Chemistry 9701 · AS & A Level · Chemistry of transition elements
Chemistry of transition elements — practice question
Cadmium ions can make complexes with primary amines as well as with 1,2-diaminoethane.
$
\text{Cd}^{2+}(aq) + 4\text{CH}_3\text{NH}_2(aq) \rightleftharpoons [\text{Cd}(\text{CH}_3\text{NH}_2)_4]^{2+}(aq) \qquad K_{\text{stab}} = 3.6 \times 10^6 \; \text{(equilibrium I)}
$
$
\text{Cd}^{2+}(aq) + 2\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2(aq) \rightleftharpoons [\text{Cd}(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2)_2]^{2+}(aq) \qquad K_{\text{stab}} = 4.2 \times 10^{10} \; \text{(equilibrium II)}
$
(a(i))[2]
Write the stability-constant expression, $K_{\text{stab}}$, for equilibrium I, and state the units.
(a(ii))[2]
In a groundwater sample, the concentration of $\text{Cd}^{2+}(aq)$ is $1.00 \times 10^{-4} \text{ mol dm}^{-3}$. Calculate the concentration of $\text{CH}_3\text{NH}_2(aq)$ needed to lower the concentration of $\text{Cd}^{2+}(aq)$ in this dilute solution by a factor of one thousand.
(b(i))[1]
Suggest a reason why the $\Delta H^{\circ}$ values for the two equilibria are so similar.
(b(ii))[1]
Calculate the value of $\Delta S^{\circ}$ for equilibrium II.
(b(iii))[1]
Suggest a reason for the difference between the $\Delta S^{\circ}$ you calculated for equilibrium II and the value for equilibrium I shown in the table.
(b(iv))[1]
Which of the two complexes is more stable? Give your reason.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$K_{\text{stab}}=\frac{[\mathrm{Cd(CH_3NH_2)_4^{2+}}]}{[\mathrm{Cd^{2+}}][\mathrm{CH_3NH_2}]^4}$; the units are $\mathrm{mol^{-4}\,dm^{12}}$” …