Why hydrogen peroxide fizzes on a cut
Decomposition of hydrogen peroxide · Decomposition · Redox · Catalysis
The foam on a scraped knee is oxygen gas, produced by one of the fastest enzymes in your body.
Reactants approach
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ΔH = -189 kJ · exothermic, releases heat
ΔH per 2 mol H2O2.
Meet the molecules
- Reactantdissolved in water
H2O2
hydrogen peroxide
- Productliquid
H2O
water
- Productgas
O2
oxygen
What you notice
Dab 3% hydrogen peroxide on a scraped knee and it foams white, while the same liquid sits calmly on unbroken skin or in its brown bottle. The foam is oxygen gas. Blood and damaged cells contain catalase, one of the fastest enzymes known: a single molecule can break down millions of H2O2 molecules per second. Cells make catalase because peroxide forms as a by-product of normal metabolism and would damage them if it built up. Doctors now advise against using peroxide to clean wounds, because it harms healthy tissue as well as bacteria.
Try it
Drop a small piece of raw potato into a spoonful of 3% hydrogen peroxide and compare it with a piece of boiled potato; cooking destroys the catalase, so the boiled piece barely fizzes.
Safety: 3% peroxide is safe on skin, but solutions stronger than about 10% burn skin and eyes.
Step by step
- 1
Slow on its own
H2O2 decomposes by itself, but slowly: the uncatalysed reaction has an activation energy of about 75 kJ/mol, so a bottle lasts for months, helped by brown plastic that blocks light.
- 2
Catalase grabs a peroxide
In the enzyme's active site, an iron atom held in a heme group binds one H2O2 and takes an oxygen atom from it, releasing a water molecule.
- 3
A second peroxide finishes the job
A second H2O2 reacts with that iron–oxygen unit, releasing O2 and another water and restoring the iron for the next cycle.
- 4
Bubbles and warmth
The O2 forms the foam. Every 2 mol of H2O2 releases about 189 kJ, so concentrated mixtures get noticeably warm.