Page 120 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

104 Chapter 7

EXAMPLES OF ELECTROLYTIC CELL TYPE PROBLEMS

Example No. 1; Chlorine is produced by the electrolysis of aqueous
sodium chloride. Draw the electrochemical cell, indicating clearly
the anode, the cathode, the direction of electron flow and the
direction of the current. Assuming that chlorine is the only species
produced at the anode, determine how long it will take to produce
1 kg of chlorine gas, in a cell operating at 950 A. (F = 96500 C
mol- ’; molar mass (M) of CI = 35.453 g mol-’1.

Solution:
1. Type of cell: electrolytic.
2. Type of electrolysis: electrolysis of aqueous sodium chloride.
Hence, water must be considered.
3. Identify all species present: Na+(aq), CI-(aq), 3320.
4. Cathode: -ve electrode (‘CNAP’): Na+(aq), €320.
Na is above Zn in the electrochemical series; therefore H2(!) is
discharged at the cathode, according to the half-reaction:
2H2O(aq) + 2e + H2(g)+ 20H-(aq)*
5. Anode: + ve electrode (‘CNAP’): Cl-(aq), H20.
In this question, it is stated that chlorine gas is evolved at the
anode. Hence, 2Cl-(aq) + C12(,) + 2e.
6. Write down the two half-reactions:

Cathode reaction: 2H,O(aq) + 2e + H2(g) + 20H -(as)
Anode reaction: 2Cl-(aq> -+ C12(,, + 2e

7. Draw the cell (Figure 7.8).
8. In this question, all that needs to be considered is the anode half-
reaction:
2Cl-(aq) C1qg) + 2e
2 F + 1 mol of chlorine gas; 1 F + 0.5 mole of chlorine gas
96 500 C + 0.5 mol of chlorine gas = 0.5 x (35.453 x 2) = 35.453 g
i.e. 35.453 g -+ 96 500 C; 1 g + (96 500/35.453) C = 2721.9135 C
1 kg = 1000 g + 2721913.5 C
9. Q = It =+ t = Q/I = 2721913.5/950 = 2865.172105 s
= (2865.172105/60) min = 47.75 min.
Answer: 47.75 min