energy of reaction

Energy released or absorbed during a chemical reaction, part of the energy transfer that takes place. In a chemical equation it may be represented by the symbol ΔH. In a chemical reaction, the energy stored in the reacting molecules is rarely the same as that stored in the product molecules. Depending on which is the greater, energy is either released (an exothermic reaction) or absorbed (an endothermic reaction) from the surroundings. The amount of energy released or absorbed by the quantities of substances represented by the chemical equation is the energy of reaction. The principle that the total amount of energy in a given chemical reaction stays the same is known as conservation of energy.

Examples of chemical reactions that release large amounts of heat energy include the combustion of fuels and explosives. For example, when a mixture of hydrogen gas and oxygen gas in a glass jar is ignited by a spark, rapid and large amounts of heat energy are released shattering the glass jar in the process. The spark supplies the activation energy required for bond breaking to occur in the reactants, enabling the reaction to begin.

2H₂ + O₂ → 2H₂O + 115.6 kJ of energy

Some oxidation reactions take place very slowly such as the rusting of iron. The heat of the chemical reaction is immediately dissipated, and the activation energy required for rusting to take place is very large. The chemical equation describing the rusting of iron is:

4Fe + 3O₂ → 2Fe₂O₃ + 380 kJ of energy