2. Production of Naoh (Caustic Soda)

What is another name commonly used for Sodium Hydroxide (NaOH)?

Caustic soda

What are the primary raw materials used in the industrial production of NaOH?

Saturated Sodium Chloride solution (brine) and Water (H2O).

What are the three historical and modern types of electrolytic cells that can be used to produce NaOH?

1. Mercury cell

2. Diaphragm cell

3. Membrane cell

What were the two primary environmental and health disadvantages of the historical Mercury cell method?

1. Dangerous release of toxic mercury into the environment.

2. Contamination of the final product (NaOH) with toxic trace mercury.

How does a Diaphragm cell mechanically prevent the products from mixing, and what is its main product purity drawback?

It uses a porous asbestos diaphragm and maintains a higher hydrostatic pressure in the anode compartment to force a one-way liquid flow. Drawback: Some Cl- ions still cross over, resulting in NaOH heavily contaminated with NaCl.

Name three major chemical impurities found in crude brine that must be removed before entering a Membrane Cell.

Calcium ions (Ca2+), Magnesium ions (Mg2+), and Sulfate ions (SO42-).

Why must Ca2+, Mg2+, and SO42- impurities be strictly removed from brine prior to membrane electrolysis?

In the alkaline cell conditions, they form insoluble precipitates (like CaCO3 and Mg(OH)2) that clog the pores of the selective polymer membrane, permanently ruining its efficiency.

What chemical reagent is added to crude brine to precipitate out Magnesium (Mg2+) ions?

Sodium Hydroxide [NaOH(aq)] to precipitate Mg(OH)2(s)

What chemical reagent is added to crude brine to precipitate out Calcium (Ca2+) ions?

Sodium Carbonate [Na2CO3(aq)] to precipitate CaCO3(s)

What chemical reagent is added to crude brine to precipitate out Sulfate (SO42-) ions?

Barium Chloride [BaCl2(aq)] to precipitate BaSO4(s)

What specific materials are used for the anode and the cathode in a modern Membrane Cell?

Anode: Titanium (Ti)

Cathode: Nickel (Ni)

Write the balanced half-reaction with state symbols that occurs at the Titanium anode in a Membrane Cell.

2Cl-(aq) -> Cl2(g) + 2e-

Write the balanced half-reaction with state symbols that occurs at the Nickel cathode in a Membrane Cell.

2H2O(l) + 2e- -> H2(g) + 2OH-(aq)

What is the structural function of the cation-exchange membrane in this process?

It is selectively permeable only to positive ions (Na+). It allows Na+ to cross from the anode to the cathode compartment to maintain neutrality, while completely blocking Cl- and OH- from crossing and contaminating the product.

Write the overall balanced chemical equation for the electrolysis of brine in the Chlor-Alkali industry.

2NaCl(aq) + 2H2O(l) -> 2NaOH(aq) + Cl2(g) + H2(g)

Why is the collected volume of Chlorine gas (Cl2) slightly lower than Hydrogen gas (H2) in real plant operations?

Chlorine gas has a significantly higher solubility in the aqueous medium (brine) compared to hydrogen gas, so a small amount remains dissolved instead of escaping as gas.

List two major industrial applications of the byproduct Chlorine gas (Cl2) generated from this industry.

1. Manufacturing of plastics like Polyvinyl Chloride (PVC).

2. Water treatment/sterilization and production of bleaching powder or hydrochloric acid.

List two major industrial applications of the byproduct Hydrogen gas (H2) generated from this industry.

1. Industrial synthesis of Ammonia gas (NH3) via the Haber process.

2. Hydrogenation of vegetable oils to produce margarine/fatty spreads.