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When the reactants are heated, the average kinetic energy of the molecules increases. This means that more molecules are moving faster and hitting each other with more energy. If more molecules hit each other with enough energy to react, then the rate of the reaction increases.
Most chemical reactions involve the breaking and formation of chemical bonds. It takes energy to break a chemical bond but energy is released when chemical bonds are formed. If more energy is released than consumed, then the chemical reaction evolves heat and is said to be exothermic. On the other hand if the reaction consumes more energy than it releases then the reaction absorbs heat and is said to be endothermic. Most reactions are exothermic. The amount of heat generated by a reaction can be quantified. The quantity that specifies the amount of heat is known as the enthalpy . If is positive then the reaction is endothermic and if it is negative the reaction is exothermic.