Page 103 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Electrochemistry I: Galvanic Cells 87
Solution:

1. There is no mention of ‘electrolysis’ or ‘electrolysed’, therefore
the electrochemical cell is a galvanic cell.
2. In this problem, the solubility product of silver chloride has to be
determined. The solubility product equilibrium reaction first has
to be written down:
AgCl(s) + Ag+(aq) + Cl-(aq), where ~sp = [Ag+(aq)I[Cl-(aq)I
since the activity, a, of AgCl,,) is unity. This is the net cell
reaction that has to be obtained. From the standard electrode
potentials, Ag(s)lAg+(aq) has the greatest E” value and therefore
would act as the cathode (RHE), whereas Ag~,>lAgCl(,)ICl-(aq)
would act as the anode (LHE) in a spontaneous galvanic cell.
However, this will not generate the desired chemical equation,
concerning the solubility product equilibrium of silver chloride.
Put in a different way, in this question, you are given the
equation in an implicit manner, and therefore, as before, you
have to examine the equation to see which substance is oxidised,
and which substance is reduced.
3. Cathode reaction (‘CROA’): Ago(s) (0); Cl-(aq) (-1); AgCl,,)
has Ag (I) and C1 (-1). There is no change in the oxidation
state of the chlorine (-I to -I), but there is a change in the
oxidation state of the silver (I + 0).
Therefore: I + 0 (reduction)
* AgCI,,, + &(s)
Ag(s) + Cl-(aq)
AgCl(s) + e -+
I -I 0 -I
Anode reaction: Ag+(,) (I), Ago(,) (0)
Hence 0 -+ 1 (oxidation), i.e. Ago -+ Ag+ + e
Hence: Cathode reaction: AgCl,,) + e Ago(,> + Cl-(aq)
Anode reaction: Ago(,) --+ Ag (aq) + e

Cell reaction: AgCl (,) + Ag+(aq) + Cl-,,,)
4. RHE is a metal-insoluble salt anion electrode, LHE is a metal-
metal-ion electrode.
5. One-line representation of the cell:
Ag(s)IAg+(aq)IIC1-(aq,IAgCl(s)
6. Diagram of the cell (Figure 6.12).