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192 6 CHEMICAL EQUILIBRIUM
E6C.2(b) (i) �e required reduction half-reactions are
R: 2H2O(l) + 2e− → H2(g) + 2OH−(aq) E−○(R) = −0.83 V
L: 2Na+(aq) + 2e− → 2Na(s) E−○(L) = −2.71 V
�e cell reaction (R−L) generated from these reduction half-reactions is
2H2O(l)+2Na(s)→ H2(g)+2OH−(aq)+2Na+(aq)which is equivalent
to the required reaction.�e cell required is
Na(s)∣NaOH(aq)∣H2(g)∣Pt(s)
�e platinum electrode is an ‘inert metal’ that acts as a source or sink
of electrons. Note that there is no interface between the half cells because
they have a common electrolyte (NaOH(aq)).�e standard cell potential
is
E−○cell = E−○(R) − E−○(L) = (−0.83 V) − (−2.71 V) = +1.88 V
(ii) �e required reduction half-reactions are
R: I2(g) + 2e− → 2I−(aq) E−○(R) = +0.54 V
L: 2H+(aq) + 2e− → H2(g) E−○(L) = 0 (by de�nition)
�e cell reaction (R−L) generated from these reduction half-reactions is
I2(g) +H2(g)→ 2I−(aq) + 2H+(aq) which is equivalent to the required
reaction.�e cell required is
Pt(s)∣H2(g)∣HI(aq)∣I2(g)∣Pt(s)
and the standard cell potential is
E−○cell = E−○(R) − E−○(L) = +0.54 V
(iii) One possible pair of reduction half-equations is
R: 2H+(aq) + 2e− → H2(g) E−○(R) = 0 (by de�nition)
L: 2H2O(l) + 2e− → H2(g) + 2OH−(aq) E−○(L) = −0.83 V
�e cell reaction (R − L) generated from these reduction half-reactions
is 2H+(aq) + 2OH−(aq) → 2H2O(l) which is equivalent to (twice) the
required reaction.�e required cell is
Pt(s)∣H2(g)∣NaOH(aq)∣∣HCl(aq)∣H2(g)∣Pt(s)
and the standard cell potential is
E−○cell = E−○(R) − E−○(L) = −(−0.83 V) = +0.83 V
An alternative pair of reduction half-equations is
R: O2(g) + 4H+(aq) + 4e− → 2H2O(l) E−○(R) = +1.23 V
L: O2(g) + 2H2O(l) + 4e− → 4OH−(aq) E−○(L) = +0.40 V