Question

Explain chlor-alkali process with chemical equation. Name the products formed at anode and cathode.

Hint:

Chlor-alkali process: Electrolysis of brine \(\Rightarrow\) NaOH + Cl₂ + H₂. Anode (+): Cl⁻ oxidized to Cl₂ gas. Cathode (-): H₂O reduced to H₂ gas and OH⁻ (forms NaOH). Products: Chlorine (anode), Hydrogen (cathode), Sodium hydroxide (in solution).

Answer 1

Step 1: Definition of Chlor-alkali process.
The chlor-alkali process is an industrial process for the electrolysis of brine (sodium chloride solution) to produce three important chemicals: chlorine, sodium hydroxide (caustic soda), and hydrogen. The name "chlor-alkali" comes from the products "chlor" (chlorine) and "alkali" (sodium hydroxide).
Step 2: Electrolysis of brine.
When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas.
Step 3: Chemical equation for the process.
\[ \boxed{2\text{NaCl (aq)} + 2\text{H}_2\text{O (l)} \xrightarrow{\text{electricity}} 2\text{NaOH (aq)} + \text{Cl}_2\text{ (g)} + \text{H}_2\text{ (g)}} \]
Step 4: Reactions at electrodes.

• At anode (positive electrode): Oxidation occurs. Chloride ions (\(\text{Cl}^-\)) lose electrons to form chlorine gas. \[ 2\text{Cl}^- \text{ (aq)} \rightarrow \text{Cl}_2\text{ (g)} + 2e^- \]
• At cathode (negative electrode): Reduction occurs. Water molecules gain electrons to form hydrogen gas and hydroxide ions. \[ 2\text{H}_2\text{O (l)} + 2e^- \rightarrow \text{H}_2\text{ (g)} + 2\text{OH}^- \text{ (aq)} \]

Step 5: Products formed at each electrode.
\[ \boxed{\text{At anode: Chlorine gas (Cl}_2\text{)}
\boxed{\text{At cathode: Hydrogen gas (H}_2\text{)}} \] Sodium hydroxide (NaOH) is formed in the solution (cathode compartment) due to the accumulation of \(\text{OH}^-\) ions.
Step 6: Uses of products.

• Chlorine: Used in water purification, PVC production, bleaching powder, disinfectants.
• Hydrogen: Used as fuel, in ammonia synthesis (Haber process), hydrogenation of oils.
• Sodium hydroxide: Used in soap and detergent manufacturing, paper industry, textile processing, drain cleaners.

Step 7: Additional information (for reference).
The chlor-alkali process is typically carried out using different cell technologies:

• Mercury cell process (older method)
• Diaphragm cell process
• Membrane cell process (modern, environmentally friendly)

The membrane cell process uses a selective membrane that allows sodium ions to pass through while preventing chloride ions and hydroxide ions from mixing, producing high-purity NaOH.