Edexcel IAL Chemistry 16.14 disproportionation and electrode potentials

Choose the two relevant half-equations, combine them into a balanced disproportionation equation and calculate whether the resulting E°cell is positive.

Exam points

Write balanced disproportionation equations from Latimer diagrams or half-equations.
Calculate E°cell for disproportionation and state whether it is feasible.
Identify which species disproportionates from a table of standard electrode potentials.

[Maximum number: 2]

This question is about manganese compounds. Some data are given below.

Table for Question 16.14—How standard electrode potentials can be used to predict the thermodynamic feasibility of disproportionation reactions question 1 — Edexcel A-Level Chemistry A2

Calculate $E_{\text {cell }}^{\ominus}$ for the disproportionation of manganate(VI) ions in acidic conditions, stating whether or not the reaction is thermodynamically feasible.

selection of appropriate values and insertion in the correct equation (1)

- completion of calculation

and

evaluation of the feasibility of the reaction (1)

$E^{\text {o }}{ }_{\text {cell }}=2.26-0.56$ $=(+) 1.70(\mathrm{~V})$ and (positive so
reaction is) feasible
Accept (if second equation given)
$E^{\mathrm{o}}{ }_{\text {cell }}=1.51-0.56$ $=(+) 0.95(\mathrm{~V})$ and (positive so
TE on alkaline disproportionation
$E_{\text {cell }}^{\text {o }}=0.59-0.56=(+) 0.03(\mathrm{~V})$ and
(positive so reaction is) feasible scores (2)
TE on equations used the wrong way
round $-1.70 /-0.95(\mathrm{~V})$ and not feasible
scores (1)
Just (+)1.70 (V) / (+)0.95 (V) scores (1)
Do not award a non-disproportionation
reaction

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16.14—How standard electrode potentials can be used to predict the thermodynamic feasibility of disproportionation reactions question 1