Redox Reactions - SS1 Chemistry Past Questions and Answers - page 3

The half-reaction method is a systematic approach used to balance redox equations by dividing the reaction into two half-reactions: one for the oxidation process and one for the reduction process. Each half-reaction represents the transfer of electrons for one species involved in the reaction. The steps involved in balancing redox equations using the half-reaction method are as follows:

 

Step 1: Identify the species undergoing oxidation and reduction.

Determine the oxidation states of each element and identify which species is losing or gaining electrons.

 

Step 2: Write the half-reactions for oxidation and reduction.

Separate the redox reaction into two half-reactions: one for the oxidation process and one for the reduction process. Write the half-reactions for each species involved.

 

Step 3: Balance the atoms in each half-reaction.

Balance the atoms other than hydrogen and oxygen in each half-reaction by adding the necessary coefficients.

 

Step 4: Balance the charges in each half-reaction.

Balance the charges in each half-reaction by adding electrons (e⁻) to the side with the higher charge. The number of electrons added should be equal to the difference in charge.

 

 

Step 5: Multiply the half-reactions to balance the electrons.

Multiply each half-reaction by an appropriate factor so that the number of electrons gained in the reduction half-reaction is equal to the number of electrons lost in the oxidation half-reaction. This ensures that the electrons cancel out when the half-reactions are combined.

 

Step 6: Combine the half-reactions and cancel out common terms.

Add the balanced half-reactions together and cancel out the common terms (species and electrons) on both sides.

 

Let's consider the example of the reaction between potassium permanganate (KMnO₄) and sodium sulfite (Na₂SO₃) in an acidic solution:

 

Step 1: Identify the oxidation states.

Mn in KMnO₄ is +7, while S in Na₂SO₃ is +4.

 

Step 2: Write the half-reactions.

Oxidation half-reaction: 8H⁺ + MnO₄⁻ → Mn²⁺ + 4H₂O

Reduction half-reaction: 2Na⁺ + SO₃²⁻ → Na₂SO₃

 

Step 3: Balance the atoms in each half-reaction.

Oxidation half-reaction: 8H⁺ + MnO₄⁻ → Mn²⁺ + 4H₂O

Reduction half-reaction: 2Na⁺ + SO₃²⁻ → Na₂SO₃

 

Step 4: Balance the charges in each half-reaction.

Oxidation half-reaction: 8H⁺ + MnO₄⁻ + 5e⁻ → Mn²⁺ + 4H₂O

Reduction half-reaction: 2Na⁺ + SO₃²⁻ + 2e⁻ → Na₂SO₃

 

Step 5: Multiply the half-reactions to balance the electrons.

Multiply the oxidation half-reaction by 2 and the reduction half-reaction by 5 to balance the electrons:

 

10Na⁺ + 5SO₃²⁻ + 10e⁻ → 5Na₂SO₃

16H⁺ + 2MnO₄⁻ + 10e⁻ → 2Mn²⁺ + 8H₂O

 

Step 6: Combine the half-reactions and cancel out common terms.

Adding the balanced half-reactions together:

 

10Na⁺ + 5SO₃²⁻ + 16H⁺ + 2MnO₄⁻ → 5Na₂SO₃ + 2Mn²⁺ + 8H₂O

 

This balanced equation represents the redox reaction between potassium permanganate and sodium sulfite in an acidic solution.