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

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.

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

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

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.

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

The reaction between methane and chlorine forms chloromethane and hydrogen chloride:

CH₄ (g) + Cl₂ (g) → CH₃Cl (g) + HCl (g)

The enthalpy change for this reaction can be estimated in different ways.

The average bond enthalpies for the relevant bonds are shown in the table below.

Calculate the enthalpy change (ΔH) for the reaction using these data.

The standard enthalpies of formation of the reactants and products are given in the table below.

Calculate the standard enthalpy change (ΔH^o) for the reaction.

Suggest one reason why the value calculated in part (a) differs from the value calculated in part (b).

Explain how a negative enthalpy change affects the thermodynamic favorability of a reaction.

The overall reaction is reversed in a later step of a thermochemical cycle.

CH₃Cl (g) + HCl (g) → CH₄ (g) + Cl₂ (g)

Explain how this affects the sign and magnitude of ΔH.

This reaction step occurs in a thermochemical cycle:

HCl (g) → H (g) + Cl (g)

Explain how this step affects the total enthalpy change for the overall reaction.

Propane (C₃H₈) undergoes complete combustion in excess oxygen, releasing energy.

Explain why the combustion of propane is an exothermic process.

The combustion reaction is represented by the balanced equation:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Bond enthalpy data:

Calculate the enthalpy change for the reaction.

The combustion of propane does not occur spontaneously at room temperature. Explain why an external energy source is required to initiate the reaction.

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.

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

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

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.

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/mol_rxn

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). 

The complete combustion of ethyne is represented by the reaction:

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

The following bond enthalpies are provided:

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

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

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

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

The combustion of propane occurs as follows:

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

Calculate the enthalpy of formation of propane.

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

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