In chemistry, there are two types of reactions involving a redistribution of energy: exothermic and endothermic reactions. The concept of exothermic and endothermic reactions deals with the laws of thermodynamics, particularly the first law which states that energy cannot be created or destroyed, only converted or transferred. The difference between exothermic and endothermic reactions lies in where the energy in a reaction goes.
In an exothermic chemical reaction, energy or heat is released. As an exothermic reaction occurs, its energy is released into its surrounding environment. The products of an exothermic reaction have less heat than the reactants. The majority of reactions that occur in everyday situations are exothermic in nature.
A few examples of exothermic chemical reactions include liquid water freezing to form ice, water vapor condensing into dew or rain drops, and the oxidation of a metal forming rust. Explosions caused by weapons or engines are strong exothermic reactions.
In an endothermic chemical reaction, energy or heat is absorbed. Over the course of an endothermic reaction, the reacting system takes energy from its surroundings to produce a reaction. The products of an endothermic reaction have more heat than the reactants.
Some examples of endothermic chemical reactions include ice cubes melting into liquid water, liquid water evaporating and becoming water vapor, and a cake baking in an oven. Water's states of matter are the clearest way to compare exothermic and endothermic reactions. As water moves from a solid to a liquid or from a liquid to a gas, the process is endothermic; when water condenses from a gas to a liquid or when liquid water freezes, the reaction is exothermic.
It should be noted that while exothermic reactions release energy and endothermic reactions absorb energy, most reactions involve both the loss and gain of energy. The key is the net result. Consider a reaction in which 100 kilojoules of energy are released and 50 kilojoules are absorbed. The net result is a 50 kilojoule release, meaning the reaction is exothermic in nature.
Think about the example of ice cubes sitting in a glass of water. The ice and the liquid water are constantly exchanging molecules, meaning some water molecules are freezing while some ice molecules are melting. Assuming the reaction is occurring at room temperature, the rate of melting exceeds the rate of freezing. The ice will eventually melt which means that the net reaction was endothermic, but the reaction involved both absorption and release of energy.
The difference between exothermic and endothermic chemical reactions lies in the distribution of energy. Reactions can involve both the release and absorption of energy, but at the end there will either be a net loss or a net gain. In an exothermic reaction, there is a net loss and energy is released. In an endothermic reaction, there is a net gain and energy is absorbed.