Exothermic reaction is a process that releases energy to the surroundings, usually in the form of heat. The energy can also be released in other forms such as sound, light etc. Because the energy is released during the reaction, the products contain less energy than the reactants. Therefore, the enthalpy change (∆H) becomes negative. Energy is released during bond formation. If the total bond formation energy is higher than bond breaking energy during the reaction, then it is exothermic. If the energy is released as heat, the surrounding temperature goes up, so sometimes the reaction can be explosive. Exothermic reactions are spontaneous. Thus, an outside energy supply is not necessary for exothermic reactions, because they produce the required energy as the reaction proceeds. However, to start the reaction, an initial energy supply may be necessary. If this released energy can be captured, lot of useful work can be done. For an example, the energy released from the combustion of fuels can be taken to operate a vehicle or a machine. Moreover, all combustion reactions are exothermic.
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