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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NEET Chemistry Mastery System

Study Redox Reactions Like a Topper

This chapter is not just for reading. Use it as a repeatable study workflow: concept map, formula conditions, easy examples, trap check, and mixed practice. That is the structure students need when moving from NCERT comfort to NEET-speed MCQs.

1. Build the Formula Map

Write every formula with units and conditions. Chemistry questions usually punish students who remember a formula but forget when it is valid.

2. Convert to the Core Quantity

For physical chemistry, convert mass, volume, concentration, or particles into moles first. For inorganic and organic chemistry, convert the question into trend, mechanism, exception, or named reaction.

3. Solve With Units Visible

Keep units beside every number. Unit tracking catches wrong molarity volume conversion, wrong gas constant, wrong oxidation number, and wrong equivalent factor.

4. Finish With the NEET Trap Check

Before selecting an option, check sign, units, approximation, limiting condition, exception, and whether the question asks atoms, molecules, moles, mass, or volume.

NCERT to MCQ Flow

1Definition

2Formula or trend

3Worked example

4NEET trap

5Timed practice

Easy Example Starters

Mole bridge

If a question gives mass, first write moles = given mass / molar mass. Most stoichiometry starts from that bridge.

Unit discipline

If volume is in mL for molarity, convert to litre before using M = n/V. A 250 mL solution is 0.25 L.

Trend questions

For periodic or inorganic trend MCQs, decide the direction first, then check exceptions instead of memorising isolated facts.

Organic logic

For reaction questions, identify the functional group, reagent role, attacking species, and major product stability.

Chemistry Mistake Clinic

Using atomic mass when the question needs molecular or formula mass.

Forgetting that molarity depends on solution volume, while molality depends on solvent mass.

Cancelling coefficients without converting the given data into moles.

Choosing a memorised exception before checking the basic trend.

Ignoring n-factor changes between acid-base, precipitation, and redox reactions.

Reading molecules as atoms in questions involving O2, N2, H2, P4, or S8.

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

Finished this topic?

Keep the practice loop moving

Move straight from chapter-wise questions into a subject test, then loop back into weaker areas instead of ending the session here.

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