Physical transformations and chemical reactions are almost always involved in heat loss or gain. Heat is one of the most common forms of energy known.

THE thermochemistry It is a part of chemistry that studies the amounts of heat released or absorbed during chemical reactions.

Most chemical reactions involve loss or gain of heat (energy).

See the table below for the types of reactions with heat loss or gain.

Reactions that release energy

Energy absorbing reactions

Coal burning

Food cooking

Burning candle

Photosynthesis of plants, the sun provides energy

Chemical reaction in a pile

Violent knock that starts detonation of an explosive

Burning of gasoline in the car

Electric bumper chrome plating

Physical transformations are also accompanied by heat, as occurs with changes in physical states of matter.

heat absorption


heat release

When the substance changes from solid to liquid to gaseous physical state, heat absorption occurs.

When the substance changes from gaseous to liquid state and then to solid state, heat is released.

This energy that comes from the chemical reactions is due to the rearrangement of the chemical bonds of the reagents into products. This stored energy is the ENTALPIA (H). It is the energy that comes from inside the molecule.

In chemical reactions, it is not necessary to calculate enthalpy. We should usually calculate the enthalpy variation (ΔH). Enthalpy variation is the difference between product enthalpy and reagent enthalpy.

Heat unit

Chemical reactions can be of two types:

- ENDOTHERMAL: absorb heat (+)
- EXothermic: release heat (-)

We will study each one below.