Redox & Disproportionation (Edexcel A Level Chemistry): Revision Note

Balancing disproportionation reactions

Balance the disproportionation reaction which takes place when chlorine is added to hot concentrated aqueous sodium hydroxide. The products are Cl^- and ClO₃^- ions and water

Answer

Step 1: Write the unbalanced equation and identify the atoms that change in oxidation number:

Unbalanced equation with oxidation numbers: Cl₂ + OH^- → Cl^- + ClO₃^- + H₂O

Cl in Cl₂ is 0; in Cl^- it is -1; in ClO₃^- it is +5 (O is -2). In OH^-, O is -2 and H is +1; in H₂O, H is +1 and O is -2.

Step 2: Deduce the oxidation number changes:

Oxidation number changes: Cl goes from 0 to -1, a change of -1; Cl goes from 0 to +5, a change of +5.

Step 3: Balance the oxidation number changes:

Balance the oxidation number changes: 5 × (-1) = -5 and 1 × (+5) = +5, so 5 Cl^- form for each ClO₃^-: Cl₂ + OH^- → 5Cl^- + ClO₃^- + H₂O

Step 4: Balance the charges

Balance the charges: total charge on the reactants is 1-; on the products it is 5 × (1-) + (1-) = 6-. So 6 OH^- ions are needed to balance the charges on both sides.

Step 5: Balance the atoms

Balance the atoms: 3Cl₂ + 6OH^- → 5Cl^- + ClO₃^- + 3H₂O

Oxidation and reduction

In each of the following equations, state which reactant has been oxidised and which has been reduced.

1. 2Na + Cl₂ →  2NaCl

2. Mg + Fe²⁺  →  Mg²⁺ + Fe

3. CO + Ag₂O  →  2Ag + CO₂

Answer 1:

• Oxidised: Na as the oxidation number has increased by 1 (0 to +1)

  • Reduced: Cl₂ (Cl) as the oxidation number has decreased by 1 (0 to -1)

Answer 2:

• Oxidised: Mg as the oxidation state has increased by 2

  • Reduced: Fe²⁺ as the oxidation state has decreased by 2

Answer 3:

• Oxidised: C as the oxidation number has increased from +2 to +4

  • Reduced: Ag as the oxidation number has decreased from +1 to 0