Free Energy (College Board AP® Chemistry): Exam Questions

A student designs an experiment to study the reaction between NaHC0₃ and HC₂H₃O₂. The reaction is represented by the equation below.

NaHCO₃ (s) + HC₂H₃O₂ (aq) → NaC₂H₃O₂ (aq) + H₂O (l) + CO₂ (g)

The student places 2.24 g of NaHCO₃ in a flask and adds 60.0 mL of 0.875 M HC₂H₃O₂. The student observes the formation of bubbles and that the flask gets cooler as the reaction proceeds.

Identify the reaction represented above as an acid-base reaction, precipitation reaction, or redox reaction. Justify your answer.

Based on the information above, identify the limiting reactant. Justify your answer with calculations.

The student observes that the bubbling is rapid at the beginning of the reaction and gradually slows as the reaction. Explain this change in the reaction rate in terms of the collisions between reactant particles.

In thermodynamic terms, a reaction can be driven by enthalpy, entropy, or both.

i) Considering that the flask gets cooler as the reaction proceeds, what drives the chemical reaction between NaHCO₃ (s) and HC₂H₃O₂ (aq) ? Answer by drawing a circle around one of the choices.

Enthalpy only / Entropy only / Both enthalpy and entropy

ii) Justify your selection in part (d)(i) in terms of ΔG°

The HCO₃⁻ ion has three carbon-to-oxygen bonds. Two of the carbon-to-oxygen bonds have the same length, and the third carbon-to-oxygen bond is longer than the other two. The hydrogen atom is bonded to one of the oxygen atoms. In the box below, draw a Lewis electron-dot diagram (or diagrams) for the HCO₃⁻ ion that is (are) consistent with the given information.

A student prepares a solution containing equimolar amounts of HC₂H₃O₂ and NaC₂H₃O₂. The pH of the solution is measured to be 4.7. The student adds two drops of 3.0 M HNO₃ (aq) and stirs the sample, observing that the pH remains at 4.7. Write a balanced, net-ionic equation for the reaction between HNO₃ (aq) and the chemical species in the sample that is responsible for the pH remaining at 4.7.

A student investigates the reactions of nitrogen oxides. One of the reactions in the investigation requires an equimolar mixture of NO(g) and NO₂(g) , which the student produces by using the reaction represented below.

2 NO (g) + O₂ (g) →  2 NO₂ (g)

The particle-level representation of the equimolar mixture of NO (g) and NO₂ (g) in the flask at the completion of the reaction between NO (g) and O₂ (g) is shown below in the box on the right. In the box below on the left, draw the particle-level representation of the reactant mixture of NO (g) and O₂ (g) that would yield the product mixture shown in the box on the right. In your drawing, represent oxygen atoms and nitrogen atoms as indicated below.

The student reads in a reference text that NO(g) and NO₂(g) will react as represented by the equation below. Thermodynamic data for the reaction are given in the table below the equation.

NO (g) + NO₂ (g)  N₂O₃ (g)

 The student begins with an equimolar mixture of NO (g) and NO₂ (g) in a rigid reaction vessel and the mixture reaches equilibrium at 298 K.

i) Calculate the value of the equilibrium constant, K, for the reaction at 298 K.

 ii) If both P_NO  and in the vessel are initially 1.0 atm, will at equilibrium be equal to 1.0 atm? Justify your answer.

The student hypothesizes that increasing the temperature will increase the amount of N₂O₃ (g) in the equilibrium mixture. Indicate whether you agree or disagree with the hypothesis. Justify your answer.

N₂O₃ (g) reacts with water to form nitrous acid, HNO₂ (aq), a compound involved in the production of acid rain. The reaction is represented below.

N₂O₃ (g) + H₂O (l) →  2 HNO₂ (aq)

The skeletal structure of the HNO₂ molecule is shown in the box below.

i) Complete the Lewis electron-dot diagram of the HNO₂ molecule in the box below, including any lone pairs of electrons.

ii) Based on your completed diagram above, identify the hybridization of the nitrogen atom in the HNO₂ molecule.

To produce an aqueous solution of HNO₂, the student bubbles N₂O₃ (g) into distilled water. Assume that the reaction goes to completion and that HNO₂ is the only species produced. To determine the concentration of HNO₂ (aq) in the resulting solution, the student titrates a 100. mL sample of the solution with 0.100 M KOH (aq). The neutralization reaction is represented below.

HNO₂ (aq) + OH⁻  (aq) →  NO₂⁻ (aq) + H₂O (l)

The following titration curve shows the change in pH of the solution during the titration.

Use the titration curve and the information above to

i) determine the initial concentration of the HNO₂ (aq) solution

ii) estimate the value of pK_a for HNO₂ (aq)

During the titration, after a volume of 15 mL of 0.100 M KOH (aq) has been added , which species, HNO₂ (aq) or NO₂^– (aq) , is present at a higher concentration in the solution? Justify your answer.

A reaction is carried out at 298 K with the following data:

• ΔH^∘ = −100.0 kJ/mol

• ΔS^∘ = −200.0 J/mol

Calculate ΔG^∘ for the reaction at 298 K.

Predict whether the reaction is spontaneous under these conditions. Justify your answer.

Explain how changes in temperature might affect the spontaneity of this reaction.

The hydrolysis of ATP to ADP is given by:

ATP → ADP + Pi, ΔG^∘ = −30.5 kJ/mol

The reduction of glutamate to glutamine is a non-spontaneous reaction:

Glutamate + NH₃ → Glutamine, ΔG^∘ = +14.2 kJ/mol

i) Explain how this reaction can proceed when coupled with ATP hydrolysis.

ii) Using the data provided, calculate the overall ΔG∘ for the coupled reaction.

Explain the role of ATP hydrolysis in biological systems, emphasising why coupling is essential for energy transfer.

For the reaction:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

At 298 K, ΔG^∘ = −33.0 kJ/mol.

Explain why this reaction favors the products at equilibrium.

Calculate the value of K for the reaction at 298 K.

Describe what happens to the equilibrium constant, K, for this reaction if the temperature is increased.

A reaction is carried out at varying temperatures with the following data:

• ΔH^∘ = −75.0 kJ mol^-1

• ΔS^∘ = 250 J mol^-1 K^-1

Calculate the temperature at which this reaction becomes thermodynamically non-spontaneous.

Determine whether the reaction is spontaneous at 400 K, showing all calculations.

Explain why ΔS^∘ > 0 for this reaction, referencing changes in particle movement or dispersal of matter.

Consider the decomposition reaction:

CaCO₃ (s) → CaO (s) + CO₂ (g)

At 298 K:

• ΔH^∘ = 178.0 kJ mol^-1

• ΔS^∘ = 160.0 J mol^-1 K^-1

Calculate ΔG^∘ at 298 K.

Determine the temperature above which the reaction becomes thermodynamically favorable.

Explain why this reaction is entropy-driven.

Electrolysis is used industrially to extract metals from their ores. The electrolysis of molten aluminum oxide occurs according to the following reaction:

2Al₂O₃ (l) → 4Al (s) + 3O₂ (g)

At 1000 K:

• ΔH^∘ = 1675.0 kJ mol^-1

• ΔS^∘ = 2050 J mol^-1 K^-1

Calculate ΔG^∘ for this reaction at 1000 K.

Explain why this process is thermodynamically non-spontaneous under standard conditions and justify why electrolysis is required

If the electrolysis process operates at 4.50 V, calculate the minimum amount of electrical energy (in kJ) required to produce 1.00 mol of aluminum.