NEET Chemistry - Chapter 6

Redox Reactions

Fresh NEET chemistry notes on oxidation number, redox definitions, balancing methods, n-factor, equivalent mass, and standard oxidising and reducing agents.

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Concept Block

1. Oxidation, Reduction, and Oxidation State Concepts

Every redox reaction involves simultaneous oxidation and reduction — electrons lost by one species are gained by another.

Key Definitions

Oxidation: loss of electrons / increase in oxidation number
Reduction: gain of electrons / decrease in oxidation number
Oxidising agent (OA): accepts electrons, gets reduced itself
Reducing agent (RA): donates electrons, gets oxidised itself

Oxidation numbers are assigned using a set of priority rules:

Rule	Assignment
Free element	0 always
Fluorine	−1 always in compounds
Oxygen	−2 (except: −1 in peroxides, −½ in superoxides, +2 in OF $_2$ )
Hydrogen	+1 in compounds (−1 in metal hydrides like NaH)
Sum rule	Sum of ONs = 0 for neutral compound, = ion charge for polyatomic ion

Quick practice: S in H

_2

_4

2(+1)+x+4(-2)=0 \Rightarrow x=+6

. Cr in K

_2

_2

_7

2(+1)+2x+7(-2)=0 \Rightarrow x=+6

. Mn in KMnO

_4

x=+7

Concept Block

2. Oxidation Number Assignment — Worked Examples and NEET Patterns

Fast oxidation-number assignment is the first skill needed in every redox question. NEET repeatedly tests unusual cases.

Compound	Element	Oxidation State	Working
H $_2$ O $_2$	O	−1	Peroxide linkage O−O
OF $_2$	O	+2	F is always −1; O adjusts
NaH	H	−1	Metal hydride
Fe $_3$ O $_4$	Fe	+8/3 (avg)	Mixed oxide: Fe $^{2+}$ + 2Fe $^{3+}$
S $_4$ O $_6^{2-}$	S	+2.5 (avg)	Thiosulfate structure

Disproportionation: An element simultaneously oxidises and reduces itself. Example: H $_2$ O $_2$ → H $_2$ O + O $_2$ (O goes from −1 to −2 and 0). Cl $_2$ + NaOH → NaCl + NaOCl (Cl: 0 → −1 and +1).

NEET trap: In CH

_4

, C has oxidation state −4 (4 H atoms each at +1, total +4; carbon must be −4 for neutral molecule). In CO

_2

, C is +4. Don't assume carbon is always some fixed state.

Concept Block

3. Balancing Redox Equations: Oxidation Number Method and Ion-Electron Method

Two methods exist. Use oxidation number method for molecular equations. Use ion-electron (half-reaction) method for ionic equations in acidic or basic medium.

Ion-Electron Method — Step by Step

Split into two half-reactions (oxidation half and reduction half).
Balance atoms other than O and H.
In acidic medium: balance O by adding H $_2$ O, then balance H by adding H $^+$ .
In basic medium: balance O by adding H $_2$ O, then balance H by adding OH $^-$ to the opposite side.
Balance charge by adding electrons to the more positive side.
Multiply half-reactions so electrons cancel, then add them.

Example — acidic medium: MnO

_4^-

+ Fe

^{2+}

→ Mn

^{2+}

+ Fe

^{3+}

Reduction half: MnO

_4^-

+ 8H

^+

+ 5e

^-

→ Mn

^{2+}

+ 4H

_2

O
Oxidation half: Fe

^{2+}

→ Fe

^{3+}

+ e

^-

(×5)
Net: MnO

_4^-

+ 5Fe

^{2+}

+ 8H

^+

→ Mn

^{2+}

+ 5Fe

^{3+}

+ 4H

_2

Concept Block

4. n-Factor, Equivalent Mass, and Titration Calculations

The n-factor is the number of electrons transferred per formula unit in a redox reaction (or the number of H $^+$ donated/accepted in acid-base). It determines equivalents and normality.

\text{Equivalent mass}=\frac{\text{Molar mass}}{n\text{-factor}},\quad N=M\times n\text{-factor}

N_1V_1=N_2V_2\quad(\text{at equivalence point})

Oxidising agent	Medium	Change in Mn/Cr/etc.	n-factor
KMnO $_4$	Acidic	Mn: +7 → +2	5
KMnO $_4$	Neutral / Basic	Mn: +7 → +4	3
K $_2$ Cr $_2$ O $_7$	Acidic	Cr: +6 → +3 (×2 Cr)	6
H $_2$ O $_2$	Acidic (OA)	O: −1 → −2	2
Na $_2$ S $_2$ O $_3$ (vs I $_2$ )	Neutral	S: +2.5 → +5 (×2 S? net 2e $^-$ per molecule)	1

Concept Block

5. Top NEET Redox Traps and Identity of Oxidising/Reducing Agents

Beyond calculation, NEET asks direct conceptual identification of oxidising and reducing agents, and their relative strengths.

In a redox reaction, the stronger oxidising agent has a greater tendency to get reduced. The product cation has weaker reducing power than the original metal (this is the basis of the electrochemical series).

Common Oxidising and Reducing Agents in NEET

Strong Oxidising Agents:

KMnO $_4$ (all media)
K $_2$ Cr $_2$ O $_7$ (acidic)
HNO $_3$ (conc. and dilute)
H $_2$ O $_2$ , O $_3$ , Cl $_2$

Strong Reducing Agents:

Metals (Na, Mg, Zn, Fe)
H $_2$ , CO, H $_2$ S
SnCl $_2$ , FeSO $_4$
SO $_2$ (reducing in acidic)

Top 3 NEET traps:

KMnO $_4$ n-factor = 5 in acidic medium, 3 in neutral/basic — ALWAYS check the medium.
H $_2$ O $_2$ can be both an OA (→ H $_2$ O) and a RA (→ O $_2$ ) depending on what it reacts with.
Disproportionation — one substance acts as both OA and RA simultaneously.

Practice Tests

5 Chapter Tests of 25 Questions Each

Each test is original, NEET-aligned, and answer-backed. Use them as sectional revision instead of a single long mock so your weak subtopics become easier to identify quickly.

Test 1: Redox Basics

Oxidation, reduction, oxidation numbers, and oxidising vs reducing agents.

Test 2: Balancing and n-Factor

Oxidation-number method, ion-electron method, equivalent mass, and medium-based changes.

Test 3: Agent Logic and Oxidation States

Species identification, disproportionation, and redox interpretation.

Test 4: Equivalent Concepts

n-factor numericals, equivalents, and redox stoichiometry.

Test 5: Mixed NEET Drill

Integrated redox practice across concepts, balancing, and numerical shortcuts.

Open Practice Tests

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