Chemistry 9701 · AS & A Level · Chemical energetics

Chemical energetics — practice question

Hydrogen peroxide is a liquid at $298\,\text{K}$. It is only moderately stable in room conditions, but it decomposes rapidly when a catalyst is introduced. Reaction 1: $\text{2H}_2\text{O}_2(l) \rightarrow \text{2H}_2\text{O}(l) + \text{O}_2(g)$
(a(i))[1]

Define entropy in terms of disorder.

(a(ii))[1]

Predict the sign of the standard entropy change for reaction 1. Explain your reasoning.

(b)[1]

Use the data in Table 3.1 to demonstrate that the enthalpy change for the reaction below is $-196\,\text{kJ mol}^{-1}$. $\text{2H}_2\text{O}_2(g) \rightarrow \text{2H}_2\text{O}(g) + \text{O}_2(g)$

(c)[3]

The enthalpy change and Gibbs free energy change for the reaction below are given. $\text{2H}_2\text{O}_2(l) \rightarrow \text{2H}_2\text{O}(l) + \text{O}_2(g)$ $\Delta H^\circ = -196\,\text{kJ mol}^{-1}$ $\Delta G^\circ = -238\,\text{kJ mol}^{-1}$ Several standard entropies, $S^\circ$, are listed. Use the information provided to calculate the standard entropy of oxygen, $S^\circ$, $\text{O}_2(g)$.

(d)[1]

The decomposition of $\text{H}_2\text{O}_2(aq)$ is catalysed by aqueous iron(III) chloride and by silver metal. Identify which catalyst is homogeneous. Explain your answer.

(e(i))[1]

An electrochemical cell is set up with these half-cells: an acidified $\text{H}_2\text{O}_2$ solution with a platinum wire, and $\text{Cr}^{2+}$ mixed with $\text{Cr}^{3+}$ with a platinum wire. Identify the positive half-cell and work out the standard cell potential, $E^{\circ}_{\text{cell}}$.

(e(ii))[2]

Calculate $\Delta G^{\circ}$ for the cell reaction that takes place, per mole of $\text{H}_2\text{O}_2$.

(f(i))[2]

Use the Nernst equation to determine $E$, the electrode potential of half-cell 2 under these conditions.

(f(ii))[1]

Write an equation for the cell reaction occurring in this cell under these conditions.

(g(i))[1]

Define the enthalpy change of hydration, $\Delta H_{\text{hyd}}$.

(g(ii))[2]

Aluminium fluoride, $\text{AlF}_3$, is an ionic solid. Complete and annotate the energy cycle to show how the enthalpy change of solution of $\text{AlF}_3$, $\Delta H^{\circ}_{\text{sol}}$, is related to the lattice energy of $\text{AlF}_3$, $\Delta H^{\circ}_{\text{latt}}$, and the enthalpy changes of hydration of $\text{Al}^{3+}$ and $\text{F}^-$, $\Delta H^{\circ}_{\text{hyd}}$. Include state symbols for all ions and substances.

(g(iii))[1]

Use the given data and the energy cycle in (g)(ii) to calculate the lattice energy, $\Delta H^{\circ}_{\text{latt}}$, of $\text{AlF}_3$.

Worked solution & mark scheme

This 17-mark question has a full step-by-step worked solution and mark scheme. One marking point: Entropy is defined as the number of possible arrangements or the degree of disorder.

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