6.3 Reactions of Transition Metal Elements

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1a4 marks

This question is about transition metals and transition metal complexes.

Describe the bonding in the element chromium and use your answer to justify why it has such a high melting temperature.

You may find it helpful to draw a labelled diagram.

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1b4 marks

When chromium(III) sulfate dissolves in water, a green solution containing the [Cr(H₂O)₆]³⁺ ion forms.

Give the shape of this complex ion.

(1)

ii)

Explain why the chromium complex ion is coloured.

(3)

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1c3 marks

The ligand ethylenediaminetetraacetate, EDTA⁴⁻, has the structure shown.

q3c-9cho-al-1-nov-2020-qp-edexcel-a-level-chem

When a solution of EDTA⁴⁻ is added to a solution of [Cr(H₂O)₆]³⁺ ions, a new complex ion is formed.

[Cr(H₂O)₆]³⁺ + EDTA⁴⁻ ⇌ [Cr(EDTA)]⁻+ 6H₂O

The equilibrium constant for this equilibrium is 2.51 × 10²³ dm³mol⁻¹.

By considering the equilibrium for this reaction and changes in entropy, comment on the value of the equilibrium constant. No calculations are required.

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5 marks

Aqueous vanadium(II) chloride, VCl₂(aq), can be oxidised by bubbling gaseous chlorine, Cl₂(g), through the solution in the absence of air.

40.0 cm³ of 0.100 mol dm⁻³ VCl₂ solution was oxidised by 144 cm³ of chlorine gas, at room temperature and pressure (r.t.p.).
The chlorine was reduced to chloride ions, according to the half-equation

Cl₂(g) + 2e⁻→ 2Cl⁻(aq)

[Molar volume of a gas at r.t.p. = 24.0 dm³ mol⁻¹]

Use these data to calculate the final oxidation state of vanadium.
You must show your working.

(5)

ii)

State the initial and final colours you would see as the chlorine bubbles through the aqueous vanadium(II) chloride, VCl₂(aq).

(2)

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1a1 mark

This question is about the properties of transition elements, their ions and their complexes.

Give the oxidation state of vanadium in the compound NH₄VO₃.

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1b7 marks

Excess zinc powder is added to an acidified solution of the compound NH₄VO₃.
Using the data in the table, explain the sequence of reactions that takes place.

In your answer, include a description of what you would see, and the relevant ionic equations with their calculated $E_{cell}^{⦵}$ values. State symbols are not required.

Electrode system	$E^{⦵} / V$
$V^{2 +} (aq) + 2 e^{-} ⇌ V (s)$	−1.18
$V^{3 +} (aq) + e^{-} ⇌ V^{2 +} (aq)$	−0.26
${VO}^{2 +} (aq) + 2 H^{+} (aq) + e^{-} ⇌ V^{3 +} (aq) + H_{2} O (l)$	+0.34
${VO}_{3}^{-} (aq) + 4 H^{+} (aq) + e^{-} ⇌ {VO}^{2 +} (aq) + 2 H_{2} O (l)$	+1.00
${Zn}^{2 +} (aq) + 2 e^{-} ⇌ Zn (s)$	−0.76

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1c2 marks

Explain how vanadium(V) oxide acts as a catalyst in one step of the contact process. The equation for this step is

2SO₂ (g) + O₂ (g)

⇌

2SO₃ (g)

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1d6 marks

Describe the reactions of separate portions of aqueous copper(II) ions with aqueous sodium hydroxide solution, with excess aqueous ammonia solution and with concentrated hydrochloric acid.

In your answer you should link observations with equations which include the formulae of any copper-containing complex ions. Include state symbols.

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Edexcel A Level Chemistry

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6.3 Reactions of Transition Metal Elements

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