The adjacent figure is not drawn to scale but depicts what happens when a free Oxygen atom combines with free Hydrogen atoms. Mole is a measure of the mass involved in chemical reactions, since it is not possible to calculate energy changes on atomic levels. We will study more about moles later in the chapter. There are two basic type of reactions, when energy changes are considered. They are exothermic reactions and endothermic reactions.
Exothermic reactions release energy as the reaction proceeds.
For endothermic reactions, the energy has to be supplied to make the reaction possible. Thus, exothermic reactions will occur spontaneously when the reaction substances are brought close together. In case of endothermic reactions, the spontaneity is not present as heat has to be supplied. Examples given below will make these concepts clear. When 1 mole of C and 1 mole of H 2 interact, they produce 1 mole of CO 2 molecule and give off 395 kJ of energy.