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

Chemistry of transition elements — practice question

The blue colour of aqueous copper(II) sulfate comes from the presence of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ complex ions.
(a(i))[1]

Write an equation for the reaction of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ ions with $\text{NaOH(aq)}$.

(a(ii))[1]

Write an equation for the reaction of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ ions with excess conc. $\text{HCl}$.

(b(i))[1]

Two stereoisomers can exist for the formula $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$. Fill in the boxes below to show both stereoisomers.

(b(ii))[1]

Use your response in (b)(i) to determine whether each isomer is polar or non-polar. Give the polarity of isomer 1 and the polarity of isomer 2.

(c(i))[1]

Define what is meant by stability constant.

(c(ii))[1]

Compare the stabilities of the $[\text{Cu(H}_2\text{O)}_6]^{2+}$ and $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ions. Explain your response.

(c(iii))[2]

Write an expression for $K_{\text{stab}}$, the stability constant of the $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ion. State the units of the stability constant.

(c(iv))[1]

In a certain solution, the concentration of the $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ion is $0.0074\ \text{mol dm}^{-3}$ and the concentration of $\text{NH}_3$ is $0.57\ \text{mol dm}^{-3}$. Use your expression in (c)(iii) together with the $K_{\text{stab}}$ value of $1.40 \times 10^{13}$ to calculate the concentration of the $[\text{Cu(H}_2\text{O)}_6]^{2+}$ complex ion in this solution.

(d)[2]

Phenanthroline, $\text{C}_{12}\text{H}_8\text{N}_2$, and ethanedioate ions, $\text{C}_2\text{O}_4^{2-}$, are bidentate ligands. Ruthenium(III) ions, $\text{Ru}^{3+}$, form an octahedral complex with phenanthroline and chloride ions. The complex ion contains two phenanthroline molecules. Iron(III) ions, $\text{Fe}^{3+}$, form an octahedral complex with ethanedioate ions only. Deduce the formula and charge of each of these complex ions.

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: The correct equation showing formation of $\mathrm{Cu(OH)_2(H_2O)_4}$

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