Chemistry 9701 · AS & A Level · Chemistry of transition elements

Chemistry of transition elements — practice question

Passing air through separate aqueous mixtures of CoCl$_2$, NH$_4$Cl and NH$_3$ gives a range of complex ions with the general formula $[\text{Co(NH}_3)_6$-$_n\text{Cl}_n]^{3-n}$.
(a(i))[1]

Determine the oxidation state of cobalt in these complex ions.

(a(ii))[2]

Name the two reaction types that the cobalt ions undergo when these complex ions are formed.

(a(iii))[3]

The complex $[\text{Co(NH}_3)_4\text{Cl}_2]$ exhibits isomerism. Draw three-dimensional structures for the two isomers, and suggest the type of isomerism that is shown here.

(b(i))[1]

What is meant by the term co-ordination number?

(b(ii))[6]

Complete the table by predicting suitable co-ordination numbers, formulae and charges for complexes C, D, E and F.

(c(i))[3]

Iron(III) forms complexes in separate reactions with both SCN$^-$ ions and Cl$^-$ ions.\n\n$\text{Fe}^{3+}(aq) + \text{SCN}^-(aq) \rightleftharpoons [\text{FeSCN}]^{2+}(aq)$\n\n$\text{Fe}^{3+}(aq) + 4\text{Cl}^-(aq) \rightleftharpoons [\text{FeCl}_4]^-(aq)$\n\nWrite the expressions for the stability constants, $K_{stab1}$ and $K_{stab2}$, for these two equilibria. Include units in your answers.

(c(ii))[1]

A further equilibrium may be established between these two complexes, as shown.\n\n$[\text{FeCl}_4]^-(aq) + \text{SCN}^-(aq) \rightleftharpoons [\text{FeSCN}]^{2+}(aq) + 4\text{Cl}^-(aq)$\n\nWrite an expression for $K_{eq3}$ in terms of $K_{stab1}$ and $K_{stab2}$.

(c(iii))[2]

The values of these stability constants are given below.\n\n$K_{stab1} = 1.4 \times 10^2$\n\n$K_{stab2} = 8.0 \times 10^{-2}$\n\nCalculate the value of $K_{eq3}$ and state its units.

Worked solution & mark scheme

This 19-mark question has a full step-by-step worked solution and mark scheme. One marking point: Answer: $+3$ or $\text{Co}^{3+}$

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