In thermodynamics, the term endothermic process describes a process or
reaction in which the system absorbs energy from its surroundings;
usually, but not always, in the form of heat.
These are reactions that take in energy from the surroundings.
The energy is usually transferred as heat energy, causing the
reaction mixture and its surroundings to get colder. The
temperature decrease can also be detected using a thermometer.
Some examples of endothermic reactions are electrolysis, the reaction between ethanoic acid and
sodium carbonate and the thermal decomposition of calcium carbonate in a blast furnace.
Endothermic reactions can be used for everyday purposes. For example, certain sports injury cold packs
use endothermic reactions.
In reversible reactions, the reaction in one direction will be
exothermic and the reaction in the other direction will be
The decomposition of ammonium chloride is a reversible reaction. Ammonium chloride
decomposes when it is heated, so the forward reaction is endothermic - energy must be
transferred from the surroundings for it to happen. The backward reaction is exothermic
- energy is transferred to the surroundings when it happens.
The reaction between anhydrous copper sulfate and water is reversible. Water is driven off from
hydrated copper sulfate when it is heated, so the forward reaction is endothermic - energy must be
transferred from the surroundings for it to happen.
The backward reaction is exothermic - energy is transferred to the surroundings when it happens.
This is easily observed. When water is added to anhydrous copper sulfate, enough heat is released to
make the water bubble and boil.
The concept is frequently applied in physical sciences to, for example,
chemical reactions, where thermal energy (heat) is converted to
chemical bond energy.
The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word
"endon" (ἔνδον) meaning "within" and the root "therm" (θερμ-) meaning "hot."
The large majority of chemical reactions are exothermic.
Exothermic reactions give out heat to their surroundings.
Breaking bonds (overcoming the force of attraction) requires energy. You have to
put heat in - it is endothermic. This is why melting and boiling are endothermic.
If you have to put in more energy than you
get out, then the reaction is endothermic.
An endothermic reaction occurs when a greater amount of energy is
required to break the existing bonds in the reactants than is released when
the new bonds form in the products.
In other words, this means an endothermic reaction requires or takes in
energy in order for it to proceed. The energy that is required by the reaction
to move forward can be in many forms, but it is typically in the form of