Definition of Endothermic and Exothermic Processes

The term endothermic refers to any process or reaction that absorbs or requires heat from its surroundings.
Conversely, an exothermic process or reaction is one that releases heat into its surroundings.
In an exothermic reaction, the energy of the products is less than the energy of the reactants. Meanwhile, in an endothermic reaction, the energy of the products is greater than the energy of the reactants.

Distinguishing Between Endothermic and Exothermic Reactions

A key characteristic of endothermic reactions is that they result in a cooling effect. This is because these processes absorb heat from the environment. Making ice cubes and photosynthesis are examples of endothermic processes.
In contrast, exothermic reactions are associated with a warming effect as they release heat to the environment. Examples of exothermic processes include combustion reactions and rusting of iron.

Using energy-level diagrams, endothermic and exothermic processes can be visually represented.
In an energy-level diagram, the vertical axis represents the energy of the reactants and products, while the horizontal axis typically represents the course of the reaction or process.
For an exothermic process, the energy of the reactants is higher than that of the products, making the energy-level line slope downwards.
For an endothermic process, the energy of the reactants is lower than that of the products, making the energy-level line slope upwards.

Understanding endothermic and exothermic reactions is important in many sectors such as chemistry, life science, geology, atmospheric science, and many industrial applications.
Exothermic reactions are utilised in heat packs, which release heat when certain chemicals react together. These packs are commonly used to warm up hands during cold months or as a heat source during camping.
Endothermic reactions, on the other hand, are used in cold packs used for treating injuries. The pack gets cold when certain chemicals absorb heat from the surroundings as they react together.

The nature (endothermic or exothermic) of a process or reaction does not necessarily dictate its feasibility or spontaneity. The Gibbs free energy, which takes into account both the enthalpy change and entropy change, is used to determine whether a process or reaction can happen spontaneously.
Exothermic reactions tend to be more spontaneous than endothermic ones; however, this is not always the case. For instance, ice melting into water is an endothermic process, but it occurs spontaneously at temperatures above 0°C.