5.4 Past Paper Questions

1. What energy changes occur when chemical bonds are formed and broken?

A. Energy is absorbed when bonds are formed and when they are broken.

B. Energy is released when bonds are formed and when they are broken.

C. Energy is absorbed when bonds are formed and released when they are broken.

D. Energy is released when bonds are formed and absorbed when they are broken.

2. The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C.

How many joules of heat energy were added? (Specific heat of Al = 0.90 J g^–1K^–1)

A. 0.36

B. 2.3

C. 9.0

D. 11

3. Using the equations below:

C_(s)+ O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s)+ O_2(g) → MnO_2(s) ∆H = –520 kJ

What is ∆H (in kJ) for the following reaction? MnO_2(s)+ C_(s) --> Mn_(s)+ CO_2(g)

A. 910

B. 130

C. –130

D. –910

4. Which statements about exothermic reactions are correct?

I. They have negative ∆H values.

II. The products have a lower enthalpy than the reactants.

III. The products are more energetically stable than the reactants.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

5. A sample of a metal is heated. Which of the following are needed to calculate the heat absorbed by the sample?

I. The mass of the sample

II. The density of the sample

III. The specific heat capacity of the sample

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

6. The average bond enthalpies for O—O and O==O are 146 and 496 kJ mol^–1 respectively.

What is the enthalpy change, in kJ, for the reaction below?

H—O—O—H_(g) --> H—O—H_(g) + ½O=O_(g)

A. – 102 B. + 102

C. + 350 D. + 394

7. When the solids Ba(OH)₂ and NH₄SCN are mixed, a solution is produced and the temperature drops.

Ba(OH)_2(s) + 2NH₄SCN_(s) → Ba(SCN)_2(aq) + 2NH_3(g) + 2H₂O_(l)

Which statement about the energetics of this reaction is correct?

A. The reaction is endothermic and ∆H is negative.

B. The reaction is endothermic and ∆H is positive.

C. The reaction is exothermic and ∆H is negative.

D. The reaction is exothermic and ∆H is positive.

8. Using the equations below

Cu(s) + 1/2 O₂(g) → CuO(s) ∆Hο = –156 kJ

2Cu(s) + 1/2 O₂(g) → Cu₂O(s) ∆Hο = –170 kJ

what is the value of ∆Hο (in kJ) for the following reaction?

2CuO(s) → Cu₂O(s) + 1/2 O₂(g)

A. 142 B. 15

C. –15 D. –142

9. Which of the quantities in the enthalpy level diagram below is (are) affected by the use of a catalyst?

Time

A. I only

B. III only

C. I and II only

D. II and III only

10. Consider the following equations.

Mg(s) + 1/2 O₂(g) → MgO(s) ∆Hο = –602 kJ

H2(g) + 1/2 O₂(g) → H₂O(g) ∆Hο = –242 kJ

What is the ∆H° value (in kJ) for the following reaction?

MgO(s) + H₂(g) → Mg(s) + H₂O(g)

A. –844 B. –360

C. +360 D. +844

11. For which of the following is the sign of the enthalpy change different from the other three?

A. CaCO₃(s) → CaO(s) + CO₂(g)

B. Na(g) → Na+(g) + e–

C. CO₂(s) → CO₂(g)

D. 2Cl(g) → Cl₂(g)

12. Separate solutions of HCl(aq) and H₂SO₄(aq) of the same concentration and same volume were completely neutralized by NaOH(aq). X kJ and Y kJ of heat were evolved respectively.

Which statement is correct?

A. X = Y B. Y = 2X

C. X = 2Y D. Y = 3X

13. Which statements are correct for an endothermic reaction?

I. The system absorbs heat.

II. The enthalpy change is positive.

III. The bond enthalpy total for the reactants is greater than for the products.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

14. The mass m (in g) of a substance of specific heat capacity c (in J g^–1 K^–1 ) increases by t°C. What is the heat change in J?

A. mct

B. mc(t + 273)

C. mct /1000

D. (mc(t + 273))/1000

15. The average bond enthalpy for the C―H bond is 412 kJ mol^–1. Which process has an enthalpy change closest to this value?

A. CH₄(g) → C(s) + 2H₂(g)

B. CH₄(g) → C(g) + 2H₂(g)

C. CH₄(g) → C(s) + 4H(g)

D. CH₄(g) → CH₃(g) + H(g)

16. The following equation shows the formation of magnesium oxide from magnesium.

2Mg(s) + O₂(g)-->2MgO(s) ∆HӨ = –1204kJ

Which statement is correct for this reaction?

A. 1204 kJ of energy are released for every mol of magnesium reacted.

B. 602 kJ of energy are absorbed for every mol of magnesium oxide formed.

C. 602 kJ of energy are released for every mol of oxygen gas reacted.

D. 1204 kJ of energy are released for every two mol of magnesium oxide formed.

17. The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide.

C(s) +O₂(g) --> CO₂(g) ∆HӨ = –x kJ mol–1

CO(g) + 1/2 O₂(g) -->CO₂(g) ∆HӨ = –y kJ mol–l

What is the enthalpy change, in kJ mol^–1, for this oxidation?

C(s) + 1/2 O2(g)--> CO(g)

A. x + y B. – x – y

C. y – x D. x – y

18. A simple calorimeter was used to determine the enthalpy of combustion of ethanol. The experimental value obtained was –920 kJ mol^–1. The Data Booklet value is –1371 kJ mol^–1. Which of the following best explains the difference between the two values?

A. incomplete combustion of the fuel

B. heat loss to the surroundings

C. poor ventilation in the laboratory

D. inaccurate temperature measurements

19. For the reaction

2H₂(g) + O₂(g) --> 2H₂O_(g)

the bond enthalpies (in kJ mol^–1) are

H–H

O=O

O–H

x

y

z

Which calculation will give the value, in kJ mol^–1, of ∆HӨ for the reaction?

A. 2x + y –2z

B. 4z – 2x – y

C. 2x + y – 4z

D. 2z –2x – y

20. Which statement about bond enthalpies is correct?

A. Bond enthalpies have positive values for strong bonds and negative values for weak bonds.

B. Bond enthalpy values are greater for ionic bonds than for covalent bonds.

C. Bond breaking is endothermic and bond making is exothermic.

D. The carbon–carbon bond enthalpy values are the same in ethane and ethene.

21. An equation for a reaction in which hydrogen is formed is

CH₄ + H₂O--> 3H₂ + CO ∆H^Ө = +210 kJ

Which energy change occurs when 1 mol of hydrogen is formed in this reaction?

A. 70 kJ of energy are absorbed from the surroundings.

B. 70 kJ of energy are released to the surroundings.

C. 210 kJ of energy are absorbed from the surroundings.

D. 210 kJ of energy are released to the surroundings.

22. The equations & enthalpy changes for 2 reactions used in the manufacture of H₂SO₄ are

S(s) + O₂(g) --> SO₂(g) ∆HӨ = –300 kJ

2SO₂(g) + O₂(g) --> 2SO₃(g) ∆HӨ = –200 kJ

What is the enthalpy change, in kJ, for the reaction below?

2S(s) + 3O₂(g) --> 2SO₃(g)

A. –100 C. –500

B. –400 D. –800

23. Approximate values of the average bond enthalpies, in kJ mol–1, of three substances are:

H–H

F–F

H–F

430

155

565

What is the enthalpy change, in kJ, for this reaction?

2HF --> H₂ + F₂

A. +545

B. +20

C. –20

D. –545

24. The standard enthalpy change of formation values of two oxides of phosphorus are:

P₄(s) + 3O₂(g) --> P₄O₆(s) ∆HӨf= –1600 kJ mol^–1

P₄(s) + 5O₂(g) --> P₄O₁₀(s) ∆HӨf= –3000 kJ mol^–1

What is the enthalpy change, in kJ mol–1, for the reaction below?

P₄O₆(s) + 2O₂(g) --> P₄O₁₀(s)

A. +4600

B. +1400

C. –1400

D. –4600

25. Which statement is correct for an endothermic reaction?

A. The products are more stable than the reactants and ∆H is positive.

B. The products are less stable than the reactants and ∆H is negative.

C. The reactants are more stable than the products and ∆H is positive.

D. The reactants are less stable than the products and ∆H is negative.

26. Which statement is correct about the reaction shown?

2SO_2(g) + O_2(g) --> 2SO_3(g) ∆H = –196 kJ

A. 196 kJ of energy are released for every mole of SO₂(g) reacted.

B. 196 kJ of energy are absorbed for every mole of SO₂(g) reacted.

C. 98 kJ of energy are released for every mole of SO₂(g) reacted.

D. 98 kJ of energy are absorbed for every mole of SO₂(g) reacted.

27. Which statements are correct for all exothermic reactions?

I. The enthalpy of the products is less than the enthalpy of the reactants.

II. The sign of ∆H is negative.

III. The reaction is rapid at room temperature.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

28. Consider the specific heat capacity of the following metals.

Which metal will show the greatest temperature increase if 50 J of heat is supplied to a 0.001 kg sample of each metal at the same initial temperature?

A. Cu

B. Ag

C. Au

D. Pt

29. Consider the following reactions.

S(s) + 1/2 O₂(g) --> SO₃(g) ∆HӨ = -395 kJ mol-1

SO₂(s) + 1/2 O₂(g) --> SO₃(g) ∆HӨ = -98 kJ mol-1

What is the ∆HӨ value (in kJ mol–1) for the following reaction?

S(s) + O₂(g) --> SO₂(g)

A. –297

B. +297

C. – 493

D. +493

30. Which statement is correct for an endothermic reaction?

A. Bonds in the products are stronger than the bonds in the reactants.

B. Bonds in the reactants are stronger than the bonds in the products.

C. The enthalpy of the products is less than that of the reactants.

D. The reaction is spontaneous at low temperatures but becomes non-spontaneous at high temperatures.

31. According to the enthalpy level diagram below, what is the sign for ∆H and what term is used to refer to the reaction?

32. When 40 joules of heat are added to a sample of solid H₂O at –16.0°C the temperature increases to –8.0°C. What is the mass of the solid H₂O sample?

[Specific heat capacity of H₂O(s) = 2.0 J g^–1K^–1]

A. 2.5 g

B. 5.0 g

C. 10 g

D. 160 g

33. The DH^Ө values for the formation of two oxides of nitrogen are given below.

1/2 N₂(g) + O₂(g) --> NO₂(g) ∆HӨ = –57 kJ mol^–1

N₂(g) + 2O₂(g) --> N₂O₄(g) ∆HӨ = +9 kJ mol^–1

Use these values to calculate ∆HӨ for the following reaction (in kJ):

2NO₂(g) --> N₂O₄(g)

A. –105 C. +66

B. – 48 D. +123

34. How much energy, in joules, is required to increase the temperature of 2.0 g of Aluminium from 25 to 30°C? (Specific heat of Al = 0.90 J g^–1 K^–1).

A. 0.36

B. 4.5

C. 9.0

D. 54

35. Which combination is correct for a chemical reaction that absorbs heat from the surroundings?

36. Using the equations below:

C(s) + O₂(g) → CO₂(g) ∆Hο = –394 kJ mol^–1

Mn(s) + O₂(g) → MnO₂(s) ∆Hο = –520 kJ mol^–1

What is ∆H, in kJ, for the following reaction?

MnO₂(s) + C(s) → Mn(s) + CO₂(g)

A. 914

B. 126

C. –126

D. –914

38. (a) Define the term average bond enthalpy, illustrating your answer with an equation for methane, CH₄.

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

(3)

(b) The equation for the reaction between methane and chlorine is

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

Use the values from Table 10 of the Data Booklet to calculate the enthalpy change for this reaction.

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

(3)

(c) Explain why no reaction takes place between methane and chlorine at room

temperature unless the reactants are sparked, exposed to UV light or heated.

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

(2)

(d) Draw an enthalpy level diagram for this reaction.

(2)

(Total 10 marks)

39. In aqueous solution, potassium hydroxide and hydrochloric acid react as follows.

KOH(aq) + HCl(aq) → KCl(aq)+ H₂O(l)

The data below is from an experiment to determine the enthalpy change of this reaction.

50.0 cm³ of a 0.500 mol dm^–3 solution of KOH was mixed rapidly in a glass beaker with 50.0 cm³ of a 0.500 mol dm^–3solution of HCl.

Initial temperature of each solution = 19.6°C

Final temperature of the mixture = 23.1°C

(a) State, with a reason, whether the reaction is exothermic or endothermic.

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

(1)

(b) Explain why the solutions were mixed rapidly.

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

(1)

(c) Calculate the enthalpy change of this reaction in kJ mol–1. Assume that the specific heat capacity of the solution is the same as that of water.

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(4)

(d) Identify the major source of error in the experimental procedure described above. Explain how it could be minimized.

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

(2)

(e) The experiment was repeated but with an HCl concentration of 0.510 mol dm^–3 instead of 0.500 mol dm^–3. State and explain what the temperature change would be.

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(2)

(Total 10 marks)

40. The data below is from an experiment used to measure the enthalpy change for the combustion of 1 mole of sucrose (common table sugar), C₁₂H₂₂O₁₁(s). The time-temperature data was taken from a data-logging software programme.

Mass of sample of sucrose, m = 0.4385 g

Heat capacity of the system, Csystem = 10.114 kJ K^–1

(a) Calculate ΔT, for the water, surrounding the chamber in the calorimeter.

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

(1)

(b) Determine the amount, in moles, of sucrose.

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

(1)

(c) (i) Calculate the enthalpy change for the combustion of 1 mole of sucrose.

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(1)

(ii) Using the Data Booklet, calculate the percentage experimental error based on the data used in this experiment.

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(1)

(d) A hypothesis is suggested that TNT, 2-methyl-1,3,5-trinitrobenzene, is a powerful explosive because it has:

• a large enthalpy of combustion

• a high reaction rate

• a large volume of gas generated upon combustion

Use your answer in part (c)(i) and the following data to evaluate this hypothesis:

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

(3)

(Total 7 marks)

41. (a) Define the term average bond enthalpy.

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

(2)

(b) Use the information from the Data Booklet to calculate the enthalpy change for the complete combustion of but-1-ene, according to the following equation.

C4H8(g) + 6O2(g) → 4CO2(g) + 4H2O(g)

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

(3)

(Total 5 marks)

42. Given the following data:

C(s) + F₂(g) → CF₄(g); ∆H1 = –680 kJ mol^–1

F₂(g) → 2F(g); ∆H2 = +158 kJ mol^–1

C(s) → C(g); ∆H3 = +715 kJ mol^–1

calculate the average bond enthalpy (in kJ mol–1) for the C–F bond.

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

.....................................................................................................................(Total 4 marks)

43. Two reactions occurring in the manufacture of sulfuric acid are shown below:

reaction I S(s) +O₂(g) --> SO₂(g) ∆HӨ = –297 kJ

reaction II SO₂(g) + 1/2 O₂(g) ⇌ SO₃(g) ∆HӨ = –92 kJ

(i) State the name of the term ∆HӨ. State, with a reason, whether reaction I would be accompanied by a decrease or increase in temperature.

(3)

(ii) At room temperature sulfur trioxide, SO₃, is a solid. Deduce, with a reason, whether the ∆H^Ө value would be more negative or less negative if SO₃(s) instead of SO₃(g) were formed in reaction II.

(2)

(iii) Deduce the ∆H^Ө value of this reaction:

S(s) + 1/2 O₂(g) --> SO₃(g)

(1)

(Total 6 marks)

44. (i) Define the term average bond enthalpy.

(3)

(ii) Explain why Br2 is not suitable as an example to illustrate the term average bond enthalpy.

(1)

(iii) Using values from the Data Booklet, calculate the enthalpy change for the following reaction:

CH₄(g) + Br₂(g) --> CH₃Br(g) + HBr(g)

(3)

(iv) Sketch an enthalpy level diagram for the reaction in part (iii).

(2)

(v) Without carrying out a calculation, suggest, with a reason, how the enthalpy change for the following reaction compares with that of the reaction in part (iii):

CH₃Br(g) + Br₂(g) --> CH₂Br₂(g) + HBr(g)

(2)

(Total 11 marks)

45. But–1–ene gas, burns in oxygen to produce carbon dioxide and water vapour according to the following equation.

C₄H₈ + 6O₂ --> 4CO₂ + 4H₂O

(a) Use the data below to calculate the value of ∆H^Ө for the combustion of but-1-ene.

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

(3)

(b) State and explain whether the reaction above is endothermic or exothermic.

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

(1)

(Total 4 marks)

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

C + 2H₂ + 1/2 O₂ --> CH₃OH ∆H4

using Hess’s Law and the following information.

CH₃OH + 1/2 O₂ --> CO₂ + 2H₂O ∆H1 = -676 kJ mol^-1

C + O₂ --> CO₂ ∆H2 = -394 kJ mol^-1

H₂ + 1/2 O₂ --> H₂O ∆H3 = -242 kJ mol^-1

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

......................................................................................................................(Total 4 marks)

47. Methylamine can be manufactured by the following reaction.

CH₃OH(g) + NH₃(g) --> CH₃NH₂(g) + H₂O(g)

(a) Define the term average bond enthalpy.

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

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

(2)

(b) Use information from the Data Booklet to calculate the enthalpy change for this reaction.

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................................................................................................................................(4)

(Total 6 marks)

48. (a) Define the term average bond enthalpy.

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

(2)

(b) Use the information from Table 10 in the Data Booklet to calculate the enthalpy change for the complete combustion of but-1-ene according to the following equation

C₄H₈(g) --> 4CO₂(g) + 4H₂O(g)

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

................................................................................................................................(3)

(c) 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.

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................................................................................................................................(1)

(d) The enthalpy level diagram for a certain reaction is shown below.

State and explain the relative stabilities of the reactants and products.

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................................................................................................................................(2)

(Total 8 marks)

49. The reaction between ethene and hydrogen gas is exothermic.

(i) Write an equation for this reaction.

(1)

(ii) Deduce the relative stabilities and energies of the reactants and products.

(2)

(iii) Explain, by referring to the bonds in the molecules, why the reaction is exothermic.

(2)

(Total 5 marks)

50. (i) Define the term average bond enthalpy.

(2)

(ii) The equation for the reaction of ethyne and hydrogen is:

C₂H₂(g) + 2H₂(g) --> C₂H₆(g)

Use information from Table 10 of the Data Booklet to calculate the change in enthalpy for the reaction. (2)

(iii) State & explain the trend in the bond enthalpies of the C–Cl, C–Br and C–I bonds.

(2)

(Total 6 marks)