Gibbs Free Energy Change, ΔG (Cambridge (CIE) A Level Chemistry): Exam Questions

State whether the standard entropy change, ΔS^θ, is positive or negative for each of the following processes. Explain your reasoning.

i) Steam condensing to water

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ii) Zinc reacting with dilute nitric acid

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iii) Solid sodium chloride dissolving in water

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Calcium carbonate decomposes according to the following equation:

CaCO₃ (s) → CaO (s) + CO₂ (g) ΔH^θ = +178 kJ mol^–1

Table 1.1 shows the standard entropies, S^θ, of the species.

Table 1.1

i) Calculate ΔS^θ for this reaction. Show your working.

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ii) Calculate the standard Gibbs free energy change, ΔG^θ, for this reaction at 298 K. Include a relevant sign in your answer. Show your working.

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ΔG^θ = .................. kJ mol^–1

Explain, with reference to ΔG^θ, why this reaction becomes more feasible at higher temperatures.

On heating, sodium hydrogencarbonate decomposes into sodium carbonate as shown.

2NaHCO₃ (s) → Na₂CO₃ (s) + CO₂ (g) + H₂O (g)

ΔH^θ = +130 kJ mol^–1 ΔS^θ = +316 J K^–1 mol^–1

Calculate the temperature above which this reaction becomes feasible. Show your working.

Temperature = ............ K

Potassium nitrate is a soluble salt. When it dissolves in water the standard enthalpy change ΔH^θ = +34.9 kJ mol^–1 and the standard entropy change ΔS^θ = +117 J K^–1 mol^–1.

Construct an equation, including state symbols, for the process that occurs when potassium nitrate dissolves in water.

Explain why ΔS^θ is positive for this process.

Calculate the temperature above which the dissolving of potassium nitrate becomes feasible. Show your working.

T = .................. K

i) State the sign of ΔG^θ for this process at a temperature lower than your answer to (c).

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ii) State what this value of ΔG^θ indicates about the solubility of potassium nitrate at this lower temperature.

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Using Table 4.1, calculate the entropy change, ΔS^θ, for the combustion of ethanol at 298 K. Show your working.

Table 4.1

ΔS^θ = ........................... (J K^-1 mol^-1)

Using the value of ΔH^θ given in part (a) and your answer to part (a), calculate the standard Gibbs free energy change, ΔG^θ, for the combustion of ethanol at 298 K. Show your working.

Explain whether changing the temperature for the combustion of ethanol will affect the feasibility of the reaction.

Carbon dioxide and hydrogen can react to form methanol.

CO₂ (g) + 3H₂ (g) ⇌ CH₃OH (g) + H₂O (g)

Data for standard enthalpy of formation and standard entropy is shown in Table 1.1.

Table 1.1

i) Calculate the standard enthalpy change, ΔH^θ, for this reaction. Show your working.

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ii) Calculate the standard entropy change, ΔS^θ, for this reaction. Show your working.

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iii) Hence, calculate the standard Gibbs free energy change, ΔG^θ, for this reaction at 790 K. Show your working.

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Use your answer to part (a)(iii) to state whether this reaction is feasible at 790 K.

Using your answers to parts (a)(i) and (a)(ii), calculate the temperature below which this reaction is feasible. Show your working.

Explain why the reaction in part (a) has a negative entropy change.

This question is about the feasibility of a reaction.

Calcium carbonate can be thermally decomposed to make calcium oxide.

CaCO₃ (s) → CaO (s) + CO₂ (g) ΔH^θ = +178 kJ mol^-1

Table 2.1 shows the standard entropies of each substance.

Table 2.1

i) Calculate the standard entropy change, ΔS^θ, for the decomposition of calcium carbonate. Show your working.

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ii) Use the data given and your answer to (a)(i) to show that the decomposition of calcium carbonate is not feasible at 298 K. Show your working.

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iii) Calculate the temperature above which the decomposition of calcium carbonate is feasible. Show your working.

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Explain how the conditions can be changed for the reaction in (a) to make the reaction feasible.

The evaporation of water is an endothermic reaction but also a feasible reaction.

Explain why.

Refer to the entropy change that occurs and the change in the arrangement of water molecules.

The equation for the reaction between hydrogen and oxygen to form gaseous water is:

H₂ (g) + ½O₂ (g) → H₂O (g)

Use the information in Table 2.2 to answer the following.

Table 2.2

i) Calculate the standard entropy change, ΔS^θ, for this reaction. Show your working.

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ii) Calculate the temperature above which this reaction is not feasible. Show your working.

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Explain what would happen to the sample of gaseous water if it was heated to a higher temperature than that calculated in part (d)(ii).

Free energy changes can be used to predict the feasibility of chemical reactions and processes.

Table 3.1 shows the equations of five different chemical reactions or processes.

Complete the table to show the sign of ΔS^θ for each chemical reaction and process.

Table 3.1

The enthalpy and entropy changes of a reaction are both negative.

Explain how the feasibility of this reaction will change as the temperature decreases.

The equation for this reversible reaction is:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

Use the information in Table 3.2 to answer the following.

Table 3.2

i) Calculate the standard enthalpy change, ΔH^θ, for the reaction. Show your working.

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ii) Calculate the standard entropy change, ΔS^θ, for the reaction. Show your working.

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iii) Calculate the temperature below which this reaction is feasible. Show your working.

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