Edexcel IAL Chemistry 16.12 limitations of standard electrode predictions

Do not treat a standard E°cell value as the whole answer: explain how concentrated acid, ion removal or activation energy can change the observed reaction.

Exam points

Explain how concentrated HCl can shift dichromate equilibria and alter Ecell.
Use precipitation or ion removal to justify a reaction despite standard potential data.
Separate thermodynamic feasibility from kinetic limits such as activation energy.

[Maximum number: 2]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

In Stage 2, $25.0 \mathrm{~cm}^{3}$ portions of the solution from Stage 1 are transferred into conical flasks.
$10 \mathrm{~cm}^{3}$ of a solution of potassium iodide is added to each flask and the mixtures are swirled. Copper(I) iodide is precipitated.

The redox equation for the reaction between copper(II) ions and iodide ions is shown.

2 \mathrm{Cu}^{2+}(\mathrm{aq})+4 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{CuI}(\mathrm{~s})+\mathrm{I}_{2}(\mathrm{aq})

The standard electrode potentials for the half-equations are
Reduction $\mathrm{Cu}^{2+}+\mathrm{e}^{-} \rightleftharpoons \mathrm{Cu}^{+} \quad E^{\ominus}=+0.15 \mathrm{~V}$
Oxidation $\mathrm{I}_{2}+2 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{I}^{-} \quad E^{\ominus}=+0.54 \mathrm{~V}$

Explain why the redox reaction does, in fact, take place.

the copper (I) iodide/ $\mathrm{Cu}^{+}$ precipitates / is removed from the equilibrium / the concentration of copper(I) in solution is very low

- the equilibrium position moves to the right-hand

(1) side

Accept the actual electrode potential (for $\mathrm{Cu}^{2+} / \mathrm{Cu}^{+}$ ) is higher/more positive than the standard electrode potential

Ignore reference to activation energy / rate of reaction/non-standard temperature/pressure Allow 1 mark for concentrations are non-standard If value in (i) for Ecell is positive then allow 1 mark in (ii) for reference to a spontaneous reaction for a positive Ecell

All question bank results loaded

16.12—The limitations of predictions made using standard electrode potentials, in terms of kinetic stability of systems question 1