Page 99 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Electrochemistry I: Galvanic Cells 83
Solution:
1. There is no mention of 'electrolysis' or 'electrolysed'; therefore
the cell in question is a galvanic cell.
- (aq) 11 Sn4+ (aq), Sn2+ (aq)lPt(,). This time,
2. Pb(s)JPbS0q,)IS042
identifying the two types of electrodes is more difficult. Re-
member however, all you need do is determine which of the four
types of electrode is involved in each case.

(a) Pb(s)IPbSOy,)lSO~-(aq) is a metal-insoluble salt anion
electrode;
(b) Sn4+(,,), Sn2f(aq)lPt(s) is a metal ion in two different valence
states (note the inert metal, Pt).
3. No balanced chemical equation is given, therefore the E" values
can be used to determine directly which electrode is acting as
the cathode:
E" (Pb IPbSO ISO:-(aq)) = -0.356 V;
8 %(
E" (Sn (as), Sn aq) ) = +0.154 V.
The latter has the more positive value +- cathode.
4. One-line representation of the cell:
llsn4+(a
P~(,)IP~SO~(~)ISO~~-(~~) (aq)lPt(si (Figure 6.10).
sn2
1,
5. Cathode reaction: ('CROA') Sn40(aq) (IV); Sn +laq) (11).
+
Hence, 4 + 2 (reduction), i.e. Sn4'(aq) + 2e -+ Sn +(as);
Anode reaction: Pb(,) (0); PbSOq,) has Pb (11) and S042-
(-IT); also S042- (-11) present. There is no change in the
oxidation state of the sulfur or oxygen, but the oxidation state
of the lead does change (0 + 11), i.e. Pbo(,) -, Pb2+(,q) + 2e.
Therefore, Pbo(,) + SO?-(aq) -+ PbSO,,,) + 2e
6. Cathode reaction: Sn4+(,,) + 2e + Sn 2+(aq)
Anode reaction: Pb,,) + S02-(aq) -+ PbS04(,) + 2e
Cell reaction:
+ Pb(,) + S02-(aq) -+ PbSOqs) + Sn2+(aq)
7. Diagram of the cell (Figure 6.10)
8. Eocell=EoRHE-EoLHE =(+0.154)-(-Oa356)V= +0.510V.
9. Nernst equation: E = Eocell - (RT/vF) In K.
Sn4+(,q) + Pb,,, + SO:-(aq) * PbSOq,) + Sn2+(,q)
K = [Sn2+(,,)]/([Sn4'(,,)) [S0,2-(aq)]), since the activity, a, of
both Pb(,) and PbS04(,) is unity (both are solid).
j E = 0.510 - [(8.314 x 298)/(2 x 96 500)] x In [0.248/(0.349
x 0.219)] = +0.495 V.
Answer: E = +0.495 V