Chemistry 9701 · AS & A Level · Electrochemistry

Electrochemistry — practice question

Gold is a metal with very low reactivity, so it is oxidised only in particular conditions. The standard electrode potential, $E^\circ$, for $\text{Au}^{3+}(aq)/\text{Au}(s)$ is $+1.50\,\text{V}$.
(a(i))[2]

Define the meaning of standard electrode potential.

(a(ii))[4]

Draw a fully labelled apparatus diagram suitable for measuring the standard cell potential, $E^\circ_{\text{cell}}$, of $\text{Au}^{3+}(aq)/\text{Au}(s)$ and $\text{HNO}_3(aq)/\text{NO}(g)$. Include every required chemical.

(a(iii))[1]

Write an ionic equation to represent the spontaneous reaction taking place when current is drawn from the cell in (a)(ii).

(a(iv))[1]

Calculate the $E^\circ_{\text{cell}}$ for the reaction in (a)(iii).

(a(v))[3]

Gold may be oxidised by a mixture of concentrated hydrochloric acid and concentrated nitric acid, called aqua regia. Concentrated hydrochloric acid is $12\,\text{mol dm}^{-3}$. Concentrated nitric acid is $16\,\text{mol dm}^{-3}$. Explain why aqua regia can dissolve gold. In your answer, state and explain the effect of using concentrated hydrochloric acid and concentrated nitric acid on the $E$ values of half-equations 2 and 3.

(b(i))[1]

Write an equation for the reaction of $\text{AuCl}_3$ with $\text{H}_2\text{O}_2$.

(b(ii))[3]

Determine the rate equation for the reaction. Support your answer by quoting data from the table.

(b(iii))[2]

Use the results from experiment 2 to calculate the value of the rate constant, $k$, for this reaction. Give the units of $k$.

(c(i))[2]

$\text{AlF}_3$ is an ionic compound. The Born-Haber cycle for the formation of $\text{AlF}_3$ is shown. Name the enthalpy changes labelled $\Delta H_4$ and $\Delta H_6$.

(c(ii))[2]

Use the table data and information from the Data Booklet to calculate the lattice energy of $\text{AlF}_3$.

(c(iii))[2]

Scandium fluoride, $\text{ScF}_3$, is an ionic compound. Use data from the Data Booklet to suggest how the lattice energy of $\text{AlF}_3$ compares with the lattice energy of $\text{ScF}_3$. Explain your answer.

(d(i))[1]

$\text{AlF}_3$ is sparingly soluble in water. The concentration of its saturated solution at $298\,\text{K}$ is $6.5 \times 10^{-2}\,\text{mol dm}^{-3}$. Write an expression for the solubility product, $K_{sp}$, of $\text{AlF}_3$.

(d(ii))[1]

Calculate the numerical value of $K_{sp}$ for $\text{AlF}_3$ at $298\,\text{K}$.

Worked solution & mark scheme

This 25-mark question has a full step-by-step worked solution and mark scheme. One marking point: Potential difference of an electrode/half‑cell measured against the standard hydrogen electrode under standard conditions

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