Redox Reactions
~7 min read
- Definitions: Oxidation = loss of electrons (or gain of O, loss of H). Reduction = gain of electrons (or loss of O, gain of H).
- OIL RIG: Oxidation Is Loss; Reduction Is Gain (of electrons).
- Agents: Oxidising agent = substance that oxidises others; itself gets reduced. Reducing agent = vice versa.
Redox reactions are everywhere — rusting, combustion, batteries, photosynthesis. NDA tests definitions, examples, and identifying agents.
What is Redox?
- Old definition (oxygen-based): Oxidation = adding oxygen. Reduction = removing oxygen.
- Modern definition (electrons): Oxidation = losing electrons. Reduction = gaining electrons.
- Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain.
- Both happen together — electrons released by one substance are gained by another. Hence "redox" — reduction-oxidation.
Classic Redox Examples
| Reaction | What's oxidised | What's reduced |
|---|---|---|
| 2Mg + O₂ → 2MgO | Mg (Mg⁰ → Mg²⁺) | O (O⁰ → O²⁻) |
| 4Fe + 3O₂ → 2Fe₂O₃ (rusting) | Iron | Oxygen |
| Zn + CuSO₄ → ZnSO₄ + Cu | Zinc | Copper ion |
| 2H₂ + O₂ → 2H₂O (combustion) | Hydrogen | Oxygen |
| CH₄ + 2O₂ → CO₂ + 2H₂O | Carbon (in methane) | Oxygen |
Oxidising and Reducing Agents
- Oxidising agent (oxidant): The substance that causes oxidation of another. It accepts electrons → itself gets reduced.
- Common oxidising agents: O₂, F₂, Cl₂, KMnO₄, K₂Cr₂O₇, H₂O₂, HNO₃ (conc).
- Reducing agent (reductant): The substance that causes reduction of another. It donates electrons → itself gets oxidised.
- Common reducing agents: H₂, C, CO, Na, Al, Mg, Zn, Fe.
Redox in Everyday Life
- Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. Iron oxidises in moist air. Painting/galvanising prevents access of O₂/water.
- Corrosion: General term — oxidation of metals by environment.
- Combustion: Rapid exothermic oxidation. Fire.
- Respiration: Glucose oxidised in body cells, producing CO₂, H₂O, and energy (ATP).
- Photosynthesis: CO₂ reduced to glucose using sunlight (reverse of respiration in a sense).
- Batteries (electrochemical cells): Redox reactions produce electrical energy. Dry cell, lead-acid, lithium-ion.
- Bleaching: Bleaching powder oxidises pigments to colourless form.
NDA PYQ Examples
Q: Oxidation involves:
(a) Gain of electrons (b) Loss of electrons (c) Gain of hydrogen (d) No electron change
Answer: (b) Loss of electrons (OIL — Oxidation Is Loss).
Q: In Zn + CuSO₄ → ZnSO₄ + Cu, the oxidising agent is:
(a) Zn (b) ZnSO₄ (c) CuSO₄ (Cu²⁺ ion) (d) SO₄²⁻
Answer: (c) Cu²⁺ ion — it gets reduced to Cu metal.
Q: Rusting of iron is an example of:
(a) Reduction (b) Sublimation (c) Oxidation/corrosion (d) Catalysis
Answer: (c) Oxidation/corrosion — iron loses electrons to oxygen.
Q: Hydrogen acts as:
(a) Oxidising agent always (b) Reducing agent always (c) Mostly reducing agent (d) Both equally
Answer: (c) Mostly a reducing agent — donates electrons to other substances.
Drill Redox Reactions for NDA
NDA-pattern items on Redox Reactions with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is rusting a redox reaction?
Iron (Fe) loses electrons → Fe²⁺ or Fe³⁺ (oxidation). Oxygen gains electrons → O²⁻ (reduction). The two combine with water to form hydrated iron oxide — rust.
How can we prevent rusting?
Block iron from oxygen and water. Methods: painting, oiling/greasing, galvanising (coating with zinc — zinc oxidises preferentially, sacrificial protection), tin-plating (tinning), chrome-plating, alloying (stainless steel).
What is a sacrificial protection?
Galvanising — coating iron with a more reactive metal (zinc). The zinc oxidises preferentially, protecting the iron underneath. Even if zinc layer is scratched, the exposed iron is protected as long as zinc remains.
How does a battery use redox?
One electrode loses electrons (oxidation) — the anode. The other gains electrons (reduction) — the cathode. Electrons flow through the external circuit from anode to cathode, doing electrical work. Internally, ions move to balance charge.
Why does food rot quickly in open air?
Many components (especially fats, oils, vitamins) are oxidised by atmospheric oxygen, often catalysed by bacteria. Oxidation produces off-flavours, off-smells, and toxic compounds. Refrigeration, vacuum packing, and antioxidant addition slow this.