A galvanic cell is prepared using the following half-cells: (i) MnO 4 - (0.40 M)

Question

A galvanic cell is prepared using the following half-cells:

(i) MnO4- (0.40 M), Mn2+(0.02 M), H+ (1.0 M);

(ii) Sn2+ (0.40 M), Sn4+ (0.01 M)

MnO4- + 8H+ + 5e- -------> Mn2+ + 4H2O   Eo = 1.507 V

Sn4+ + 2e- --------> Sn2+                                            Eo = 0.151 V

(a) Calculate the galvanic cell voltage.

(b) Write the complete cell description.

(c) Write the cell reaction indicating clearly the direction of spontaneous reaction.

(d) Name the electrode (anode or cathode), give the polarity (positive or negative)

of the electrode, and state the reaction (oxidation or reduction) occurring in the

tin half-cell.

Explanation / Answer

Note that the more positive the standard reduction potential (E0) of a reaction, the easier is the reduction process. Conversely, the less positive is the standard reduction potential, the more is the tendency of the element/ion to be oxidized.

MnO4-/Mn2+ forms the cathode and Sn4+/Sn2+ forms the anode. The half reactions are:

Reduction: MnO4- + 8 H+ + 5 e- ------> Mn2+ + 4 H2O ; E0red = +1.507 V

Oxidation: Sn2+ ------> Sn4+ + 2 e-; E0ox = -0.151 V (the sign of E0 reverses for the oxidation half)

The overall cell reaction is

2 MnO4- + 5 Sn2+ + 16 H+ -------> 2 Mn2+ + 5 Sn4+ + 8 H2O …..(1)

E0cell = E0red + E0ox = (+1.507 V) + (-0.151 V) = 1.356 V

Since the reactants and the products are under non-standard conditions, we must have

Ecell = E0cell – 0.0591/10*log [Mn2+]2[Sn4+]5/[MnO4-][Sn2+][H+]16 (10 moles of electrons are involved).

===> Ecell = 1.356 – (0.00591)*log [(0.02)2(0.01)5/(0.40)2(0.40)5(1.0)16]

===> Ecell = 1.356 – (0.00591)*log [4.0*10-14/1.6384*10-3]

===> Ecell = 1.356 – (0.00591)*log (2.4414*10-11)

===> Ecell = 1.356 – (0.00591)*(-10.6123)

===> Ecell = 1.356 – 0.0627 = 1.2933 1.30

The emf of the cell is +1.30 V (ans, a)

b) The cell consists of MnO4-/Mn2+ at the cathode and Sn2+/Sn4+ at the anode (ans).

c) Since Ecell is positive, the reaction (1) is spontaneous (ans).

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