Oxidation–Reduction (Redox) Reactions - Chemistry

1. Fundamental Concepts

Definition: Oxidation–Reduction (Redox) reactions involve the transfer of electrons from one species to another.
Oxidation: The loss of electrons, resulting in an increase in oxidation state.
Reduction: The gain of electrons, resulting in a decrease in oxidation state.
Oxidizing Agent: A substance that gains electrons and is reduced.
Reducing Agent: A substance that loses electrons and is oxidized.

2. Key Concepts

Oxidation State: $$\text{Oxidation state} = \text{Charge on the atom} - \sum (\text{Bonded atoms' electronegativity})$$

Half-Reactions: $$\text{Oxidation half-reaction: } \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$$ $$\text{Reduction half-reaction: } \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$$

Balancing Redox Reactions: $$\text{Balance atoms, then balance charge by adding electrons.}$$

Application: Used in batteries, corrosion, and biological processes like respiration.

3. Examples

Example 1 (Basic)

Problem: Identify the oxidation and reduction half-reactions in the following reaction: $$\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$$

Step-by-Step Solution:

Identify the species being oxidized and reduced:
- Zn is oxidized: $$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$
- Cu²⁺ is reduced: $$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$$

Validation: Check the overall charge and mass balance.

Charge: Left side: 0 + 2+ = 2+; Right side: 2+ + 0 = 2+ ✓
Mass: Left side: 1 Zn + 1 Cu²⁺; Right side: 1 Zn²⁺ + 1 Cu ✓

Example 2 (Intermediate)

Problem: Balance the following redox reaction in acidic solution: $$\text{MnO}_4^- + \text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + \text{Fe}^{3+}$$

Step-by-Step Solution:

Write the half-reactions:
- Oxidation: $$\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$$
- Reduction: $$\text{MnO}_4^- + 8H^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4H_2O$$
Balance the number of electrons:
- Multiply the oxidation half-reaction by 5: $$5\text{Fe}^{2+} \rightarrow 5\text{Fe}^{3+} + 5e^-$$
Add the half-reactions:
- $$\text{MnO}_4^- + 8H^+ + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 4H_2O + 5\text{Fe}^{3+}$$

Validation: Check the overall charge and mass balance.

Charge: Left side: 1- + 8+ + 10+ = 17+; Right side: 2+ + 15+ = 17+ ✓
Mass: Left side: 1 Mn, 4 O, 8 H, 5 Fe; Right side: 1 Mn, 4 O, 8 H, 5 Fe ✓

4. Problem-Solving Techniques

Identify Half-Reactions: Separate the reaction into oxidation and reduction half-reactions.
Balance Atoms: Balance all atoms except oxygen and hydrogen first.
Balance Oxygen and Hydrogen: In acidic solutions, add H⁺ to balance hydrogen and H₂O to balance oxygen.
Balance Charge: Add electrons to balance the charge in each half-reaction.
Combine Half-Reactions: Multiply the half-reactions by appropriate factors to equalize the number of electrons, then add them together.
Check Overall Balance: Ensure the overall reaction is balanced in terms of both mass and charge.