Chemistry 9701 · AS & A Level · Electrochemistry

Electrochemistry — practice question

When dilute sulfuric acid undergoes electrolysis, water is decomposed into hydrogen and oxygen. The equation is $2\text{H}_2\text{O}(l) \rightarrow 2\text{H}_2(g) + \text{O}_2(g)$. A current of $x\,\text{A}$ is passed through the solution for $14.0$ minutes. $462\,\text{cm}^3$ of hydrogen is collected at the cathode, measured under room conditions.
(a)[2]

Calculate how many hydrogen molecules are formed during the electrolysis.

(b)[1]

Calculate the total number of electrons needed to produce this number of hydrogen molecules.

(c)[1]

Calculate the charge, in coulombs, carried by the total number of electrons found in (b).

(d)[1]

Calculate the current, $x$, flowing during this experiment.

(e(i))[1]

Calculate $\Delta S^\circ$ for the reaction $2\text{H}_2\text{O}(l) \rightarrow 2\text{H}_2(g) + \text{O}_2(g)$ by using the standard entropies provided.

(e(ii))[2]

For the reaction $2\text{H}_2\text{O}(l) \rightarrow 2\text{H}_2(g) + \text{O}_2(g)$, $\Delta H^\circ$ is $+572\,\text{kJ mol}^{-1}$. Calculate $\Delta G^\circ$ for this reaction at $298\,\text{K}$.

(e(iii))[1]

Predict the effect of raising the temperature on how spontaneous this reaction is. Explain your answer.

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: moles of $\mathrm{H_2}$ = $\frac{462}{24000} = 0.01925$

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