Oxidation Numbers and Redox Agents

• Define oxidation and reduction in terms of electron transfer (OIL RIG)
• Apply all oxidation number rules systematically including tricky cases
• Assign oxidation numbers to transition metals in complex compounds
• Identify oxidising and reducing agents in any reaction
• Recognise disproportionation reactions
• Understand variable oxidation states of Mn, Cr, and Fe

Level 1 — Foundations

Oxidation and Reduction:

• Oxidation = Loss of electrons (OIL)
• Reduction = Gain of electrons (RIG)
• OIL RIG: Oxidation Is Loss, Reduction Is Gain

Oxidising agent: accepts electrons → itself gets reduced → oxidation number decreases Reducing agent: donates electrons → itself gets oxidised → oxidation number increases

Basic oxidation number rules:

1. Pure element: oxidation number = 0 (e.g., O₂, Fe, Na all = 0)
2. Monoatomic ion: equals ionic charge (e.g., Na⁺ = +1, Fe³⁺ = +3)
3. O in most compounds: −2
4. H in most compounds: +1
5. Sum of ON in neutral molecule = 0
6. Sum of ON in polyatomic ion = charge of ion

Level 2 — JEE depth

Complete rule set for tricky cases:

Variable oxidation states — transition metals:

Manganese (Mn): +2 (MnO, MnSO₄), +4 (MnO₂), +6 (MnO₄²⁻ — manganate), +7 (MnO₄⁻ — permanganate) KMnO₄: K = +1, O = −2×4 = −8 → Mn = 7−(+1) = +7 (actually: +1 + Mn + 4(−2) = 0 → Mn = +7) ✓

Chromium (Cr): +2 (CrO, CrCl₂), +3 (Cr₂O₃, CrCl₃), +6 (CrO₄²⁻, Cr₂O₇²⁻, K₂CrO₄) K₂Cr₂O₇ (dichromate): 2(+1) + 2(Cr) + 7(−2) = 0 → 2 + 2Cr − 14 = 0 → Cr = +6

Iron (Fe): +2 (FeO, FeCl₂, FeSO₄), +3 (Fe₂O₃, FeCl₃, Fe₂(SO₄)₃) Fe₃O₄ is a mixed oxide: contains Fe²⁺ and Fe³⁺; average ON = +8/3

Identifying oxidising and reducing agents:

1. Assign ON to all atoms in reactants and products
2. Species whose ON increases → oxidised → is the reducing agent
3. Species whose ON decreases → reduced → is the oxidising agent

Example: 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺

• Fe: +3 → +2 (decreases) → Fe³⁺ is the oxidising agent
• Sn: +2 → +4 (increases) → Sn²⁺ is the reducing agent

Disproportionation reactions: One element simultaneously undergoes both oxidation and reduction. Example: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O Cl₂ (ON = 0): one Cl goes to −1 (in NaCl, reduced) and one Cl goes to +1 (in NaOCl, oxidised)

Another example: 2H₂O₂ → 2H₂O + O₂ O in H₂O₂ (ON = −1): goes to −2 in H₂O (reduced) and 0 in O₂ (oxidised)

JEE trap: In H₂O₂, oxygen is −1 (peroxide), NOT −2. Students often write ON(O) = −2 for peroxides — this is wrong.

JEE trap: In S₄O₆²⁻ (tetrathionate), not all S atoms have the same ON. Use average: 2(−1) + 4×ON(S) + 6(−2) = −2 → ON(S) = +2.5 (fractional ON is allowed).

Find oxidation number of Cr in K₂Cr₂O₇ and Mn in KMnO₄

K₂Cr₂O₇:
Let ON of Cr = x
2(+1) + 2x + 7(−2) = 0
2 + 2x − 14 = 0
2x = 12
x = +6

Cr in K₂Cr₂O₇ (potassium dichromate) = +6

KMnO₄:
Let ON of Mn = y
(+1) + y + 4(−2) = 0
1 + y − 8 = 0
y = +7

Mn in KMnO₄ (potassium permanganate) = +7

Identify oxidising and reducing agents in: 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺

Assign oxidation numbers:
Reactants:  Fe³⁺ (Fe = +3),  Sn²⁺ (Sn = +2)
Products:   Fe²⁺ (Fe = +2),  Sn⁴⁺ (Sn = +4)

Track changes:
Fe: +3 → +2  (decrease of 1 per Fe, total 2 for 2 Fe atoms) → REDUCTION
Sn: +2 → +4  (increase of 2) → OXIDATION

Electrons transferred: 2 electrons (2×1 from Fe = 1×2 from Sn ✓)

Fe³⁺ is the OXIDISING AGENT (it oxidises Sn²⁺ by accepting electrons)
Sn²⁺ is the REDUCING AGENT (it reduces Fe³⁺ by donating electrons)

• Q1: Find ON of S in H₂SO₄, SO₂, S₂O₃²⁻, and S₄O₆²⁻.
• Q2: Find ON of N in NH₃, NO, NO₂, HNO₃, and N₂H₄.
• Q3: Identify the disproportionation reaction: (a) 2H₂O₂ → 2H₂O + O₂, (b) Fe + 2HCl → FeCl₂ + H₂.
• Q4: Find ON of Fe in Fe₃O₄. Is this a whole number?
• Stretch: Q5: In the reaction 3Cl₂ + 6NaOH (hot conc.) → 5NaCl + NaClO₃ + 3H₂O, assign all oxidation numbers and show this is a disproportionation. Calculate electrons transferred per Cl₂ molecule.

NCERT Chapter 8 link: Chapter 8 (Class 11) — "Redox Reactions" covers classical and electron-transfer definitions, oxidation number concept and rules, redox reactions in terms of ON change, oxidising/reducing agents, and disproportionation. Pay special attention to Table 8.1 (rules) and the extensive worked examples.

Exam connections: JEE Mains tests ON assignments especially for Mn, Cr, S, and N in unusual compounds; identification of oxidising/reducing agents; and recognition of disproportionation. JEE Advanced asks for multi-step ON tracking in complex reactions and electron-change balancing.

Study strategy: Build a master table of ON for Mn, Cr, Fe, S, N, Cl in 5 common compounds each. Practise assigning ON in 20 compounds before moving to identifying redox agents. Once ON tracking is automatic, balancing redox equations (next topic) becomes much easier.

• Use the platform-native live simulation or PhET-style tool for this topic.
• Mirror / body / home activity: observe iron rusting (Fe → Fe²⁺/Fe³⁺) or a copper coin turning green — photograph, identify the oxidation number change, and name the oxidising agent (O₂).
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.