AP®ChemistryCollege BoardExam QuestionsUnit 6: ThermodynamicsEnthalpy Changes

Enthalpy Changes (College Board AP® Chemistry)

Exam Questions

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Enthalpy Changes

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Enthalpy Changes

11 mark

Which of the following statements correctly describes the energy changes associated with bond breaking and bond formation?

  • Bond breaking is an endothermic process, and bond formation is also an endothermic process.

  • Bond breaking is an endothermic process, and bond formation is an exothermic process.

  • Bond breaking is an exothermic process, and bond formation is an endothermic process.

  • Bond breaking is an exothermic process, and bond formation is also an exothermic process.

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21 mark

Reaction 1: HNO3 (aq) + NH3 (aq) → NH4NO3 (aq)

Reaction 2: CuCO3 (s) → CuO (s) + CO2 (g)

Reaction 3: S (s) + O2 (g) → SO2 (g)

Which enthalpy changes correctly describe the reactions above? 

ΔHco

ΔHfo

ΔHneuto

ΔHro

A

2

2

1

3

B

3

2

1

2

C

3

3

1

2

D

2

3

1

3

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    3
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    The following equation shows a reaction between the mineral rutile (TiO2) and carbon:

    TiO2 (s) + 2C (s) → Ti (s) + 2CO (g)

    The standard enthalpy changes of formation are as follows:

    • ΔHfo​ TiO2 (s) = –890.0 kJ mol⁻¹

    • ΔHfo​ CO (g) = –110.5 kJ mol⁻¹

    Which of the following is the standard enthalpy change for the extraction of titanium?

    • + 669.0 kJ mol–1

    • + 779.5 kJ mol–1

    • – 779.5 kJ mol–1

    • – 669.0 kJ mol–1

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    41 mark

    Which of the following correctly describes the possible values for the standard enthalpy changes of formation, combustion, and neutralization?

    • The enthalpy of formation and combustion are positive, and the enthalpy of neutralization can be positive or negative.

    • The enthalpy of formation and neutralization can be positive or negative, and the enthalpy of combustion is positive.

    • The enthalpy of formation can be positive or negative, and the enthalpy of combustion and neutralization are negative.

    • The enthalpy of formation and combustion are negative, and the enthalpy of neutralization is always positive.

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    5
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    C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)

    The combustion of ethanol (C2H5OH) is increasingly being used to fuel cars. Enthalpies of combustion for the substances involved in the reaction are shown in the table below.

    Substance

    ΔHco (kJ mol-1)

    H2 (g)

    -286

    C (s)

    -394

    C2H5OH (l)

    -1367

    Using the information above, determine the value of ΔHfo for the formation of ethanol.

    • 687 kJ mol-1

    • -279 kJ mol-1

    • -2047 kJ mol-1

    • -3013 kJ mol-1

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    6
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    The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation.

    4NH3 (g) + 5O2 (g) rightwards harpoon over leftwards harpoon 4NO (g) + 6H2O (g)

    Enthalpies of combustion for the substances involved in the reaction are shown in the table below.

    Substance

    ΔHco (kJ mol-1)

    NH3 (g)

    -51.3

    NO (g)

    +92.2

    H2O(g)

    -239.6

    Using the information above, what is the value of the standard enthalpy change for this reaction?

    • +863.6 kJ mol–1

    • +1274.0  kJ mol–1

    • –863.6 kJ mol–1

    • –1274.0 kJ mol–1

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    7
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    Bond

    Average bond enthalpy / kJ mol-1

    C−H

    412

    C=C 

    612

    O=O

    496

    C=O

    805

    O−H 

    463

    Using the information in the table above, what is the enthalpy change for the complete combustion of ethene?

    • +1324 kJ mol–1

    • -1324 kJ mol–1

    • -1936 kJ mol–1

    • +219 kJ mol–1

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    81 mark

    Which of the following has the highest bond enthalpy?

    • C–H single bond

    • C=O double bond

    • N≡N triple bond

    • O=O double bond

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    9
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    Nitrogen reacts with fluorine according to the following equation:

    N2 (g)  +  3F2 (g)  →  2NF3 (g)   ΔHr = -246 kJ

    The bond energy of  N≡N is 945 kJ mol-1 and the bond energy of F–F is 159 kJ mol-1.

    What is the bond energy of the N–F bond?

    • 225 kJ mol-1

    • 278 kJ mol-1

    • 1350 kJ mol-1

    • 1668 kJ mol-1

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    10
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    ∆H / kJ mol−1 

    C (s) + O2 (g) → CO2 (g)

    −394 

    H2 (g) + ½O2 (g) → H2O (g)

    −286

    4C (s) + 5H2 (g) → C4H10 (g)

    −125

    Using the information in the table above, what is the enthalpy change for the complete combustion of butane?

    • -555 kJ mol-1

    • -1699 kJ mol-1

    • -1737 kJ mol-1

    • -2881 kJ mol-1

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