Core
Reactions: Redox
Slide 33 of 42
Half-reactions
(equations): Another example
For the
disproportionation reaction of iodine which you encountered in one of the
previous slides:
I2 (s) + OH – (aq) ® I
– (aq) + OI – (aq) + H2O (l)
Oxidation half-equation: I2 (s) ® 2IO – (aq) + 2e –
–2 electrons are needed on the right hand side to account
for the change in the oxidation
state of iodine from ‘0’ to ‘1+’.
Reduction half-equation: I2 (s) + 2e – ® 2I – (aq)
–2 electrons are needed on the left hand side to account for
the change in the oxidation
state of iodine from ‘0’ to ‘1–’.
Similarly
for the following reaction:
Cr2O72- (aq) + Cl- (aq) ® Cr3+ (aq) + Cl2 (g)
Oxidation half-equation: 2Cl- ® Cl2 +
2e-
Reduction half-equation: Cr2O72- + 6e- ®
2Cr3+
NB: In the half equations, the change in
the oxidation state of the oxidized and reduced species must be balanced by adding electrons.