Chemistry 9701 · AS & A Level · Chemistry of transition elements
Chemistry of transition elements — practice question
The ions $\text{Ni}^{2+}$ can make several distinct complex ions, among them $\left[\text{Ni(H}_2\text{O)}_6\right]^{2+}$, $\left[\text{Ni(NH}_3)_6\right]^{2+}$ and $\left[\text{Ni(en)}_3\right]^{2+}$. The abbreviation $en$ denotes 1,2-diaminoethane. Table 5.1 gives the numerical values of two stability constants, $K_{\text{stab}}$.
(a)[1]
Write the expression for the $K_{\text{stab}}$ of $\left[\text{Ni(en)}_3\right]^{2+}$.
(b(i))[1]
Predict which complex ion, $\left[\text{Ni(NH}_3)_6\right]^{2+}$ or $\left[\text{Ni(en)}_3\right]^{2+}$, is at the higher concentration in the mixture produced. Explain your answer.
(b(ii))[1]
Complete the equation for the ligand exchange reaction in (i).
$\left[\text{Ni(H}_2\text{O)}_6\right]^{2+} + \ldots \rightarrow \ldots + \ldots$
(c)[3]
Complete Fig. 5.1 to show the three-dimensional structures of the two isomers of $\left[\text{Ni(en)}_3\right]^{2+}$. Use N-N to represent the $en$ ligand. Name the type of isomerism shown.
Worked solution & mark scheme
This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: “$K_{stab} = \frac{[[\text{Ni(en)}_3]^{2+}]}{[\text{Ni}^{2+}][\text{en}]^3}$” …