Chapter 2 · Question 11

Describe the chlor-alkali process. Why is it so named? List the three main products obtained and give one important use of each.

Q11

Describe the chlor-alkali process. Why is it so named? List the three main products obtained and give one important use of each.

Answer Revealed

Direct Answer:

The chlor-alkali process is the electrolysis of brine (aqueous sodium chloride solution) to produce sodium hydroxide, chlorine gas, and hydrogen gas:

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

. It is named 'chlor-alkali' because the products are chlorine ('chlor') and sodium hydroxide ('alkali'). At the anode (+), chlorine gas is released. At the cathode (-), hydrogen gas is released, and NaOH solution is formed near the cathode. Uses: NaOH — making soaps and detergents; Cl₂ — water treatment and PVC production; H₂ — fuel and manufacturing ammonia for fertilisers.

Simple Explanation

When you pass electricity through salty water (brine), it splits into three valuable products. Chlorine gas bubbles out at one electrode, hydrogen gas at the other, and sodium hydroxide (caustic soda) builds up in the remaining solution. The process gets its name from the two key products: chlorine and alkali (NaOH). Chlorine is used to disinfect water and make PVC pipes. Hydrogen is used as fuel and to make fertilisers. Sodium hydroxide is used to make soap — literally the chemical that turns oils into soap.

Exam-Ready Structure

The chlor-alkali process is the industrial-scale application of electrolysis that turns common salt — one of the cheapest and most abundant raw materials — into three commercially valuable chemicals. 1. Sources of common salt: (a) Seawater — contains many dissolved salts; sodium chloride is purified and separated. (b) Rock salt — large brownish crystalline deposits formed from ancient dried-up seas, mined like coal. The Dandi March led by Mahatma Gandhi used salt as a symbol of resistance, highlighting its historical importance. 2. Chlor-alkali process: When electric current is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas:

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

. 3. The name: 'Chlor' refers to chlorine, and 'alkali' refers to sodium hydroxide — the two prominent products. 4. Product distribution (Figure 2.8): (a) At the anode (+): Chlorine gas (

\text{Cl}_2

) is released. (b) At the cathode (-): Hydrogen gas (

\text{H}_2

) is released. (c) Near the cathode: Sodium hydroxide (

\text{NaOH}

) solution is formed. A membrane separates the anode and cathode compartments. 5. Product uses: (a) Chlorine: water treatment, swimming pool disinfection, manufacture of PVC (polyvinyl chloride), disinfectants, CFCs (historically), and pesticides. (b) Hydrogen: used as fuel, in hydrogenation of oils to make margarine, and in manufacturing ammonia (

\text{NH}_3

) for fertilisers. (c) Sodium hydroxide: degreasing metals, manufacturing soaps and detergents, paper making, and producing artificial fibres.

Key Points

Chlor-alkali process: electrolysis of brine — $2\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$
Named 'chlor-alkali' for its two prominent products: chlorine (chlor) and sodium hydroxide (alkali)
Anode (+): $\text{Cl}_2$ gas; Cathode (-): $\text{H}_2$ gas; NaOH formed near the cathode
NaOH uses: soaps, detergents, paper, artificial fibres
Cl₂ uses: water treatment, PVC, disinfectants, pesticides
H₂ uses: fuels, margarine, ammonia for fertilisers

Describe the chlor-alkali process. Why is it so named? List the three main products obtained and give one important use of each.