Transition Metal Reactions (Edexcel International A Level (IAL) Chemistry): Exam Questions

The diagram summarises some reactions of copper compounds.

Identify, by name (including the oxidation state) or formula, the species in the sequence that contain copper.

P ......................................................
Q ......................................................
R ......................................................
S ......................................................
T ......................................................
V ......................................................
W ......................................................

T and V are the same type of chemical species.

i) Name this type of chemical species.

(1)

ii) Explain why T is coloured while V is colourless.

A detailed explanation of the fact that T is coloured is not required.

(3)

iii) Suggest an explanation for the change of V into T on shaking.

(2)

The reaction between copper(I) iodide and sulfuric acid is a disproportionation.

i) Write the ionic equation for this disproportionation reaction.

State symbols are not required.

(1)

ii) Show that the reaction in (c)(i) is thermodynamically feasible.

Use the standard electrode potentials of the relevant half-cells from the Data Booklet.

(2)

The rare mineral mitscherlichite has the chemical formula K₂CuCl₄⦁nH₂O.

4.26 g of mitscherlichite was dissolved in distilled water and the solution made up to 250.0 cm³. Excess potassium iodide solution was added to a 25.0 cm³ portion of this solution and the iodine formed was titrated against a solution of sodium thiosulfate with a concentration of 0.0500 mol dm⁻³.

This procedure was repeated until concordant results were obtained. The mean accurate titre was 26.65 cm³. The equations for the reactions are

   $$\begin{gathered} 2{\mathrm{Cu}}^{2+}+4{\mathrm{I}}^{-}\to 2\mathrm{CuI}+{\mathrm{I}}_2 \\ 2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{I}}_2\to {\mathrm{S}}_4{\mathrm{O}}_6^{2-}+2{\mathrm{I}}^{-} \end{gathered}$$

Calculate the value of n, the number of moles of water of crystallisation per mole of mitscherlichite.

Iron(II) ions catalyse the reaction between iodide ions and peroxodisulfate ions:

S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

Without a catalyst, this reaction is extremely slow. The reaction profile for this reaction is shown.

Explain why the uncatalysed reaction between I⁻ and S₂O₈²⁻ ions is very slow.

Fe²⁺ ions catalyse this reaction in two steps. Write ionic equations for each step of the catalytic cycle.

Explain why Fe²⁺ ions are a particularly effective catalyst for this reaction.

The reaction between manganate(VII) ions and ethanedioate ions is also slow when the reactants are first mixed, but the rate increases significantly as the reaction proceeds:

2MnO₄^- (aq) + 5C₂O₄^2- (aq) + 16H⁺ (aq) → 2Mn²⁺ (aq) + 10CO₂ (g) + 8H₂O (l)

i) Name the type of catalysis occurring in this reaction.

[1]

ii) Identify the catalyst in this reaction.

[1]

iii) Sketch a graph of rate against time for the MnO₄^-/C₂O₄^2- reaction on the axes, starting from the moment the reactants are first mixed. Label the axes.

[2]

(*) Compare and contrast the homogeneous catalysis of the reaction between I^- and S₂O₈^2- by Fe²⁺ ions with the autocatalysis of the reaction between MnO₄^- and C₂O₄^2- by Mn²⁺ ions.

In your answer, you should include:

• similarities between the two catalytic systems

• differences, including how the rate changes over time in each case

A student investigates the rate of the Fe²⁺-catalysed reaction using the iodine clock method. A small, fixed amount of sodium thiosulfate solution and starch indicator are added to the reaction mixture at the start.

Describe what the student observes and explain what this shows.

The same experiment is repeated with the concentration of I⁻ doubled but all other concentrations kept constant.

Predict the effect on the time taken for the blue-black colour to appear.