Chapter 7 – Redox Reactions and Electrochemistry
1. Meaning of Redox Reactions
Redox reactions are chemical reactions in which **oxidation and reduction occur simultaneously**.
Oxidation and reduction never occur independently.
REDOX = REDuction + OXidation
2. Oxidation and Reduction
| Oxidation | Reduction |
|---|---|
| Loss of electrons | Gain of electrons |
| Increase in oxidation number | Decrease in oxidation number |
| Addition of oxygen | Removal of oxygen |
3. Oxidising and Reducing Agents
- Oxidising agent: Causes oxidation and gets reduced
- Reducing agent: Causes reduction and gets oxidised
4. Oxidation Number (Oxidation State)
Oxidation number is the **imaginary charge** assigned to an atom assuming complete transfer of electrons.
Rules for Assigning Oxidation Number
- Free element → 0
- Monoatomic ion → its charge
- Oxygen → usually −2 (except peroxides, superoxides)
- Hydrogen → +1 (−1 in metal hydrides)
- Sum of oxidation numbers = charge on species
5. Identification of Redox Reactions
$$\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$$
Zn: 0 → +2 (oxidation)
Cu: +2 → 0 (reduction)
6. Balancing Redox Reactions (Oxidation Number Method)
Steps:
- Assign oxidation numbers
- Identify oxidised and reduced species
- Equalise increase and decrease in oxidation numbers
- Balance remaining atoms
7. Balancing Redox Reactions (Ion–Electron Method)
Used mainly in **aqueous solutions** (acidic or basic medium).
Steps
- Split into oxidation and reduction half-reactions
- Balance atoms and charge
- Add electrons
- Multiply to equalise electrons
- Add half-reactions
8. Electrochemistry – Introduction
Electrochemistry deals with the relationship between **chemical energy and electrical energy**.
9. Electrochemical Cell
A device that converts chemical energy into electrical energy using a redox reaction.
Example: Daniell Cell
10. Daniell Cell
$$\text{Zn}|\text{Zn}^{2+}||\text{Cu}^{2+}|\text{Cu}$$
| Anode | Cathode |
|---|---|
| Zn (oxidation) | Cu (reduction) |
11. Electrode Potential
Electrode potential is the tendency of an electrode to lose or gain electrons.
12. Standard Electrode Potential
Measured under standard conditions:
- 1 M concentration
- 1 atm pressure
- 298 K temperature
13. Standard Hydrogen Electrode (SHE)
$$\text{E}^\circ_{\text{SHE}} = 0.00 \text{ V}$$
14. EMF of a Cell
$$E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$$
15. Gibbs Free Energy and EMF
$$\Delta G^\circ = -nFE^\circ_{\text{cell}}$$
Negative $\Delta G^\circ$ → reaction is spontaneous
16. Nernst Equation
$$E = E^\circ - \frac{0.0591}{n}\log Q$$
Used to calculate electrode potential under non-standard conditions.
17. Electrolysis
Electrolysis is the process of using electrical energy to drive a non-spontaneous reaction.
18. Faraday’s Laws of Electrolysis
First Law
$$m \propto Q$$
Second Law
$$\frac{m_1}{m_2} = \frac{E_1}{E_2}$$
19. Corrosion
Corrosion is the slow destruction of metals due to redox reactions with environment.
Rusting of iron is an electrochemical process.
20. Common JEE Mistakes
- Forgetting oxidation and reduction occur together
- Wrong oxidation number assignment
- Incorrect sign of EMF
- Misuse of Nernst equation
21. Final Revision Checklist
You are exam-ready if you can:
- Identify oxidising and reducing agents
- Balance redox reactions confidently
- Calculate EMF and $\Delta G^\circ$
- Apply Nernst equation correctly
- Solve electrolysis numericals