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AP®ChemistryCollege BoardExam QuestionsUnit 6: ThermochemistryEnthalpy Changes

Enthalpy Changes (College Board AP® Chemistry): Exam Questions

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Enthalpy Changes

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Enthalpy Changes

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1a1 mark

The decomposition of hydrogen peroxide (H2O2​) is represented as:

2H2O2 (l) → 2H2O (g) + O2 (g)

The enthalpy change for this reaction can be calculated using the following data:

  1. H2O2 (l) → H2O (l) + 1/2O2 (g), ΔH = −98.2 kJ

  2. H2O (l) → H2O (g), ΔH = +44.0 kJ

Calculate the overall ΔH for the reaction.

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1b2 marks

Use the provided enthalpy values to draw a labeled enthalpy diagram for the decomposition of H2O2 (l), including intermediate steps.

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1c1 mark

State whether this reaction is exothermic or endothermic and justify your answer.

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2a1 mark

A student investigates the combustion of methane, CH4, in oxygen under standard conditions. The reaction is represented below:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)space space space space space space space space space spaceΔH∘=−890  kJ/mol

Explain the meaning of ΔH∘ = −890 kJ/mol.

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2b1 mark

Calculate the total energy released when 3.00 mol of CH₄ combusts.

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2c2 marks

Methane combustion is exothermic. Explain this in terms of bond enthalpies, referencing the bonds broken in the reactants and the bonds formed in the products.

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3a2 marks

The formation of ammonia (NH3​) is represented as:

N2 (g) + 3H2 (g) → 2NH3 (g)

The enthalpy change for this reaction can be calculated using the following bond enthalpy data:

  • N≡N: 945 kJ/mol

  • H–H: 436 kJ/mol

  • N–H: 391 kJ/mol

Calculate the overall ΔH for the reaction.

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3b2 marks

Justify whether the reaction is exothermic or endothermic. In your explanation, discuss how bond enthalpies relate to the potential energy of the reactants and products

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1a1 mark

A student places a mixture of plastic beads consisting of polypropylene (PP) and polyvinyl chloride (PVC) in a 1.0 L beaker containing distilled water. After stirring the contents of the beaker vigorously, the student observes that the beads of one type of plastic sink to the bottom of the beaker and the beads of the other type of plastic float on the water. The chemical structures of PP and PVC are represented by the diagrams below, which show segments of each polymer.

q6-june-2014---ap-chemistry

Given that the spacing between polymer chains in PP and PVC is similar, the beads that sink are made of which polymer? Explain.

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1b1 mark

PP is synthesized from propene, C3H6 , and PVC is synthesized from vinyl chloride, C2H3Cl . The structures of the molecules are shown below.

q6a-june-2014---ap-chemistry

The boiling point of liquid propene (226 K) is lower than the boiling point of liquid vinyl chloride (260 K). Explain this difference in terms of the types and strengths of intermolecular forces present in each liquid.

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1c2 marks

In a separate experiment, the student measures the enthalpies of combustion of propene and vinyl chloride. The student determines that the combustion of 2.00 mol of vinyl chloride releases 2300 kJ of energy, according to the equation below.

2 C2H3Cl(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g) + 2 HCl(g)      ∆H° = − 2300 kJ/molrxn

Using the table of standard enthalpies of formation below, determine whether the combustion of 2.00 mol of propene releases more, less, or the same amount of energy that 2.00 mol of vinyl chloride releases. Justify your answer with a calculation. The balanced equation for the combustion of 2.00 mol of propene is  

2 C3H6(g)  +  9 O2(g)  →  6 CO2(g)  +  6 H2O(g). 

Substance

C2H3Cl(g)

C3H6(g)

CO2(g)

H2O(g)

HCl(g)

O2(g)

Standard Enthalpy of Formation (kJ/mol)

 37

21

−394

 −242

−92

0

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2a1 mark

The complete combustion of ethyne is represented by the reaction:

2C2H2 (g) + 5O2 (g) → 4CO2 (g) + 2H2O (g)

The following bond enthalpies are provided:

Bond

Bond Enthalpy (kJ/mol)

C≡C

839

C–H

414

O=O

498

C=O

804

O–H

463

Identify the bonds broken in the reactants and the bonds formed in the products for this reaction.

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2b2 marks

Calculate the overall enthalpy change (ΔH) for the reaction, using the bond enthalpy values provided.

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2c1 mark

Justify whether the reaction is exothermic or endothermic, referencing the relative stability of the products and reactants.

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3a2 marks

The standard enthalpy of combustion for propane is −2220.0 kJ/mol. The following standard enthalpy of formation (ΔHf∘​) values are provided:

Substance

ΔHf∘​ (kJ/mol)

CO2 (g)

-393.5

H2O (l)

-285.8

The combustion of propane occurs as follows:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)

Calculate the enthalpy of formation of propane.

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3b1 mark

Explain why the reaction is thermodynamically favourable at standard conditions, referencing the sign of ΔH∘ and the relative stability of the products and reactants.

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3c1 mark

Predict how increasing the temperature would affect the thermodynamic favourability of this reaction. Justify your answer.

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