Question

The following question is a source-based/case-based question that follows. Read the case carefully and answer the questions that follow.

Seawater contains many salts dissolved in it. Common salt is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt and is mined like coal. The common salt is an important raw material for chemicals of daily use.

1. Write balanced chemical equations to show the products formed during the electrolysis of brine.  (1)
2. List two uses of any one product obtained during the electrolysis of brine.  (1)
3. 1. A mild, non-corrosive basic salt ‘A’, used for faster cooking, is strongly heated to produce a compound ‘B’, that is used for removing the permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated.  (2)
      OR
   2. Define water of crystallisation. Give two examples of salts that have water of crystallisation.  (2)

Answer 1

i. During the electrolysis of brine (an aqueous solution of NaCl), the following half-reactions occur:

• At the anode (Oxidation):
  \[\ce{2Cl^-(aq) -> Cl2(g) + 2e^-}\]
• At the cathode (reduction):
  \[\ce{2H2O(l) + 2e^- -> H2(g) + 2OH^-(aq)}\]

The overall balanced equation is:

\[\ce{2NaCl(aq) + 2H2O(l) -> Cl2(g) + H2(g) + 2NaOH(aq)}\]

ii. Two common uses of chlorine obtained from the electrolysis of brine are:

• Water Disinfection: Chlorine kills bacteria and other germs that are dangerous to drinking water.
• Production of Plastics: Polyvinyl chloride (PVC), which is used in pipes, wires, and many other plastic items, can't be made without chlorine. 

iii. a. Conversion of a Basic Salt

• Identification:
  • Salt A: The first salt is sodium bicarbonate(NaHCO₃) which is a mild, non-corrosive basic salt used in cooking and is also known as baking soda. 
  • Salt B: Sodium carbonate(Na₂CO₃) which is often called cleaning soda and is used to make water less hard in the long term.
• Chemical Reaction (upon heating Salt A): When strongly heated, sodium bicarbonate decomposes as follows:
  \[\ce{2NaHCO3(S) -> Na2CO3(S) + CO2(g) + H2O(g)}\]

OR

b. Water of crystallisation refers to the water molecules that are integrated into the crystal lattice of a salt. These water molecules are present in fixed ratios and are an essential part of the compound’s structure. When a hydrated salt is heated, these water molecules are usually driven off. 

Examples: 

1. Copper (II) Sulfate Pentahydrate: CuSO₄ ⋅ 5H₂
2. Sodium Carbonate Decahydrate: Na₂CO₃ ⋅ 10H₂O