which are characteristics of ions in an ionic compound with a large lattice enthalpy value?

A. Large ionic radius and high ionic charge

B. Small ionic radius and low ionic charge

C. Large ionic radius and low ionic charge

D. Small ionic radius and high ionic charge

11. The following reaction is spontaneous only at temperatures above 850^°C.

CaCO_3(s) → CaO_(s) + CO_2(g)

Which combination is correct for this reaction at 1000^°C?

12. Consider the following information.

C₆H_6(l) + 7.5 O_2(g)→ 6CO_2(g) + 3 H₂O_(l)

Which expression gives the correct value of the standard enthalpy change of combustion for Benzene _(l), in kJ mol^–1?

A. 12(-394) + (-286) -2(49)

B. 12(394) + 6(286) -2(-49)

C. 6(-394) + 3(-286) - (-49)

D. 6(394) + 3(286) - (-49)

13. Which equation represents the lattice enthalpy of Magnesium Oxide?

A. Mg_(s) + 0.5 O_2(g) → MgO_(s)

B. Mg²⁺_(g) + O^2–_(g)→ MgO_(g)

C. Mg²⁺_(g) + 0.5 O_2(g) → MgO_(s)

D. Mg²⁺_(g) + O^2-_(g) → MgO_(s)

14. The ∆H^Ө and ∆S^Ө values for a reaction are both negative. What will happen to the spontaneity of this reaction as the temperature is increased?

A. The reaction will become more spontaneous as the temperature is increased.

B. The reaction will become less spontaneous as the temperature is increased.

C. The reaction will remain spontaneous at all temperatures.

D. The reaction will remain non-spontaneous at any temperature.

15. Which reaction has the most negative ∆H^ο value?

A. LiF_(s) → Li⁺_(g) + F^–_(g)

B. Li⁺_(g) + F^–_(g) → LiF_(s)

C. NaCl_(s)→ Na⁺_(g) + Cl^–_(g)

D. Na⁺_(g) + Cl^–_(g) → NaCl_(s)

16. Which equation represents the electron affinity of Calcium?

A. Ca_(g) →Ca⁺_(g)+ e^–

B. Ca_(g) →Ca^–(g) + e^–

C. Ca_(g) + e^– → Ca^–_(g)

D. Ca⁺_(g) + e^– → Ca_(g)

17. Which reaction causes a decrease in the entropy of the system?

A. CaCO_3(s) → CaO_(s) + CO_2(g)

B. 2H_2(g) + O_2(g) → 2H₂O_(l)

C. 2C_(s) + O_2(g) → 2CO_(g)

D. 2SO_3(g) → 2SO_2(g) + O_2(g)

18. What are the signs of ∆H^ο and ∆S^ο for a reaction that is non-spontaneous at low temperature but spontaneous at high temperature?

19. The equation for the decomposition of Calcium Carbonate is given below.

CaCO_3(s) → CaO_(s)+ CO_2(g)

At 500 K, ∆H for this reaction is +177 kJ mol^–1 and ∆S is 161 J K^–1 mol^–1.

(a) Explain why ∆H for the reaction above cannot be described as ∆H_f^ο.

.....................................................................................................................................

(2)

(b) State the meaning of the term ∆S.

.....................................................................................................................................

(1)

(c) Calculate the value of ∆G at 500 K and determine, giving a reason, whether or not the reaction will be spontaneous.

.....................................................................................................................................

(3)

(Total 6 marks)

20. The standard enthalpy change for the combustion of Phenol, C₆H₅OH_(s), is –3050 kJ mol^–1 at 298 K.

(a) Write an equation for the complete combustion of Phenol.

.....................................................................................................................................

(1)

(b) The standard enthalpy changes of formation of Carbon Dioxide, CO_2(g), and of water, H₂O_(l), are –394 kJ mol^–1 and –286 kJ mol^–1 respectively.

Calculate the standard enthalpy change of formation of Phenol, C₆H₅OH_(s).

.....................................................................................................................................

(3)

(c) The standard entropy change of formation, ∆S^ο, of Phenol, C₆H₅OH_(s) at

298 K is –385 J K^–1 mol ^–1. Calculate the standard free energy change of formation, ∆G^ο, of Phenol at 298 K.

.....................................................................................................................................

(3)

(d) Determine whether the reaction is spontaneous at 298 K, and give a reason.

.....................................................................................................................................

(2)

(e) Predict the effect, if any, of an increase in temperature on the spontaneity of this reaction.

.....................................................................................................................................

(2)

(Total 11 marks)

21. Explain in terms of ∆G^ο, why a reaction for which both ∆H^ο and ∆S^ο are positive is sometimes spontaneous and sometimes not.

……………………………………………………………………………………………

(Total 4 marks)

22. Consider the following reaction.

N_2(g) + 3H_2(g) → 2NH_3(g)

(i) Using the average bond enthalpy values in Table 10 of the Data Booklet, calculate the standard enthalpy change for this reaction.

………………………………………………………………………………………

(4)

(ii) The absolute entropy values, S, at 300 K for N_2(g), H_2(g) and NH_3(g)are 193, 131 & 192 JK^–1 mol^–1 respectively. Calculate ∆S^ο for the reaction and explain the sign of ∆S^ο.

………………………………………………………………………………………

(3)

(iii) Calculate ∆G^ο for the reaction at 300 K.

………………………………………………………………………………………

(1)

(iv) If the Ammonia was produced as a liquid and not as a gas, state and explain the effect this would have on the value of ∆H^ο for the reaction.

………………………………………………………………………………………

(2)

(Total 10 marks)

23. Define the term standard enthalpy of formation, and write the equation for the standard enthalpy of formation of Ethanol.

……………………………………………………………………………………………

(Total 5 marks)

24. Throughout this question, use relevant information from the Data Booklet.

(a) Define the term standard enthalpy change of formation, and illustrate your answer with an equation, including state symbols, for the formation of Nitric acid.

(4)

(b) Propyne undergoes complete combustion as follows:

C₃H_4(g) + 4O_2(g) → 3CO_2(g) + 2H₂O_(l)

Calculate the enthalpy change of this reaction, given the following additional values:

∆H_f^Ө of CO_2(g) = –394 kJ mol^–1

∆H_f^Ө of H₂O_(l) = –286 kJ mol^–1

(4)

(c) Predict and explain whether the value of ∆S^Ө for the reaction in part (b) would be negative, close to zero, or positive.

(3)

(Total 11 marks)

25. (a) Propyne reacts with Hydrogen as follows:

C₃H_4(g) + 2H_2(g)→ C₃H_8(g) ∆H^Ө = –287 kJmol^-1

Calculate the standard entropy change of this reaction, given the following additional information:

S^Ө of H_2(g) = 131 J K^–1 mol^–1

(3)

(b) Calculate the standard free energy change at 298 K, ∆G^Ө, for the reaction in part (a). Use your answer and relevant information from part (d). If you did not obtain an answer to part (a), use ∆S^Ө = –360 J K^–1 (this is not the correct value).

(3)

(Total 6 marks)

26. (a) The lattice enthalpy of an ionic compound can be calculated using a Born-Haber cycle. Using Lithium Fluoride as the example, construct a Born-Haber cycle, labelling the cycle with the formulas and state symbols of the species present at each stage.

(6)

(b) Two values of the lattice enthalpies for each of the Silver Halides are quoted in the Data Booklet. Discuss the bonding in Silver Fluoride and in Silver Iodide, with reference to these values.

(2)

(Total 8 marks)

27. Hex-1-ene gas, C₆H₁₂, burns in Oxygen to produce Carbon Dioxide and water vapour.

(a) Write an equation to represent this reaction.

....................................................................................................................................

(1)

(b) Use the data below to calculate the values of DH_c^Ө and DS_c^Ө for the combustion of Hex-1-ene.

(i) Value of ∆H_c^Ө

.........................................................................................................................

(2)

(ii) Value of ∆S_c^Ө

.........................................................................................................................

(2)

(c) Calculate the standard free energy change for the combustion of Hex-1-ene.

....................................................................................................................................

(2)

(d) State and explain whether or not the combustion of Hex-1-ene is spontaneous at 25^°C.

....................................................................................................................................

(1)

(Total 8 marks)

28. Calculate the enthalpy change, ∆H₄ for the reaction

C + 2H₂ + 0.5 O₂ → CH₃OH ∆H₄

using Hess’s Law, and the following information.

CH₃OH + 1.5 O₂ → CO₂ + 2H₂O ∆H₁ = -676 kJ mol^-1

C + O₂ → CO₂ ∆H₂ = -394 kJ mol^-1

H₂ + 0.5 O₂ → H₂O ∆H₃ = -242 kJ mol^-1

..............................................................................................................................................

(Total 4 marks)

29. Methylamine can be manufactured by the following reaction.

CH₃OH_(g) + NH_3(g) → CH₃NH_2(g) + H₂O_(g)

(a) Define the term standard enthalpy change of formation.

....................................................................................................................................

(2)

(b) The values of standard enthalpy changes of formation for some compounds are shown in the table.

Compound

NH_3(g)

H₂O_(g)

∆H_f^Ө / kJ mol^–1

– 46

– 242

Predict, with a reason, whether the value of ∆H_f^Ө for H₂O_(l) is less than, greater than, or equal to, the value of ∆H_f^Ө for H₂O_(g).

....................................................................................................................................

(2)

(c) Use information from the table in (b) and from Table 11 of the Data Booklet to calculate the enthalpy change for the reaction used to manufacture Methylamine.

....................................................................................................................................

(3)

(Total 7 marks)

30. (a) Define the term standard enthalpy change of formation, ∆H_f^Ө.

(2)

(b) (i) Use the information in the following table to calculate the enthalpy change for the complete combustion of But-1-ene according to the following equation.

C₄H_8(g) + 6O_2(g) → 4CO_2(g) + 4H₂O_(g)

(3)

(ii) Deduce, giving a reason, whether the reactants or the products are more stable.

(2)

(iii) Predict, giving a reason, how the enthalpy change for the complete combustion of But-2-ene would compare with that of But-1-ene based on average bond enthalpies.

(1)

(Total 8 marks)

31. (i) Define the term standard enthalpy change of formation, ∆H_f^Ө.

(2)

(ii) Construct a simple enthalpy cycle and calculate the value of ∆H_f^Ө (C₂H₅OH_(l)) given the following data.

(5)

(Total 7 marks)

32. Consider the following reaction:

N_2(g) + 3H_2(g) → 2NH_3(g)

(i) Suggest why this reaction is important for humanity.

(1)

(ii) Using the average bond enthalpy values in Table 10 of the Data Booklet, calculate the standard enthalpy change for this reaction.

(4)

(iii) The absolute entropy values, S, at 238 K for N_2(g), H_2(g) and NH_3(g) are 192, 131 & 193 J K^-1 mol^–1 respectively. Calculate ∆S^ο for the reaction and explain the sign of ∆S^ο.

(2)

(iv) Calculate ∆G^ο for the reaction at 238 K. State and explain whether the reaction is spontaneous.

(3)

(v) If Ammonia was produced as a liquid and not as a gas, state and explain the effect this would have on the value of ∆H^ο for the reaction.

(2)

(Total 12 marks)

33. (i) Define the terms lattice enthalpy and electron affinity.

(2)

(ii) Use the data in the following table and from the data booklet to construct the Born-Haber cycle for Sodium Chloride, NaCl, and determine the lattice enthalpy of NaCl_(s).

Na_(s) + 0.5 Cl_2(g) → NaCl_(g) ∆H^ο = –411 kJ mol^–1

Na_(s) → Na_(g) ∆H^ο = +108 kJ mol^–1

(4)

(iii) Describe the structure of Sodium Chloride.

(2)

(Total 8 marks)