The chemical equation for the combustion of a fossil fuel, such as methane (CH4), with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O) is:

CH4 + 2O2 → CO2 + 2H2O

In this equation, one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. This reaction releases energy in the form of heat and light, which is why fossil fuels are used as a source of energy.