Very pure water is an extremely poor conductor of electricity. However, once hydrogen chloride gas dissolves in water, ions are produced and electrolysis allows a current to pass.
$\text{HCl(g)} + \text{aq} \rightarrow \text{H}^+(\text{aq}) + \text{Cl}^-(\text{aq})$
The net effect of the electrolysis is that hydrogen chloride gas is changed into hydrogen and chlorine.
$2\text{HCl(g)} \rightarrow \text{H}_2(\text{g}) + \text{Cl}_2(\text{g})$
If a current of $3.10\ \text{A}$ is passed through the solution for $Y$ minutes, $351\ \text{cm}^3$ of chlorine are obtained at the anode, measured under room conditions.
(a)[2]
Calculate the number of chlorine molecules formed during the electrolysis.
(b)[1]
Calculate the total number of electrons needed to produce this number of chlorine molecules.
(c)[1]
Calculate the charge, in coulombs, of the total number of electrons calculated in (b).
(d)[1]
Calculate the time, $Y$, in minutes, for which the current is passed.
(e(i))[1]
The standard entropies, $S^\circ$, of three species are supplied. Calculate $\Delta S^\circ$ for the reaction $2\text{HCl(g)} \rightarrow \text{H}_2(\text{g}) + \text{Cl}_2(\text{g})$.
(e(ii))[2]
Given that $\Delta H^\circ$ for $2\text{HCl(g)} \rightarrow \text{H}_2(\text{g}) + \text{Cl}_2(\text{g})$ is $+185\ \text{kJ mol}^{-1}$, calculate $\Delta G^\circ$ for this reaction at $298\ \text{K}$.
(e(iii))[1]
Predict the effect of increasing temperature on the spontaneity of this reaction. 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: “Using the molar gas volume, $\frac{0.351}{24} = 0.015\,\mathrm{mol}$” …