1️⃣ Definition of Redox Reaction
A redox reaction is a chemical reaction in which reduction and oxidation occur simultaneously.
- Oxidation = loss of electrons
- Reduction = gain of electrons
So, in a redox reaction:
🔄 One substance gets oxidized, the other gets reduced.
2️⃣ Oxidation and Reduction:
| Term | Definition of Oxidation | Definition of Reduction |
|---|---|---|
| Electrons | Loss of electrons (OIL) | Gain of electrons (RIG) |
| Oxygen | Gain of oxygen | Loss of oxygen |
| Hydrogen | Loss of hydrogen | Gain of hydrogen |
| Oxidation Number | Increase in oxidation number | Decrease in oxidation number |
🔁 Use the acronym OIL RIG:
Oxidation Is Loss, Reduction Is Gain (of electrons)
3️⃣ Oxidation and Reduction Half Equations
A half equation shows only the oxidation or reduction process.
🔸 Example 1: Oxidation Half Equation
Magnesium loses electrons: Mg→Mg2++2e−
🔸 Example 2: Reduction Half Equation
Oxygen gains electrons: O2+4e−→2O2−
4️⃣ Oxidation Number (Oxidation State)
The oxidation number of an atom is a number that represents how many electrons an atom has lost or gained.
✅ Rules for Assigning Oxidation Numbers:
| Atom/Group | Oxidation Number |
|---|---|
| Uncombined element | 0 |
| Group 1 metals | +1 |
| Group 2 metals | +2 |
| Oxygen (usually) | -2 |
| Hydrogen (usually) | +1 |
| Fluorine | -1 |
| Compound (neutral) | Sum = 0 |
| Polyatomic ion | Sum = charge |
🔸 Example: Oxidation Number of Mn in KMnO₄
- K = +1
- O = -2 × 4 = -8
Let Mn = x
+1+x+(−8)=0⇒x=+7+1 + x + (-8) = 0 \Rightarrow x = +7+1+x+(−8)=0⇒x=+7
5️⃣ Oxidizing Agents and Reducing Agents
✅ Oxidizing Agent:
- Causes oxidation
- Gains electrons
- Gets reduced
✅ Reducing Agent:
- Causes reduction
- Loses electrons
- Gets oxidized
6️⃣ Common Oxidizing Agents (Gain Electrons):
| Chemical | Observation When Reacts |
|---|---|
| Acidified potassium manganate (VII) KMnO₄ | Purple → Colourless |
| Acidified potassium dichromate (VI) K₂Cr₂O₇ | Orange → Green |
| Chlorine (Cl₂) | Displaces halides |
| Oxygen (O₂) | Supports combustion |
| Hydrogen peroxide (H₂O₂) | Bubbles of O₂ in presence of catalyst |
7️⃣ Common Reducing Agents (Lose Electrons):
| Chemical | Observation When Reacts |
|---|---|
| Zinc (Zn), Iron (Fe) | Forms positive ions |
| Hydrogen (H₂) | Reduces metal oxides |
| Carbon (C) | Reduces metal oxides in blast furnace |
| Sulfur dioxide (SO₂) | Reduces potassium manganate |
| Iodide ions (I⁻) | Turns purple KMnO₄ colourless |
8️⃣ How to Identify Oxidizing and Reducing Agents in a Reaction
🔍 Steps:
- Assign oxidation numbers to all relevant elements.
- Identify which substance is oxidized (oxidation number ↑) and which is reduced (oxidation number ↓).
- The substance that gets oxidized is the reducing agent.
- The substance that gets reduced is the oxidizing agent.
🔸 Example:
Zn+Cu2+→Zn2++Cu
- Zn: 0 → +2 (oxidized) → Reducing agent
- Cu²⁺: +2 → 0 (reduced) → Oxidizing agent
9️⃣ Tests for Oxidizing and Reducing Agents
✅ Test for Oxidizing Agents
- Use potassium iodide (KI) solution
- Oxidizing agent oxidizes I⁻ to I₂
- Positive result: solution turns brown (due to iodine)
✅ Test for Reducing Agents
- Use acidified potassium manganate (VII) (purple)
- Reducing agent reduces Mn⁷⁺ to Mn²⁺
- Positive result: colour changes from purple to colourless
✅ Summary Table
| Concept | Oxidation | Reduction |
|---|---|---|
| Electrons | Loss | Gain |
| Oxygen | Gain | Loss |
| Hydrogen | Loss | Gain |
| Oxidation number | Increase | Decrease |
| Agent Type | Gains/Loses e⁻ | Gets… | Example |
|---|---|---|---|
| Oxidizing Agent | Gains e⁻ | Reduced | KMnO₄ |
| Reducing Agent | Loses e⁻ | Oxidized | Zn, H₂ |
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