Thermochemistry: Units of Energy
To measure the amount of energy involved in chemical reactions, we must have a unit of energy. Over the decades many units have emerged, some more useful than others. The SI energy unit is derived from the SI base units. The unit is the joule, J, and it is based upon 1 J = 1 kg m2/s2.
 
If you dropped 2 kg of butter on your foot from a height of about 10 cm, you would deliver about 1 J of energy to your foot. This is actually a very small amount of energy, especially when we consider chemicals reacting on a mole scale. A more common unit is the kilojoule, kJ, which is equal to 1000 J.
 
The Special Place of Heat
One of the important facts about our world is that all forms of energy can be converted quantitatively into heat. For example, the mechanical energy of a moving car is converted entirely into heat by the frictional action of the brakes, and the brake shoes and drums become very hot indeed. When a current of electrons is directed into a poor conductor, something with high resistance such as the heating element in a toaster, the electrical energy changes into mostly heat with some light. The full convertibility of energy in other forms into heat gives us a way to measure the other kinds of energy, and the measurement of heat is relatively simple.
 
The traditional unit of heat energy was the calorie, abbreviated cal, and was originally defined as the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. The equivalent amount in Joules is 4.184 J.

ie. 1 cal = 4.184 J