Question

Name and explain the process used to obtain sodium hydroxide from the aqueous solution of sodium chloride (Brine). Name the two gases which are given at the anode and the cathode respectively. List two uses of each of these gases.

Answer 1

The process used to obtain sodium hydroxide from brine is called the chlor-alkali process. It involves the electrolysis of brine (concentrated aqueous solution of sodium chloride) using a diaphragm cell or a mercury cell.

In the chlor-alkali process, brine is electrolyzed using inert electrodes (e.g., graphite for anode and steel for cathode). During the electrolysis:

At the cathode (reduction): Sodium ions (\(\text{Na}^+\)) are reduced to sodium metal (\(\text{Na}\)), which immediately reacts with water to form sodium hydroxide (\(\text{NaOH}\)) and hydrogen gas (\(\text{H}_2\)).

\[ 2\text{Na}^+(\text{aq}) + 2e^- \rightarrow 2\text{Na}(\text{s}) \] \[ 2\text{Na}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaOH}(\text{aq}) + \text{H}_2(\text{g}) \]

At the anode (oxidation): Chloride ions (\(\text{Cl}^-\)) are oxidized to chlorine gas (\(\text{Cl}_2\)).

\[ 2\text{Cl}^-(\text{aq}) \rightarrow \text{Cl}_2(\text{g}) + 2e^- \]

The overall reaction is:

\[ 2\text{NaCl}(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaOH}(\text{aq}) + \text{H}_2(\text{g}) + \text{Cl}_2(\text{g}) \]

Gases produced:

• Anode: Chlorine gas (\(\text{Cl}_2\))
• Cathode: Hydrogen gas (\(\text{H}_2\))

Uses of gases:

Chlorine (\(\text{Cl}_2\)):

1. Used as a disinfectant for water purification.
2. Used in the manufacture of PVC (polyvinyl chloride) and other chlorinated organic compounds.

Hydrogen (\(\text{H}_2\)):

1. Used as a fuel in fuel cells to generate electricity.
2. Used in the manufacture of ammonia (Haber process) and other chemicals.