Page 111 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Electrochemistry II: Electrolytic Cells 95
(b) If the species is below zinc, the positive MI+ cation will be
reduced to the metal:

ex. &+(a,) + e + Ago(,,
(c) If the electrolysis involves an acid (i.e. a proton donor), write
down the appropriate ionisation reaction for the acid with
water. This.wil1 then generate H30+ ions at the cathode:

e.g. H20 + H2SO4 + H30+ + HS04-
H20 + HS04- -+ H30+ + SO,'-
11. Anode reaction (oxidation):
Examine the position of the anion in the following series:

\
Ease of Oxidation
Anions on the left-hand side of the above series such as iodide
will be easily oxidised, whereas those on the right-hand side, such
as the sulfate oxyanion, SO:-, will not be easily oxidised. In
sulfate, S is in a (VI) oxidation state, having the stable pe]sopo
inert gas core configuration (cf. S(0): Pe]3s23p4). For this
reason, the oxidation of water, H20, will occur instead, with
oxygen gas evolved at the anode:

2~20(aq) -+ 02(g) + 4e + 4~+(aq)
I -11 0 I
u

-2 + 0 (increase in oxidation number + oxidation)

If the anion is chloride, this 'sits on the fence' within the series,
and which oxidation occurs is concentration dependent, i.e. if the
solution is concentrated, chloride will be oxidised to chlorine gas,
according to the reaction: 2C1-(aq1 + C12(,) + 2e. If, however,
the solution is dilute, water will be more easily oxidised, and the
half-reaction above will take place. All this relates to the E"
values. In the case of chloride, E" = - 1.36 V, and in the case of
water the corresponding value is - 1.23 V. As stated previously,
it is not necessary to memorise these values. All that is required is