Energetics II: Entropy (Edexcel A Level Chemistry): Exam Questions

Prop-2-en-1-ol is an unsaturated alcohol with the structure shown.

A student planned to use bond enthalpy data to calculate a value for the enthalpy change of combustion of prop-2-en-1-ol.

i) When researching the bond enthalpy data, the student claimed that it was not necessary to find the value for the C=C bond as they could use the value for a C–C bond and multiply it by two. Explain why the student is incorrect.

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ii) Calculate a value for the enthalpy of combustion of prop-2-en-1-ol using the data shown.

C₃H₆O (g) + 4O₂ (g) → 3CO₂ (g) + 3H₂O (g)

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iii) Explain, in terms of entropy, why the combustion of prop-2-en-1-ol is always feasible in the gaseous state.

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Chemists are researching a process to make ethanol and ethene directly from carbon dioxide and water.

4CO₂ (g) + 5H₂O (l) → CH₃CH₂OH (l) + C₂H₄(g) + 6O₂ (g)       ∆H ^⦵ = +2778 kJ mol⁻¹

Calculate ∆S^⦵ _total for the reaction and hence determine whether the reaction is feasible under standard conditions.

Sodium hydride, NaH, can be used to generate hydrogen for fuel cells.

In order to calculate the first electron affinity of hydrogen, a student was asked to draw a Born-Haber cycle for sodium hydride.

The cycle had two errors but the numerical data were correct.

i) Identify and correct the two errors in this Born-Haber cycle.

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ii) Calculate the first electron affinity, in kJ mol^–1, of hydrogen, using the values given in the cycle.

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The equation for the formation of sodium hydride is

Na (s) + ½H₂ (g) → NaH (s) 

The standard entropy change of the system, , for this reaction is  –76.5 J K^–1 mol^–1

i) Deduce the feasibility of this reaction at 298 K by calculating the free energy change, .

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ii) Calculate the temperature at which   = 0.

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The sodium hydride is crushed in the presence of water to release the hydrogen gas for a fuel cell.

The overall equation for the reaction occurring in the fuel cell is

H₂ (g) + ½O₂ (g) → H₂O (l)

In an alkaline fuel cell the oxidation half-equation is

H₂ (g) + 2OH^– (aq) → 2H₂O (l) + 2e^– 

Deduce the reduction half-equation for the alkaline fuel cell.

State symbols are not required.

Lattice energies provide an indication of ionic bond strength.

Which are the lattice energies of the hydrides NaH, KH and MgH₂?

This question is about entropy and free energy.

Complete the table by giving the sign of the entropy change of the system, ∆S_system , for each reaction.

Calculate the total entropy change, ∆S_total, for the thermal decomposition of calcium carbonate at 298K.

CaCO₃ (s) → CaO (s) + CO₂ (g)

[Data: ∆_rH = +178 kJ mol⁻¹ ∆S_system = +160J K⁻¹ mol⁻¹]

Sulfur dioxide reacts with oxygen to form sulfur trioxide.

2SO₂(g) + O₂ (g) ⇌ 2SO₃ (g)   Δ_rH = -288.4 kJ mol⁻¹

The standard molar entropy values at 298 K are given in the table.

i) Calculate the entropy change of the system, ΔS_system, for the forward reaction. Include a sign and units in your answer.

2SO₂ (g) + O₂( g) → 2SO₃ (g)

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ii) Calculate the free energy change, ΔG, at 298 K and hence deduce whether the reaction is feasible.

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iii) In industry, the reaction is carried out at about 700 K using a vanadium(V) oxide catalyst.

Calculate the value of the equilibrium constant, K, at 700 K.

ΔG at 700K is -60 kJ mol⁻¹

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iv) The equilibrium constant has a larger value at 298 K than at 700K.

Explain why the reaction is carried out at 700 K and not at 298 K.

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