Redox Reactions
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Direct answer
A redox reaction couples oxidation (loss of electrons, oxidation number rises) with reduction (gain of electrons, oxidation number falls). Because electrons transferred must balance exactly, the ion-electron method balances any redox equation: split into half-reactions, balance atoms, then balance electrons. Electrons transferred per formula unit define the n-factor, which underpins equivalent-mass stoichiometry, titrations and electrochemistry.
What you must remember
- Oxidation number rules: free elements 0; fluorine always -1; oxygen usually -2 but -1 in peroxides and +2 in OF2; hydrogen +1 with non-metals, -1 with metals; the sum over a neutral species is zero.
- Key half-reactions in acid: MnO4^- + 8H+ + 5e^- → Mn^2+ + 4H2O; Cr2O7^2- + 14H+ + 6e^- → 2Cr^3+ + 7H2O. For basic medium, neutralise H+ with OH^- into water.
- Balanced classics: 5Fe^2+ + MnO4^- + 8H+ → 5Fe^3+ + Mn^2+ + 4H2O.
- Disproportionation — one element both oxidised and reduced: 2H2O2 → 2H2O + O2; Cl2 + 2NaOH (cold) → NaCl + NaOCl + H2O, but hot concentrated alkali gives NaClO3; P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2.
- Oxidising power of halogens falls F2 > Cl2 > Br2 > I2, mirroring standard electrode potentials.
- Equivalent mass = molar mass / n-factor: n = 5 for MnO4^- in acid, 6 for Cr2O7^2-, 1 for Fe^2+ → Fe^3+.
- H2O2 acts as oxidant and reductant — oxygen sits at the intermediate state -1, free to fall to -2 or rise to 0.
Common confusion
The classic confusion is oxidation number versus valency: valency is combining capacity, always positive, while oxidation number is a formal count that can be negative or fractional (average iron in Fe3O4 is +8/3). Students miss disproportionation unless they check every element on both sides, misassign oxygen in peroxides and superoxides (KO2, where it is -1/2), and add H+ by habit when balancing in basic medium.
Exam-focused takeaway
JEE Main tests fast identification: oxidation numbers in unusual compounds, spotting disproportionation, one-step balancing and n-factor values feeding titration numericals. JEE Advanced disguises the same skills inside titration chains — permanganate against oxalate, dichromate against iron(II) — and comparative questions built on standard potentials. Write the half-reactions before touching stoichiometry; the electron count is the skeleton of every such problem.
Frequently asked questions
How do I balance equations in basic medium?
Balance as in acid, then add OH^- to both sides equal to the H+ present, combining H+ and OH^- into water.
What is disproportionation, with an example?
One element undergoing both oxidation and reduction, as when chlorine in cold alkali gives chloride (-1) and hypochlorite (+1) from Cl2 (0).
What is oxygen's oxidation state in H2O2, KO2 and OF2?
-1 in peroxides, -1/2 in the superoxide KO2, and +2 in OF2 because fluorine is more electronegative.
What is the n-factor of KMnO4?
It depends on medium: 5 in acid (to Mn^2+), 3 in neutral to weakly alkaline (to MnO2), 1 in strongly alkaline (to MnO4^2-).
Why can H2O2 act as both oxidant and reductant?
Oxygen at -1 can drop to -2 (oxidising others) or rise to 0 in O2 (reducing them).
Which halogen is the strongest oxidising agent?
Fluorine, with the highest standard reduction potential among the halogens.