Chemistry 9701 · AS & A Level · Group 2

Group 2 — practice question

(a)[3]

The solubility of the Group 2 hydroxides rises as you go down the group. Explain this pattern.

(b(i))[2]

At $298\,\text{K}$, the solubility of $\text{Be(OH)}_2$ in water is $2.40 \times 10^{-6}\,\text{g dm}^{-3}$. Write the solubility product expression, $K_{sp}$, for $\text{Be(OH)}_2$ and give its units.

(b(ii))[2]

Calculate the numerical value of $K_{sp}$ for $\text{Be(OH)}_2$ at $298\,\text{K}$.

(c(i))[1]

$\text{Be(OH)}_2$ dissolves in alkaline solutions that contain excess hydroxide ions and produces the complex ion $[\text{Be(OH)}_4]^{2-}$. Define the term complex ion.

(c(ii))[1]

Draw a three-dimensional diagram to show the structure of the complex ion $[\text{Be(OH)}_4]^{2-}$. Name the shape of the $[\text{Be(OH)}_4]^{2-}$ complex ion.

(d(i))[1]

Explain why transition elements can form complex ions.

(d(ii))[2]

Complete Table 1.1 to show the coordination number of each metal ion, and the shapes and overall polarities of the complex ions listed: cis-$[\text{Pt(H}_2\text{NCH}_2\text{CH}_2\text{NH}_2)\text{Cl}_2]$, $[\text{Ag(NH}_3)_2]^+$, and $[\text{Fe(C}_2\text{O}_4)_3]^{3-}$.

(e(i))[1]

Define stability constant, $K_{stab}$.

(e(ii))[2]

Nickel can form complexes with the ligands $en$, $\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2$, and $tn$, $\text{H}_2\text{NCH}_2\text{CH}_2\text{CH}_2\text{NH}_2$, as shown. Equilibrium 1: $[\text{Ni(H}_2\text{O})_6]^{2+} + 3en \rightleftharpoons [\text{Ni(en)}_3]^{2+} + 6\text{H}_2\text{O}$, $K_{stab} = 6.76 \times 10^{17}$. Equilibrium 2: $[\text{Ni(H}_2\text{O})_6]^{2+} + 3tn \rightleftharpoons [\text{Ni(tn)}_3]^{2+} + 6\text{H}_2\text{O}$, $K_{stab} = 1.86 \times 10^{12}$. Construct an expression for the stability constant, $K_{stab}$, for equilibrium 1 and state its units.

(e(iii))[1]

Describe what the $K_{stab}$ values show about the equilibrium position for equilibrium 1 and 2. Use the $K_{stab}$ values to decide which complex, $[\text{Ni(en)}_3]^{2+}$ or $[\text{Ni(tn)}_3]^{2+}$, is more stable.

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