The chemical equation for the reaction of oxygen (O2) with hydrogen (H2) to produce water (H2O) is:

2H2 + O2 -> 2H2O

In this reaction, two molecules of hydrogen gas (H2) react with one molecule of oxygen gas (O2) to produce two molecules of water (H2O). This reaction is highly exothermic, meaning that it releases a large amount of energy in the form of heat and light. This reaction is also the basis for many combustion reactions, such as the burning of fossil fuels.