Page 117 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Electrochemistry II: Electrolytic Cells 101
is present, sodium is above zinc in the electrochemical series, and
therefore hydrogen gas is evolved at the cathode. Chloride ions
will accumulate at the anode, and chlorine gas will consequently
be evolved here.
There is one further factor which must be addressed in these
cells, i.e. concentration. The electrolysis of hydrochloric acid
illustrates this effect, as shown in example (c) below.

Electrolysis of Concentrated Aqueous HC1
1. Determine whether or not the electrolysis involves (a) a molten
or (b) an aqueous substance. In this case, the electrolysis involves
aqueous hydrochloric acid and therefore additional reactions
involving water need to be considered.
2. Identify all species present: H+(aq), Cl-(aq) and H20.
3. Having identified all the species present, determine which species
accumulate at the cathode, and which species accumulate at the
anode:
Cathode -ve H+, H20 Anode +ve C1-, H20
4. At the cathode, two species appear to be present. However, since
hydrochloric acid is a strong acid (i.e. a good proton donor,
which dissociates readily in solution), it will first ionise in the
presence of water to undergo the following reaction: H20 + HC1
4 H30+ + C1- i.e. generating H30+ ions (H+) at the cathode.
The cathode reaction then becomes much simpler: 2H+(,,) + 2e
-+ H2(g). Hence, hydrogen gas is evolved at the cathode.
At the anode there are two possible half-reactions:

(a) Cl-(aq)+@lZ(g) + e E" = -1.36V
-1 0
02(g) + 4H+,,,) + 4e E" = - 1.23 V
(b) 2H20(aq) -+
I -11 0 I
u

-2 to 0 (oxidation)
This time, since E" for the chloride is - 1.36 V and E" for the
water is - 1.23 V, the values are so close that it will be concentra-
tion dependent as to which half-reaction will actually occur, i.e.
if the solution is concentrated hydrochloric acid, the concentra-
tion of chloride will be large, and chlorine gas will be evolved at
the anode, but if the solution is very dilute, then oxygen gas will