Much more important in chemistry is the potential energy known as electrostatic potential energy, Eel, arising from the interactions between charged particles.
The SI unit for energy is the joule: J = 1 kg-m2/s2
A 2 kg mass moving at 1 m/s possesses a kinetic energy of 1 J:
Ek = 1/2 mv2 = 1/2 (2 kg) (1 m/s)2 = 1 kg-m2/s2 = 1 J
Since a joule is a small amount of energy, the unit kilojoules, kJ, is often used when describing chemical reactions.
Traditionally, energy changes in chemical reactions have been expressed in calories, a non-SI unit.
A calorie was originally defined as the amount of heat energy required to raise the temperature of 1 g of water from 14.5 oC to 15.5 oC. It is now defined in terms of the joule:
1 cal = 4.184 J (exactly)
A related energy unit used in nutrition is the nutritional Calorie (note the capital letter).
1 Cal = 1000 cal = 1 kcal