What happens during the electrolysis of an aqueous NaCl solution?

Electrolysis of sodium chloride is an important process to manufacture many bulk chemicals of commercial utility, like chlorine, sodium hydroxide etc. Sodium chloride is electrolyzed either in a molten state or in aqueous solutions. Electrolysis is, also done in the presence of additional salts to aid the redox reactions.

Electrolysis of Molten Sodium Chloride

Electrolysis involves the movement of ions to the electrode. Solid-state does not allow the movement of ions and unsuitable for electrolysis. When melted at high temperature, sodium chloride separates into sodium and chloride ions, so that, electrolysis can take place to form sodium atom and chlorine gas.

Read More In Detail: Electrolysis

NaCl → Na +(l) + Cl–(l)

At cathode: reduction of 2Na+(l) + e– → Na(l)

At anode: oxidation of 2Cl–(l) → Cl2(g) + 2e–

Net Reaction is written as: 2Na +(l) + 2Cl–(l) → 2Na(l) + Cl2(g)

Down’s Process: Sodium chloride melts at a very high temperature of 801°C. Addition of anhydride calcium chloride in the ratio of CaCl2: NaCl = 3:2 reduces the melting point to 580°C. Electrolysis is done with an iron cathode and graphite anode and iron gauze preventing the mixing of products chlorine and sodium. The products of molten sodium chloride are sodium metal and chlorine gas.

Electrolysis of Aqueous Sodium Chloride

Sodium chloride is dissociated and exists as sodium and chloride ions in aqueous solution. Electrolysis of sodium chloride is easier in aqueous solution. But, water itself can undergo reduction and oxidation reactions at different potentials. So, the substance that is oxidized or reduced is not sodium and chloride ions alone but it may involve the water molecule also.

Also Read: Water Electrolysis

Two competing reactions are possible at both cathode and anode. At Cathode: reduction reaction: at pH =7

Water can be reduced to hydrogen gas or Sodium ions reduced to sodium metal.

2H2O(l) + 2e– → H2(g) + 2OH– E° = -1.0 V

Na+(l) + e– → Na(l) E° = -2.71V

At anode: Oxidation reaction: at pH =7. Water can be oxidized to oxygen or chloride ion oxidized to chlorine molecule.

2H2O → O2(g) + 4H+ + 4e– E° =-1.42 V

2Cl– → Cl2 + 2e– E =- 1.36V

Hence, the product of electrolysis of aqueous sodium chloride can be anything between,

i) sodium metal, or hydrogen gas at the cathode and

ii) chlorine or oxygen gas at the anode,

With a side product of sodium hydroxide resulting from the reaction of sodium and water. The product of electrolysis depends on the concentration of sodium chloride aqueous solution.

a) Very Dilute Aqueous Sodium Chloride Solution

Water has very low conductivity and the small amount of ionic sodium chloride helps the ionic conductivity of the solution. In small concentration, the electrolysis of water becomes more predominant yielding hydrogen at cathode and oxygen at the anode.

At cathode: 2H2O + 2e– → H2(g) + 2OH– E° = -1.0 V

At anode: 2H2O → O2(g) + 4H+ + 4e– E° = +1.4 V

Net reaction of electrolysis of very dilute aqueous sodium chloride is given as;

2H2O → H2(g) + 2OH– + O2(g) E° = – 2.4 V

b) High Concentration of Sodium Chloride

At Cathode: reduction reaction: at pH =7

2H2O(l) + 2e– → H2(g) + 2OH– E° = -1.0 V

Na+(l) + e– → Na(l) E° = -2.71V

Reduction (over) potential of water being more positive than the reduction of sodium ions, water is decomposed to liberate hydrogen at the cathode.

At anode: Oxidation reaction: at pH =7

2H2O → O2(g) + 4H+ + 4e– E° =-1.4 V

2Cl– → Cl2 + 2e– E =- 1.36V

The thermo-dynamical reduction potential of water and chloride is +0.82V and 0.1.36V respectively. 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 -1.4V. Chloride oxidation is more positive than the net voltage of water oxidation. So, chloride is, oxidized to chlorine at the anode. The net reaction taking place is given as;

At cathode – 2H2O(l) + 2e– → H2(g) + 2OH–

At anode – 2Cl– → Cl2 + 2e–

The overall reaction is as follows;

2H2O(l) + 2Cl– → H2(g) + Cl2 + 2OH–

Or \

2H2O(l) + 2NaCl → H2(g) + Cl2 + 2NaOH

At high salt concentration, hydrogen and chlorine is the product of electrolysis with sodium hydroxide as a by product.

In the Castner-Kellner process, brine (aqueous sodium chloride) is, electrolyzed in a cell having two compartments. Graphite anode is in the side compartments and iron cathode in the central compartment. Mercury is at the bottom and separates, the two compartments and acts as a conduit for carrying sodium formed at the end compartments to the central compartment.

Brine, added to the end compartment is, electrolyzed to sodium and chlorine. While chlorine escapes at the top of the cell, sodium forms sodium amalgam with mercury at the bottom. Sodium transferred to the central compartment reacts with water to form sodium hydroxide and hydrogen.

The product of electrolysis of concentrated aqueous sodium chloride are sodium hydroxide, hydrogen gas and chlorine gas.

Mercury used in the Caster-Kellner process contaminates the products and is an environmental hazard due to sublimation. Mercury being carcinogenic is eased out for the electrolysis of aqueous sodium chloride.

Presently all the Chlor-alkali industries producing chlorine and sodium hydroxide uses a semipermeable membrane ‘Nafion’ to separate the anode and cathode compartments.

Nafion is a sulfonated ionic tetra fluoro-ethylene copolymer. The membrane allows the sodium ion to flow across the membrane into the cathode cell. Additional reinforcing with other semipermeable membrane prevents any back mixing of incoming chloride and hydroxide ions across, Nafion.

c) Electrolysis of Aqueous Sodium Chloride in Intermediate Concentration

At cathode: reduction of water to hydrogen is more feasible than sodium hydrogen. In all aqueous solutions reduction of water to hydrogen takes place at cathode compared to the reduction of sodium ion. At anode: Oxidation potential of water and chloride ions are almost the same (-1.4V and 1.36V respectively).

At higher concentration of sodium chloride, chloride oxidation is favoured kinetically, while at low concentrations oxidation of water to hydrogen is favoured kinetically. At intermediate concentrations, both water and chloride ions are oxidized. So, both oxygen and chlorine gases evolve at the anode, simultaneously.

At Cathode: reduction reaction: at pH =7, 2H2O(l) + 2e– → H2(g) + 2OH–

At Anode: Oxidation reaction: at pH =7 2H2O → O2(g) + 4H+ + 4e–

2Cl– → Cl2 + 2e–

Reduction reaction x 3 6H2O(l) + 6e– → 3H2(g) + 6OH–

So, the net reaction is;

4H2O(l) + 2Cl– → 3H2(g) + 2OH– + Cl2 + O2

Or

4H2O(l) + 2NaCl → 3H2(g) + 2NaOH + Cl2 + O2

Sodium chloride electrolysis in aqueous solution yields different products, depending on the relative concentration of sodium chloride and water. The electrolysis is thermodynamically controlled at very low and high concentrations of sodium chloride and kinetically controlled at inter-mediatory concentrations.

Also Check Out:

• Electrochemistry
• Electrochemical Series
• EMF of a cell