What energy does the oxygen release?

Oxyhydrogen is an explosive mixture of gaseous hydrogen and oxygen. When it comes into contact with an open flame (embers or sparks) the so-called oxyhydrogen reaction occurs. In air under atmospheric pressure, the volume fraction of hydrogen must be between 4 and 77%. If these limit values ​​are fallen below or exceeded, an explosion no longer occurs. A controlled combustion creates an oxyhydrogen flame. The greatest reaction occurs with a ratio of two hydrogen and one oxygen. If the oxygen is removed from the air, a volume ratio of around 2: 5 leads to a particularly large explosion.

Oxyhydrogen reaction

The Oxyhydrogen reaction is the explosive (exothermic) reaction of hydrogen with oxygen. It is a form of combustion (oxidation).

The reaction equation is:

It is a highly branched chain reaction (chain branching explosion) with the participation of hydrogen, oxygen and hydroxyl radicals as chain carriers.

H2 → H + H (chain start)
H + O2 → OH + O
O + H2 → OH + H
OH + H2 → H2O + H
and other reactions

The reaction product is water.

The energy released per molar formula conversion is ΔrH0 = -571.6 kJ / mol. This changes the enthalpyH for one mole of the resulting water by -286 kJ / mol.

Hydrogen peroxide is also formed as a side reaction according to:

H2 + O2 → H2O2

In the mitochondria of living cells, the end oxidation in complex IV in the respiratory chain leads to an analogous but strictly controlled exergonic reaction (biological oxyhydrogen reaction), which is used to generate energy in the cell, i.e. to form ATP molecules:

O2 + 4 e + 4 H.+ → 2 H2O

The free enthalpy ΔG ° 'of the reaction results from its redox potential (+0.5 V) and under physiological conditions (pH 7) is -193 kJ / mol.

The same reaction also takes place in the fuel cell. In the construction of fuel cells, attempts are made to use the energy released during the oxyhydrogen reaction (enthalpy difference) here, more precisely: free enthalpy or Gibbs energy ΔG, without causing an explosion. The enthalpy that is released is partly released as electrical current and partly as heat. However, the reaction in the fuel cell takes place slowly and in a controlled manner.

Oxyhydrogen can be produced by the electrolytic decomposition of water, e.g. B. by passing electricity through a dilute acid or alkali, or by thermal decomposition of water.

A small amount of oxyhydrogen is constantly leaking from batteries. If many batteries are stored in one room (e.g. in workshops), the concentration of the gas can be harmful to health and highly explosive. Therefore, such rooms must be under constant control of the oxyhydrogen concentration.

Known oxyhydrogen explosions are the explosion of the reactor core in the Chernobyl disaster and the false start of the Challenger space shuttle.

Oxyhydrogen sample

With the term Oxyhydrogen sample In chemistry, this is the term used to denote evidence of hydrogen.

For the detection, the gas to be checked is usually held in a test tube with the opening facing downwards (so that hydrogen cannot escape because of its lower density than air) near an ignition source (Bunsen burner, lighter).

Case 1: The gas collected is pure hydrogen. There is a calm combustion or possibly a weak deflagration (negative oxyhydrogen gas sample).

Case 2: The gas collected is a mixture of hydrogen and oxygen (oxyhydrogen). The combustion takes place with a whistling noise (positive detonating gas sample).

Due to the different noises of pure hydrogen and the mixture of hydrogen and oxygen, the oxyhydrogen sample is also used to check the purity of a hydrogen atmosphere in order to avoid an explosion in a closed vessel.

Similar to oxyhydrogen, the chlorinated oxyhydrogen explodes.


  • Jander, G., Spandau, H. (1987): Short textbook of inorganic and general chemistry, Springer-Verlag Berlin Heidelberg.

Categories: Mixture of substances | Gas | Chemical analysis method | Hydrogen chemistry