4.3 Redox Equations — IAL Edexcel AS Chemistry

এই টপিকটা basically electron transfer নিয়ে। Redox reaction মানে একসাথে oxidation আর reduction হবে—একটা ছাড়া আরেকটা possible না।

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Oxidation & Reduction (Exam Definitions)

• Oxidation → Electron হারানো, oxidation number বাড়ে
• Reduction → Electron পাওয়া, oxidation number কমে

মনে রাখার ট্রিক: OIL RIG
Oxidation Is Loss, Reduction Is Gain (of electrons)

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Oxidation Number Rules (Must Memorise)

• Free element → 0
• Group 1 metals → +1
• Group 2 metals → +2
• Oxygen → −2 (peroxide এ −1)
• Hydrogen → +1 (metal hydride এ −1)
• Compound এর total oxidation number = 0
• Ion এর total oxidation number = charge

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Method: Identify Oxidation & Reduction

1. Reactant আর product এ oxidation number বের করো
2. Compare করো
3. Oxidation number বাড়লে → oxidation
4. Oxidation number কমলে → reduction

Exam-এ marks mostly এখান থেকেই আসে, তাই steps skip করো না।

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Worked Example

Reaction:

$$\mathrm{Zn}+{\mathrm{Cu}}^2\to {\mathrm{Zn}}^2+\mathrm{Cu}$$

Oxidation numbers:

• Zn: 0 → +2 (oxidation)
• Cu: +2 → 0 (reduction)

মানে Zn electron হারাচ্ছে, Cu electron নিচ্ছে—simple।

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Half-Equations (Very Important)

Oxidation half-equation:

$$\mathrm{Zn}\to {\mathrm{Zn}}^2+2e-$$

Reduction half-equation:

$${\mathrm{Cu}}^2+2e-\to \mathrm{Cu}$$

Electron balance না করলে exam-এ mark কাটা যাবে।

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Oxidising & Reducing Agent

• Oxidising agent → নিজে reduce হয়, অন্যকে oxidise করে
• Reducing agent → নিজে oxidise হয়, অন্যকে reduce করে

এই reaction-এ Cu²⁺ oxidising agent আর Zn reducing agent।

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MCQ Practice (AS Exam Style)

Q1. Which species is oxidised in the reaction?

$$\mathrm{Mg}+{\mathrm{Fe}}^2\to {\mathrm{Mg}}^2+\mathrm{Fe}$$

A) Mg
B) Fe²⁺
C) Fe
D) Mg²⁺

Answer: A (Mg oxidation number 0 → +2)

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Exam-Style Structured Question

Q. Consider the following reaction:

$$\mathrm{Al}+{\mathrm{Fe}}^3\to {\mathrm{Al}}^3+\mathrm{Fe}$$

(a) Identify the species that is oxidised.
(b) Write the oxidation half-equation.
(c) Identify the reducing agent.

Worked Answer

(a) Al is oxidised (0 → +3)

(b) Oxidation half-equation:

$$\mathrm{Al}\to {\mathrm{Al}}^3+3e-$$

(c) Reducing agent = Al

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Final Exam Tip

Redox question দেখলেই আগে oxidation number লেখো, তারপর electron loss/gain দেখো। Guess করলে চলবে না—logic দেখাতে হবে।