Chemistry 9701 · AS & A Level · Chemistry of transition elements
Chemistry of transition elements — practice question
A solution is prepared by dissolving $\text{CuSO}_4\cdot5\text{H}_2\text{O}$ in excess aqueous ammonia. The resulting mixture contains the copper complex $[\text{Cu(NH}_3)_4]^{2+}$.
This $[\text{Cu(NH}_3)_4]^{2+}$ solution is then heated gently in a fume cupboard, causing $\text{NH}_3$ to be driven off. Some $\text{NH}_3$ remains dissolved and some becomes $\text{NH}_3$(g). The solution changes colour; a precipitate of $\text{Cu(OH)}_2$ is produced and collected.
A portion of $\text{Cu(OH)}_2$ is added to concentrated hydrochloric acid. A reaction occurs to give a coloured copper complex, $Y$.
A portion of $\text{Cu(OH)}_2$ is added to dilute sulfuric acid. A reaction occurs to give a coloured copper complex, $Z$.
$[\text{Cu(NH}_3)_4]^{2+}$, $Y$ and $Z$ have different colours.
(a(i))[1]
State an expression for the $K_{stab}$ of $[\text{Cu(NH}_3)_4]^{2+}$.
(a(ii))[1]
State the colour of the solution containing $[\text{Cu(NH}_3)_4]^{2+}$.
(b)[1]
Suggest an equation for the reaction of $[\text{Cu(NH}_3)_4]^{2+}$ to form $\text{Cu(OH)}_2$ when the aqueous solution of $[\text{Cu(NH}_3)_4]^{2+}$ is heated.
(c)[2]
Suggest an equation for the reaction of $\text{Cu(OH)}_2$ with concentrated hydrochloric acid, to form $Y$.
(d)[2]
Complete the table by giving the colour and geometry of complex $Y$, together with the colour, geometry and formula of complex $Z$.
(e)[5]
Explain why complexes $Y$ and $Z$ are coloured and why their colours are not the same.
Worked solution & mark scheme
This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: “accurate expression for $K_{stab}$” …