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Edexcel IAL Chemistry 16.10 feasibility from electrode potentials

Choose the relevant half-cells from the Data Booklet, calculate the cell potential and link a positive or negative value to feasibility under standard conditions.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Calculate E°cell to decide whether a redox reaction is thermodynamically feasible.
  • Compare reducing-agent strength by ordering ions or metals from electrode potentials.
  • Select a reagent that reduces one oxidation state but not a further half-reaction.

16.10—Standard electrode potentials to predict the thermodynamic feasibility of a reaction question 1

[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 cm325.0 \mathrm{~cm}^{3} portions of the solution from Stage 1 are transferred into conical flasks.
10 cm310 \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.

2Cu2+(aq)+4I−(aq)→2CuI( s)+I2(aq)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 Cu2++e−⇌Cu+E⊖=+0.15 V\mathrm{Cu}^{2+}+\mathrm{e}^{-} \rightleftharpoons \mathrm{Cu}^{+} \quad E^{\ominus}=+0.15 \mathrm{~V}
Oxidation I2+2e−⇌2I−E⊖=+0.54 V\mathrm{I}_{2}+2 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{I}^{-} \quad E^{\ominus}=+0.54 \mathrm{~V}

Use the electrode potential data to explain why the redox reaction might not be expected to be feasible.

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