What defines an exothermic reaction?

An exothermic reaction is defined as a chemical reaction that releases energy, usually in the form of heat.

In more detail, an exothermic reaction is a type of chemical reaction where the energy of the products is less than the energy of the reactants. This difference in energy is released, typically as heat, but sometimes as light or sound. The term 'exothermic' comes from the Greek words 'exo', meaning 'out', and 'thermic', meaning 'heat'. So, in essence, an exothermic reaction is one where heat is 'sent out' or released.

During an exothermic reaction, the bonds in the reactants are broken and new bonds are formed to create the products. The energy required to break the bonds in the reactants is less than the energy released when the new bonds are formed in the products. This excess energy is then released into the surroundings, causing an increase in temperature. This is why exothermic reactions often feel hot to the touch.

Common examples of exothermic reactions include combustion (like burning wood or petrol), neutralisation reactions (where an acid and a base react to form water and a salt), and many oxidation reactions. In fact, most chemical reactions are exothermic. This is because the formation of new bonds releases more energy than is needed to break the original bonds, leading to a net release of energy.

In terms of energy diagrams, exothermic reactions are represented by a downward slope, showing that the energy of the products is less than the energy of the reactants. The difference in energy levels is known as the 'enthalpy change' and is usually negative for exothermic reactions, indicating a release of energy.

Understanding exothermic reactions is crucial in chemistry as it helps us predict the energy changes that occur during chemical reactions, which is important in many areas, from industrial processes to understanding natural phenomena.