Redox reactions

Oxidation and reduction

Older wording: oxidation = gain of O / loss of H; reduction = reverse. Electron wording is cleaner for Class 11: oxidation = loss of electrons; reduction = gain of electrons.

They always happen together. That pair is a redox reaction.

The species that is oxidised is the reducing agent. The species that is reduced is the oxidising agent.

Zn + Cu²⁺ → Zn²⁺ + Cu: Zn is oxidised (reducing agent); Cu²⁺ is reduced (oxidising agent).

Oxidation number rules

Element alone (Na, O₂, Cl₂): 0.

Monoatomic ion: charge on the ion (Na⁺ → +1, Cl^- → −1).

F in compounds: −1. O usually −2; in peroxides (H₂O₂) −1; in OF₂, O is +2.

H usually +1; in metal hydrides (NaH) −1.

Alkali metals in compounds +1; alkaline earth +2.

Sum of oxidation numbers in a neutral molecule = 0. In a polyatomic ion = ion charge.

Common redox types

Combination: A + B → AB (e.g. Na + Cl₂ → NaCl).

Decomposition: AB → A + B (e.g. 2HgO → 2Hg + O₂).

Displacement: more reactive metal pushes out a less reactive one from its salt (Zn + CuSO₄).

Disproportionation: same element is oxidised and reduced in one go (2H₂O₂ → 2H₂O + O₂).

Balancing — oxidation number method

Write the skeleton. Assign oxidation numbers. Find what goes up and what goes down.

Multiply species so the total increase in ox. no. equals the total decrease.

Balance other atoms (not H/O yet). Add H₂O for oxygen. In acid medium, add H⁺ for hydrogen. In base, balance H with H₂O/OH^- as your teacher taught.

Check atoms and total charge on both sides.

Balancing — ion–electron (half-reaction) method

Split into oxidation half and reduction half.

Balance the element that changes ox. no., then add electrons to fix charge, then O with H₂O, then H with H⁺ (acid) or H₂O/OH^- (base).

Scale halves so electrons cancel. Add them. Recheck charge.

Link to KMnO₄ titration

In acid medium, MnO₄^- → Mn²⁺ (Mn from +7 to +2). That is a strong oxidising change; five electrons per Mn in the half-reaction you learn in school.

Oxalate / oxalic acid is oxidised to CO₂. The balanced equation in your practical file is the one your teacher and manual use — do not invent coefficients here.

Viva angle: KMnO₄ is self-indicating in many acid–oxalate titrations because the first lasting pink means excess MnO₄^-.

Galvanic cell in one breath

Chemical energy → electrical energy via a spontaneous redox split into two half-cells.

Oxidation at the anode, reduction at the cathode. Salt bridge keeps charge balance.

Cell notation sketch: Zn | Zn²⁺ || Cu²⁺ | Cu. Standard hydrogen electrode is defined as E° = 0.00 V by convention.