Entropy (A Level Only) (CIE A Level Chemistry)

The spontaneity of a chemical reaction depends on the standard Gibbs free energy change, ΔG^θ. 

State and explain whether the following processes will lead to an increase or decrease in entropy.

Calcium carbonate thermally decomposes to form calcium oxide and carbon dioxide, as shown below:

CaCO₃ (s) → CaO (s) + CO₂ (g)

The enthalpy change of the above reaction is ΔH^θ = +178 kJ mol^-1

Standard entropies are shown in Table 1.1

Table 1.1

Calculate the free energy change, ΔG^θ, at 298 K for this process. Include a relevant sign and give your answer to three significant figures. Show your working. 

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.

Calculate the minimum temperature at which the reaction becomes spontaneous. Show your working.

Ammonia, NH₃, is produced by the Haber process and is an important chemical in the manufacture of fertilisers and clearing products. 

Ammonia gas can react with oxygen to produce nitrogen monoxide and steam, and is the first step in the Ostwald process which produces nitric acid. 

Use the data in Table 2.1 to determine the entropy for change for this reaction at 298 K.

Explain why the standard entropy change for the reaction in part (a) is positive.

The second step in the Ostwald process produces nitrogen dioxide as shown in the equation 

2NO (g) + O₂ (g) → 2NO₂ (g)             ΔH^θ = -112 kJ mol^-1   

The standard entropy for NO₂ (g) is 240.0 J K^-1 mol^-1

Use Table 2.1 to determine the value for the free energy change, ΔG^θ , for this reaction at 298 K. Show your working.

ΔG^θ = ......................... kJ mol^-1

Explain whether changing the temperature for the production of nitrogen dioxide will alter the spontaneity of the reaction.

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

Write 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 minimum temperature at which the reaction will become feasible. Show your working.

Ethanol is used in large quantities in the production of alcoholic beverages and as a fuel.

The combustion of ethanol is represented by the equation         

 
  CH₃CH₂OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (g)    ΔH^θ = -1367 kJ mol^-1

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

Table 4.1

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

Calculate the free energy change, ΔG^θ , for the combustion of ethanol using ΔS^ϴ determined in part (a).

Explain whether changing the temperature for the combustion of ethanol will alter the spontaneity of the reaction. 

Carbon dioxide and hydrogen can react to form methanol. 

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

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

Table 1.1

Calculate the Gibbs free-energy change (ΔG), in kJ mol^–1, for this reaction at 790 K.

Explain whether this reaction is feasible at 790 K.

Use your values of ΔH and ΔS from part (a) to calculate the temperature above which this reaction is feasible.

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. 

Table 2.1 shows the standard entropies of each substance. 

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 spontaneous reaction. 

Explain why.

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

Use the information in Table 2.2 to calculate the temperature above which the reaction between hydrogen and oxygen to form gaseous water is not feasible.

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

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 by predicting the sign of ΔS 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 a reversible reaction is shown below:

 2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

Use the information in Table 3.2 to determine the temperature at which the reaction becomes feasible. Show your working.