Page 122 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

106 Chapter 7
7. Draw the cell (Figure 7.8).
8. In this question, all that has to be considered is the anode half-
reaction:
2Cl-(aq) + Clq,) + 2e
Per minute, Q = It = 0.650 x 60 = 39 C;
2 F 4 1 mol of chlorine gas
96 500 C + 0.5 mol of chlorine gas = (0.5 x 24.8 dm3) at 1 bar
pressure and 298 K
1 C --+ (0.5 x 24.8)/96 500 dm3; 39 C --+ 0.00501 14 dm3

V2 = (1 x 0.00501 14 x 300)/(1.2 x 298) = 0.0042 dm3 min-'
i.e. 4.2 cm3 min-', since 1000 cm3 = 1 dm3.
Answer = 4.2 em3 m'n-'

~ ~~
Example No. 3: A slightly acidified solution of copper(I1) sulfate is
electrolysed using inert platinum electrodes. Oxygen gas is evolved at
one electrode, and copper metal is deposited at the other. Draw a
fully labelled diagram of the cell, indicating clearly the cathode, the
anode, the direction of current, the direction of electrons and ion
flow. Write down the cathode, anode and cell reactions. How long
would it take this cell to deliver 2 dm3 of 02(g, at 1.5 bar pressure and
65 "C, using an electrolysis current of 475 mA? (F = 96 500 C mol- *;
1 mol of an ideal gas at 25 "C and 1 bar pressure occupies 24.8 dm3).

Solution:
1. Type of cell: electrolytic.
2. Type of electrolysis: electrolysis of aqueous copper(I1) sulfate.
3. Identify all species present: Cu2+(aq), SO:-(aq), H20.
4. Cathode: - ve electrode ('CNAP'): CU2+(aq), H2O.
Cu is below Zn in the electrochemical series; therefore copper
metal is deposited at the cathode:
cu2+(aq) + 2e --+ CUO(~)
5. Anode: +ve electrode ('CNAP'): S02-(,q), H20. But S042-(aq)
is not easily oxidised:
I- > OH- > ci- > NO^- > so,'-
It
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Ease of Oxidation