Academia Aeternum
तमसो मा ज्योतिर्गमय
Wisdom · Knowledge · Eternity
Chapter 07
The Dance of Electrons
Every oxidation demands a reduction. In chemistry, electrons are the currency of transformation.
9 key topics in this chapter — each linked to NCERT exercises and exam questions.
Everything you need to master this chapter — from concept notes to previous year questions.
Essential expressions — understand derivations, not just results.
Important points to remember — curated from CBSE Board and entrance exam question patterns.
CBSE always asks "calculate the oxidation state of X in compound Y" — apply the sum rule systematically.
Know all exceptions to oxidation number rules: O in Na₂O₂ = −1; O in OF₂ = +2; H in NaH = −1.
Balancing by half-reaction method: separate into oxidation and reduction half-reactions, balance atoms and charge separately, then combine.
Disproportionation = same element is both oxidised and reduced. Common example: Cl₂ → Cl⁻ + ClO⁻ in basic medium.
For CBSE 5-mark balancing questions: always state the change in oxidation number before balancing electron transfer.
Electrode processes: oxidation occurs at anode (−ve in electrolytic cell); reduction at cathode.
Targeted tips for JEE Main, JEE Advanced, NEET, and BITSAT.
JEE tests complex oxidation state assignments in polyoxoanions like Cr₂O₇²⁻, MnO₄⁻, S₂O₃²⁻ — work methodically using the sum rule.
Disproportionation vs comproportionation identification is a quick JEE MCQ — one element, two different oxidation states in products (or reactants).
NEET tests biological redox: NAD⁺/NADH, FAD/FADH₂ in respiration. Know that NADH is the reduced form (gains electrons).
BITSAT gives rapid oxidation state questions — memorise Mn (+7 in KMnO₄, +4 in MnO₂, +2 in MnSO₄) and Cr (+6 in K₂Cr₂O₇, +3 in Cr₂O₃).
Students consistently lose marks on these — know them before your exam.
Confusing the oxidising agent (gets reduced) with the reducing agent (gets oxidised).
Forgetting the oxidation number of H = −1 in metal hydrides like NaH, KH, CaH₂.
Ignoring the sign of the ion when applying the sum rule (e.g., MnO₄⁻: sum = −1, not 0).
Not multiplying half-reactions by the LCM to equalise electrons before adding them.
The non-negotiable concepts every student must carry out of this chapter.
Oxidation = loss of electrons = increase in oxidation number.
Reduction = gain of electrons = decrease in oxidation number.
The oxidising agent is reduced; the reducing agent is oxidised.
Oxidation number of a free element is always zero.
The algebraic sum of oxidation numbers in a neutral compound is zero; in an ion it equals the charge.
Both the oxidation number method and the half-reaction (ion-electron) method must be mastered.
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