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Exothermic vs. Endothermic
All chemical processes are either endothermic or exothermic. Endothermic reactions require a net input of energy --- in other words, they absorb energy, usually in the form of heat, from their surroundings. Exothermic reactions lead to a net release of energy, usually in the form of heat but sometimes as light, sound or electricity.
Combustion of Hydrogen: Reactants
Neither hydrogen or oxygen exists as individual atoms under naturally occurring conditions. Instead, both form diatomic gases, meaning gases in which each molecule contains two atoms. Gaseous hydrogen has a chemical formula of H2, while gaseous oxygen has a chemical formula of O2.
Activation Energy
Whether a reaction is endothermic or exothermic, most reactions have an "activation energy." This term refers to the amount of energy required to get a reaction started. Even through the combustion of hydrogen releases a great deal of energy overall, it does not occur without an initial input of energy. In other words, hydrogen gas and oxygen gas will not react unless they are exposed to a spark, a flame or to high temperatures.
Combustion of Hydrogen: Products
Once a spark or flame jump starts the reaction between O2 and H2, the reaction proceeds spontaneously. The only physical product is water, or H2O, so the overall chemical equation can be written as 2H2 + O2 = 2H2O, meaning each molecule of diatomic oxygen reacts with two molecules of diatomic hydrogen to produce two molecules of water. Because the reaction also releases energy, it could also be written as: 2H2 + O2 = 2H2O + Energy.