Heat comes from the release of energy, like a campfire. The reactants, such as the wood in the campfire, have a certain amount of energy stored in their chemical bonds. This energy can be released when these materials react with the oxygen in the air (combustion) and are converted to the combustion products (usually carbon dioxide and water). Chemists use a quantity called the heat of formation as a measure of the energy stored in these compounds. The energy change associated with a combustion reaction is the difference between the energy (heat of formation) of the products and the energy (heat of formation) of the reactants.
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