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 solution contains the copper complex $[\text{Cu}(\text{NH}_3)_4]^{2+}$. The $[\text{Cu}(\text{NH}_3)_4]^{2+}$ solution is then warmed gently in a fume cupboard so that $\text{NH}_3$ is given off. Some $\text{NH}_3$ remains dissolved and some is present as $\text{NH}_3(g)$. The solution changes colour; a precipitate of $\text{Cu(OH)}_2$ forms and is collected.
A sample of $\text{Cu(OH)}_2$ is added to concentrated hydrochloric acid. A reaction occurs and a coloured copper complex, Y, is produced.
A sample of $\text{Cu(OH)}_2$ is added to dilute sulfuric acid. A reaction occurs and a coloured copper complex, Z, is produced.
$[\text{Cu}(\text{NH}_3)_4]^{2+}$, Y and Z are not the same colour.
(a(i))[1]
Write an expression for the $K_{stab}$ of $[\text{Cu}(\text{NH}_3)_4]^{2+}$.
(a(ii))[1]
State the colour of the solution of $[\text{Cu}(\text{NH}_3)_4]^{2+}$.
(b)[1]
Suggest an equation for the reaction of $[\text{Cu}(\text{NH}_3)_4]^{2+}$ to form $\text{Cu(OH)}_2$ as the aqueous solution of $[\text{Cu}(\text{NH}_3)_4]^{2+}$ is heated.
(c)[2]
Suggest an equation for the reaction of $\text{Cu(OH)}_2$ with concentrated hydrochloric acid, forming Y.
(d)[2]
Fill in the table with the colour and geometry of complex Y, and the colour, geometry and formula of complex Z.
(e)[5]
Explain why complexes Y and Z are coloured and why their colours are different.
Worked solution & mark scheme
This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$K_{stab} = [Cu(NH_3)_4^{2+}] / ([Cu(H_2O)_6^{2+}][NH_3]^4)$” …