# Hydrogen Peroxide

> Hydrogen peroxide for JEE Chemistry with open-book structure, volume-strength arithmetic, oxidising and reducing reactions, storage and manufacture status.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/hydrogen-peroxide
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Hydrogen Peroxide", PrepElephant, https://prepelephant.com/topics/jee/chemistry/hydrogen-peroxide

## Direct answer

Hydrogen peroxide sits at oxygen's odd oxidation state of −1, able to climb to 0 (acting as reductant, releasing O2) or fall to −2 (acting as oxidant, becoming water) — and to do both at once by disproportionation. The molecule is an open book: two O–H planes twisted to a dihedral angle of 111.5° in the gas phase and about 90° in the solid, with the O–O bond near 145 pm. Commercial strength is quoted in volumes: 20-volume H2O2 means one litre liberates twenty litres of O2 at STP, which converts to 6.07 per cent w/v, 1.79 molar and 3.57 normal.

## What you must remember

- **Structure numbers:** skewed open-book geometry, dihedral 111.5° gas versus 90° solid; O–O about 145 pm; extensive hydrogen bonding makes the pure liquid denser and far more viscous than water, boiling near 423 K.
- **Volume-strength conversions:** molarity = V/11.2, normality = V/5.6, per cent w/v = 0.3036 × V — so 20 volume is 1.79 M, 3.57 N, 6.07%; the 30% laboratory grade runs near 100 volume.
- **Disproportionation:** 2H2O2 → 2H2O + O2, catalysed by light, MnO2, alkali and rough metal surfaces — hence storage in wax-lined amber bottles, cool and dark, with stabilisers.
- **As oxidant (best in acid):** liberates I2 from KI, blackens→ whitens PbS to PbSO4 (PbS + 4H2O2 → PbSO4 + 4H2O, the painting-restoration reaction), oxidises Fe2+ to Fe3+; E°(H2O2/H2O) = +1.77 V in acid against about +0.87 V in alkali.
- **As reductant (against stronger oxidants):** decolourises acidified permanganate (5H2O2 + 2MnO4− + 6H+ → 5O2 + 2Mn2+ + 8H2O) and reduces chlorine to chloride; the signal is oxygen gas leaving.
- **Preparations:** laboratory BaO2·8H2O with cold dilute H2SO4; industrial electrolysis of 50% sulphuric acid through the peroxodisulphate route, and the 2-ethylanthraquinol auto-oxidation cycle used in modern plants.
- **Uses:** rocket-propellant oxidant, bleaching of hair, textiles and paper pulp, dilute antiseptic (perhydrol is the concentrated grade), and pollution-control oxidation of effluents.
- **Redox bookkeeping rule:** when H2O2 oxidises something it becomes water; when it reduces something it becomes O2 — the single sentence that answers half the questions.

## Decoding 20-volume strength

The label is a stoichiometry problem in disguise. Twenty litres of O2 at STP is 20/22.4 = 0.893 mol; the disproportionation 2H2O2 → 2H2O + O2 demands two hydrogen peroxides per oxygen, so the litre holds 1.786 mol H2O2 — 1.79 M, weighing 60.7 g, hence 6.07 per cent w/v, and with two electrons transferred per molecule, 3.57 N. Every other grade scales linearly, which makes molarity-from-volume a ten-second division. Then the redox logic: bubble hydrogen peroxide chemistry through one reaction pair and both exits appear. With acidified KI, peroxide oxygen falls from −1 to −2 (water formed) and iodide rises to iodine — starch turns blue. With acidified permanganate, that same oxygen climbs from −1 to 0 and streams off as O2 while manganese falls — the pink vanishes. One reagent, two fates, decided by the opponent's electrode potential.

## Where H2O2 stands in the syllabus

The dedicated hydrogen chapter was struck off the JEE Main syllabus in the 2023-24 rationalisation, and detailed manufacture processes sit outside the current Main scope; what survives in practice is H2O2 as a redox cameo — iodide oxidations, permanganate titrations, effluent chemistry — while JEE Advanced retains hydrogen-family chemistry and keeps a qualitative window on preparation and properties. Main-only aspirants therefore meet this page mostly through redox numericals and volume-strength one-liners; Advanced candidates should hold the structure, the conversions and the oxidant-versus-reductant discrimination. That discrimination is exactly where marks die: asked what becomes of H2O2 when it oxidises lead sulphide, candidates answer oxygen — wrong, it is reduced to water; oxygen is released only in its reducing role. For the record, pure H2O2 is a pale blue, syrupy liquid while its dilute solutions appear colourless — a colour pair worth one mark somewhere.

## Frequently asked questions

### What does 20-volume hydrogen peroxide mean?

One litre of it fully decomposes to liberate twenty litres of oxygen at STP — equivalently 6.07 per cent w/v, 1.79 molar and 3.57 normal.

### Why is H2O2 stored in dark, wax-lined bottles?

Disproportionation to water and oxygen is catalysed by light, alkali and metal-rich glass surfaces; wax-lined amber glass kept cool slows the loss.

### How does hydrogen peroxide restore discoloured oil paintings?

Black lead sulphide is oxidised to white lead sulphate: PbS + 4H2O2 → PbSO4 + 4H2O, returning the original tone without abrading the paint.

### Is H2O2 a stronger oxidant in acidic or alkaline medium?

Acidic — the H2O2/H2O couple runs at +1.77 V in acid against about +0.87 V in alkali, so acid conditions sharpen its oxidising power.

### Which process dominates modern industrial production?

Auto-oxidation of 2-ethylanthraquinol: hydrogenation to the anthraquinol, air oxidation regenerating the quinone and releasing H2O2, extracted with water.