Exothermic reactions are reactions that release energy into the environment in the form of heat. Exothermic reactions feel warm or hot or may even be explosive.
More energy is released making chemical bonds than is used breaking them. In an exothermic reaction, the enthalpy change has a negative value: ΔH < 0
Terms Related to Exothermic Reaction
Exergonic Reaction – A reaction that releases energy in any form (e.g., light, sound, heat). An exothermic reaction is a type of exergonic reaction.
Endothermic Reaction – An endothermic reaction is the opposite of an exothermic reaction. Heat is absorbed in an endothermic reaction. In an exothermic reaction, the enthalpy change has a positive value: ΔH > 0
Exothermic reactions may be written as chemical reactions. Exothermic processes may be written in reaction format, but are more general and often include a combination of chemical reactions or involve nuclear reactions. Here are examples of exothermic reactions and processes:
Examples of Exothermic Reactions
Examples of exothermic reactions include combustion reactions, oxidation reactions (such as rusting), and phase transitions from the liquid to solid state.
- any combustion reaction
- a neutralization reaction
- rusting of iron (rust steel wool with vinegar)
- the thermite reaction
- reaction between water and calcium chloride
- reaction between sodium sulfite and bleach (dilute sodium hypochlorite)
- reaction between potassium permanganate and glycerol
- reaction between sodium and chlorine to make sodium chloride (table salt)
- reaction between water and any strong acid
- reaction between water and any anhydrous salt
- dissolving laundry detergent in water
- adding water to anhydrous copper(II) sulfate
- crystallization of sodium acetate or “hot ice” (try the demonstration)
- freezing water into ice cubes
- snow forming inside clouds
- rain forming from water vapor in clouds
- formation of ion pairs
- making a gas molecule from atoms
- burning sugar
- dehydrating sugar with sulfuric acid
- a burning candle
- nuclear fission
Simple Exothermic Reactions To Try
Probably the easiest, most readily available reaction is the one between laundry detergent and water. The next time you do laundry, put a bit of powdered detergent in your hand. Add water. Feel the heat? Be sure to rinse your hands thoroughly, since detergent dries out skin.
Another simple example of an exothermic reaction is combustion, such as lighting a candle. An initial input of energy causes oxygen and wax react to produce carbon dioxide, water, and heat.