CID 784
H2O2
Hydrogen peroxide
- H 2
- O 2
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- IUPAC name
- hydrogen peroxide
- Molar mass
- 34.0140(24) g/mol
- Atoms
- 4
- SMILES
- OO
- InChIKey
- MHAJPDPJQMAIIY-UHFFFAOYSA-N
- PubChem CID
- 784
Left in its bottle, hydrogen peroxide slowly comes apart into water and oxygen; a relatively stable sample loses about 0.5 per cent a year. The 3 per cent household solution is mostly water, with H2O2 the working part. Made of hydrogen and oxygen alone, it has a molar mass of 34.014 g/mol. Pure, it is a colourless liquid that mixes with water in any proportion. OpenStax files it with the peroxides, compounds built around an oxygen–oxygen single bond, and describes them as strong oxidisers. In water it also behaves as a weak acid.
In concentrated grades the heat of the breakdown feeds back, because it runs faster in the warm. PubChem’s experimental data give a rate that climbs about 2.2 times per 10 °C rise between 20 and 100 °C. Light and warming speed it as well. The same data add agitation, rough surfaces, alkalis, finely divided metals and many organic solvents, while a mineral acid makes the liquid steadier.
Structure and bonding
The model is a short chain, H–O–O–H, and the readout gives 4 atoms, 3 single bonds. In Ball-and-stick, the two oxygens sit in the middle joined by one stick, each holding a hydrogen at its outer end. With two central atoms there is no one shape word, and OpenStax treats each central atom separately in such molecules. Labels prints O on the inner pair and H on the outer two.
Turned so that the O–O bond points straight at the viewer, the model shows its twist. The two hydrogens do not lie in one plane: in PubChem’s computed arrangement they stand about 94° apart around that bond, and each H–O–O angle measures about 96°. Those are calculated figures, not measurements on the real molecule. The Stick view shows the twist most plainly.
Space-filling merges the two oxygens into one lumpy core, a smaller hydrogen bulging from each end. Every bond drawn is single, and here the drawing matches the real bonding: OpenStax describes the oxygen–oxygen link of the peroxide ion as a covalent single bond. The model shows the neutral molecule.
Where it turns up
Small amounts of hydrogen peroxide gas occur naturally in the air. Indoors, US health data put household solutions at 3 to 9 per cent, kept for medical uses and to bleach clothes and hair. Household disinfecting uses 3 to 5 per cent, and some hair-bleaching solutions reach 10. Labels sometimes give strength in volumes instead: 10 volume is roughly 3 per cent by weight, 30 volume about 9 and 100 volume about 30.
A US consumer product database lists 704 products that contain it. The US Food and Drug Administration (FDA) affirms it as generally recognised as safe for adding directly to food. Haz-Map, a database of hazardous chemicals and occupational diseases, lists a use in photography. It even forms unbidden: a drop of water bursts into flame in fluorine gas, and hydrogen peroxide is among the products, alongside oxygen and ozone.
Why it matters
Industry uses it to bleach, to disinfect and to make other chemicals. A breakdown in PubChem’s data gives chemical synthesis 24 per cent of consumption and pulp and paper 23. Environmental uses such as water treatment take 18, and textiles 14. Other listed uses run from leather dyeing and semiconductor manufacture to an oxygen source in respiratory protective equipment. Reports to the US Environmental Protection Agency (EPA) put the country’s 2023 volume between 1 and 2.5 billion pounds.
Keeping the product intact is part of the chemistry. Solutions free of every contaminant are highly stable, but traces of iron, copper and other heavy metals catalyse the breakdown, speeding it without being used up. That is why makers usually add a little stabiliser, acetanilide or sodium stannate for example. The EPA’s Safer Choice programme gives hydrogen peroxide a green circle, verified as of low concern. PubChem’s manufacturing data call it an environmentally friendly oxidant, used in wastewater treatment.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | H2O2 | ||
| Molar mass | 34.0140(24) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Hydrogen atoms | 2 | ||
| Oxygen atoms | 2 | ||
| Bonds | 3 | ||
| PubChem molecular weight | 34.015 g/mol | g/mol | PubChem |
Safety
Hydrogen peroxide carries the signal word Danger under EU classification, as a strong oxidiser that is harmful if swallowed or inhaled and causes severe skin burns and eye damage. How much it irritates depends on concentration, from mild for household strengths to corrosive for concentrated grades, and it can set organic material alight on contact. The PubChem Laboratory Chemical Safety Summary and the NIOSH (US National Institute for Occupational Safety and Health) Pocket Guide entry are the authoritative sheets.
Questions
What shape is a hydrogen peroxide molecule?
It has no single shape word from VSEPR (valence shell electron pair repulsion), the model that predicts shape from the electron pairs around one central atom. Hydrogen peroxide is a chain, H–O–O–H, with two oxygens in the middle, so OpenStax's approach treats each oxygen on its own. In the 3D model the chain is twisted rather than flat, its hydrogens pointing different ways, in a computed arrangement from PubChem rather than a measured one.
Does hydrogen peroxide go bad over time?
Slowly, breaking down into water and oxygen. Light and warmth both hasten the loss, and the international chemical safety card for solutions stronger than 60 per cent advises keeping them in the dark in vented containers. Leaving a container open causes a different problem: water evaporates from the solution, which becomes more concentrated and a greater fire hazard.
What is the difference between 3 per cent hydrogen peroxide and stronger solutions?
Concentration changes both the job and the hazard. The 3 to 5 per cent strength used for household disinfecting is mildly irritating to skin and mucous membranes, according to the Haz-Map occupational health database. Some hair bleaches use 10 per cent, which Haz-Map calls strongly irritating and possibly corrosive, and solutions of 30 to 70 per cent are corrosive to skin. Commercial grades are marketed anywhere from 3 to 90 per cent by weight.
Is hydrogen peroxide an acid?
A weak one. Its pKa, the number chemists use to express how strong an acid is, stands at 11.62. Solutions of 35, 50, 70 and 90 per cent by weight have true pH values of 4.6, 4.3, 4.4 and 5.1 in PubChem's experimental data, so all four are mildly acidic. The 3D model draws the neutral molecule, with both hydrogens still attached to their oxygens.
Made of the same elements
- H2O Water 3 atoms, 2 single bonds, bent.
Sources
- PubChem CID 784, hydrogen peroxide
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for hydrogen peroxide (CID 784)
- NIOSH Pocket Guide to Chemical Hazards, Hydrogen peroxide
- OpenStax, Chemistry 2e, §18.9 Occurrence, Preparation, and Compounds of Oxygen
- OpenStax, Chemistry 2e, §7.6 Molecular Structure and Polarity
- OpenStax, Chemistry 2e, §18.11 Occurrence, Preparation, and Properties of Halogens