What are the factors affecting the product of electrolysis?

Products of electrolysis mostly depend on the redox potentials of the ions present in the electrolyte. The products of electrolysis for the same electrolyte is also affected by factors like:

i) Nature and state of the electrolyte

ii) Nature and electrode potential of ions present in the electrolyte

iii) Nature of the electrode and

iv) Overvoltage at the electrodes

i) Nature and state of the electrolyte

Electrolysis involves movement of ions towards the oppositely charged electrodes. Naturally, the electrolyte should have mobile ions. In solids, ions are in specific positions and cannot move at ordinary temperatures. Hence solids are unsuitable for electrolysis.

For electrolysis, electrolyte should be in the liquid form- molten or in solution with a suitable polar solvent. Sodium chloride will undergo electrolysis in the molten state or in aqueous solution.

ii) Nature and electrode potential of ions present in the electrolyte

Electrolysis of electrolytes of two elemental ions is straight forward giving the two elements on electrolysis. Molten sodium chloride gives sodium atoms and chlorine molecule.

Electrolysis of radical ions do not give the elemental atoms.

Electrolytes containing more than one ionic compound, depends on the relative redox potentials.

Electrolysis of aqueous solutions of electrolytes is examples. Water molecules also can undergo redox reactions and will compete with redox reactions of the electrolyte ions.

Electrolysis of molten sodium chloride gives sodium and chlorine. But electrolysis of aqueous sodium chloride gives hydrogen and chlorine and not sodium.

iii) Nature of the electrode

For the same electrolyte, nature of the electrolyte may give different products.

When aqueous copper sulphate solution is, electrolyzed, the following redox reactions are possible.

At cathode: Reduction at pH =7

Cu2+ (aq) + 2e →Cu (s)E° = 0.34V and 2H2O + 2e→H2 + 2OH E° = -1.02V

At anode: Oxidation at pH = 7

Cu(s) →Cu2+ (aq) + 2e E° = – 0.34V and 2H2O → O2(g) + 4H+ + 4e E° = +1.4 V

At cathode, out of the two electrodes, reduction potential of copper ions is more positive than reduction of water. So, irrespective of electrode, copper ions from the electrolyte will be reduced and deposited on the cathode, increasing its mass. But the reaction at anode depends on the electrode. Electrolysis with inert electrodes like Platinum, graphite etc. Inert electrodes dos not react with the electrolyte or the products and so does not under go any changes.

Since, oxidation of water has more positive potential, oxygen will be evolved at the anode. But, if the copper is, used as anode, copper will react with the sulphate ion to retain the electrolyte concentration. So, there will not be any gas evolution. Instead, the anode mass slowly decreases going into the solution.

iv) Overvoltage at the electrodes.

Redox potential of electrolyte ions, decide the electrolysis reactions and products. Sometimes, redox potentials of some half reactions during the electrolysis is more than the thermodynamic potentials. This excess voltage (over voltage) of the half-reaction may make the reaction unfavorable and change the product of electrolysis.

In the hydrolysis of aqueous sodium chloride, at the anode, two oxidation reactions can take place. Reduction potential of water and chloride is +0.82V and 0.1.36V respectively.

2H2O→O2(g) + 4H+ + 4eE° = -0.82 V

2Cl→ Cl2 + 2e E = – 1.36V

 Oxidation of water being more positive is more feasible and so, the evolution of oxygen gas should happen at the anode. But, the evolution of oxygen from water has an overvoltage of -0.6V making the voltage for the oxidation of water as -1.42V. Chloride oxidation is more positive than the net voltage of water oxidation.

So, chloride is, oxidized to chlorine at the anode. Chlorine is liberated and not oxygen because of over voltage.

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