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

Chemistry of transition elements — practice question

Copper is classified as a transition element and has atomic number $29$.
(a)[2]

Fill in the electronic configurations for a Cu atom and a Cu$^{+}$ ion. Each one begins $ ext{1s}^2\text{2s}^2\text{2p}^6\,\ldots$

(b)[2]

Cu$^{+}$ ions make a linear complex with $ ext{Cl}^-$ ions, which act as monodentate ligands. Draw this complex and include its overall charge.

(c)[2]

In aqueous solution, Cu$^{2+}$ ions are present as $[\text{Cu(H}_2\text{O)}_6]^{2+}$ complex ions. Complete a three-dimensional diagram to show the shape of this complex. Name the shape. Label and state the value of one bond angle.

(d)[4]

When $\text{NH}_3(\text{aq})$ is introduced to Cu$^{2+}(\text{aq})$, first dropwise and then in excess, two reactions take place: $[\text{Cu(H}_2\text{O)}_6]^{2+} \xrightarrow{\text{dropwise }\text{NH}_3} A \xrightarrow{\text{excess }\text{NH}_3} B$. For each stage, describe the observation and write an equation.

(e(i))[1]

EDTA$^{4-}$ is a polydentate ligand. When a solution of EDTA$^{4-}$ is added to a solution containing $[\text{Cu(H}_2\text{O)}_6]^{2+}$, a new complex with formula $[\text{CuEDTA}]^{2-}$ is formed. Name the type of reaction taking place here.

(e(ii))[1]

Write an expression for the stability constant, $K_{\text{stab}}$, of $[\text{CuEDTA}]^{2-}$ in this reaction.

(e(iii))[1]

The numerical value of $K_{\text{stab}}$ for $[\text{CuEDTA}]^{2-}$ is $6.3 \times 10^{19}$ at $298\,\text{K}$. State what this indicates about the $[\text{CuEDTA}]^{2-}$ complex ion.

(f(i))[2]

Ethanedioate ions, $\text{C}_2\text{O}_4^{2-}$, can act as a bidentate ligand. Explain what the term bidentate ligand means.

(f(ii))[2]

When ethanedioate ions are added to a solution of zirconium ions, $\text{Zr}^{4+}$, a complex ion containing four $\text{C}_2\text{O}_4^{2-}$ ions and one $\text{Zr}^{4+}$ ion is formed. In this complex, all four ethanedioate ions behave as bidentate ligands. Give the formula of this complex ion and explain why it is not octahedral.

Worked solution & mark scheme

This 17-mark question has a full step-by-step worked solution and mark scheme. One marking point: Electron configuration $3s^23p^63d^{10}4s^1$

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