By Clare Jackson | Updated Aug 30, 2022
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Hydrogen and oxygen combine explosively to form water, releasing a significant amount of heat.
At ambient temperature, hydrogen (H₂) and oxygen (O₂) gases coexist without reacting because their molecules lack sufficient kinetic energy to break existing bonds during collisions. The mixture is stable until an external energy source initiates the reaction.
Introducing a spark or heat raises the temperature of some molecules, increasing their collision energy. Once the energy exceeds the modest activation threshold for hydrogen (about 0.3 eV per bond), the H₂ and O₂ molecules form new covalent bonds, producing water.
Hydrogen and oxygen possess higher internal energy than the resulting water molecules. Consequently, the reaction releases energy as heat, light, and sound. The exothermic nature drives a chain reaction, rapidly propagating the combustion through the remaining mixture.
On an electronic level, hydrogen atoms share their single electrons to fill the inner shell of two electrons, while oxygen atoms share four electrons to complete their outer shell of eight. The most stable arrangement is achieved when two hydrogen atoms each share one electron with a single oxygen atom, forming the H₂O molecule. This realignment releases the energy difference that fuels the reaction.
The primary products are water (H₂O) and heat. The released heat can be harnessed for work—such as driving turbines—though the reaction is essentially irreversible under normal conditions.
