3.1.8.2 Free energy change + entropy change (A-level only)

A reaction is exothermic and has a negative entropy change.

Which statement is correct?

• The reaction is always feasible

• The reaction is feasible above a certain temperature

• The reaction is feasible below a certain temperature

• The reaction is never feasible

Which one of the equations below represents a reaction that is feasible at all temperatures?

• P(s) → Q(s) + R(g) endothermic

• 2L(g) + M(g) → 2N(g) exothermic

• S(g) → 2T(g) exothermic

• A(g) + B(g) → C(g) endothermic

Which one of the following reactions in aqueous solution has the most positive change in entropy?

• [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

• [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O

• [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O

• [Cu(H₂O)₆]²⁺ + 2H₂NCH₂CH₂NH₂ → [Cu(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺ + 4H₂O

Refer to the reaction in the image

Which one of the following statements is correct?

• This is a redox reaction.

• The reaction is not feasible below 298 K

• At equilibrium, the yield of hydrogen iodide is changed by increasing the pressure.

• At equilibrium, the yield of hydrogen iodide increases as the temperature is increased.

This question is about the reaction given below.

CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g)

Enthalpy data for the reacting species are given in the table

Which one of the following statements is not correct?

• The value of K_p changes when the temperature changes.

• The activation energy decreases when the temperature is increased.

• The entropy change is more positive when the water is liquid rather than gaseous.

• The enthalpy change is more positive when the water is liquid rather than gaseous.

Using the information below, answer this question.

Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g)

ΔH= +96 kJ mol⁻¹, ΔS = +138 J K⁻¹ mol⁻¹

The standard entropy value for steam is

• +332 J K⁻¹ mol⁻¹

• +189 J K⁻¹ mol⁻¹

• +145 J K⁻¹ mol⁻¹

• +85 J K⁻¹ mol⁻¹

In which one of the following reactions is there a decrease in entropy?

• [Fe(H₂O)₆]³⁺(aq) + 3C₂O₄^2- (aq) → [Fe(C₂O₄)₃]³⁻(aq) + 6H₂O(l)

• [Cu(H₂O)₆]²⁺(aq) + EDTA⁴⁻(aq) → [Cu(EDTA)]²⁻(aq) + 6H₂O(l)

• [CoCl₄]²⁻(aq) + 6H₂O(l) → [Co(H₂O)₆]²⁺(aq) + 4Cl⁻ (aq)

• Na₂CO₃(s) + 2H⁺(aq) → 2Na⁺(aq) + CO₂(g) + H₂O(l)

This question relates to the equilibrium gas-phase synthesis of sulphur trioxide:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Thermodynamic data for the components of this equilibrium are given in the table.

This equilibrium, at a temperature of 585 K and a total pressure of 540 kPa, occurs in a vessel of volume 1.80 dm³. At equilibrium, the vessel contains 0.0500 mol of SO₂(g), 0.0800 mol of O₂(g) and 0.0700 mol of SO₃(g).

The standard entropy change for this reaction is

• −222 J K⁻¹ mol⁻¹

• −195 J K⁻¹ mol⁻¹

• −186 J K⁻¹ mol⁻¹

• +198 J K⁻¹ mol⁻¹

The following information concerns the equilibrium gas-phase synthesis of methanol.

CO(g) + 2H₂(g) ⇌ CH₃OH(g)

At equilibrium, when the temperature is 68 °C, the total pressure is 1.70 MPa.

The number of moles of CO, H₂ and CH₃OH present are 0.160, 0.320 and 0.180, respectively.

Thermodynamic data are given.

The standard entropy change for this reaction is

• −220 J K⁻¹ mol⁻¹

• +220 J K⁻¹ mol⁻¹

• −89 J K⁻¹ mol⁻¹

• +89 J K⁻¹ mol⁻¹

The compound lithium tetrahydridoaluminate(III), LiAlH₄, is a useful reducing agent. It behaves in a similar fashion to NaBH₄. Carbonyl compounds and carboxylic acids are reduced to alcohols. However, LiAlH₄ also reduces water in a violent reaction so that it must be used in an organic solvent.

Which one of the following concerning the violent reaction between LiAlH₄ and water is false?

• A gas is produced.

• The activation energy for the reaction is relatively high.

• The reaction has a negative free-energy change.

• Aqueous lithium ions are formed.