Question

Why do chemical reaction involve absorption/evolution of heat

Answer 1

As it is a form of energy, heat plays multiple important roles in chemical reactions. In some cases, reactions need heat to begin; for example, a camp fire requires a match and kindling to get it started. Reactions consume heat or produce it depending on the chemicals involved. Heat also determines the speed at which reactions occur and whether they proceed in a forward or reverse direction.Many familiar chemical reactions, such as the burning of coal, rusting and exploding gunpowder, give off heat; chemists call these reactions exothermic. Because reactions liberate heat, they increase the ambient temperature. Other reactions, such as combining nitrogen and oxygen to form nitric oxide, take in heat, reducing the ambient temperature. As they remove heat from their environment, these reactions are endothermic. Many reactions both consume and produce heat, but if the net result is to give off heat, the reaction is exothermic; otherwise, it is endothermic.Heat energy manifests itself as the random jostling motions of molecules in matter; as the temperature of a substance increases, its molecules vibrate and bounce with more energy and at faster speeds. At certain temperatures, vibrations overcome the forces that make molecules stick to one another, causing solids to melt into liquids, and liquids to boil into gases. Gases respond to heat with an increase in pressure as molecules collide against their container with greater force.