What causes some reactions to be exothermic while others are endothermic?

When a reaction takes place there is a change in the
reactants, this could be either in terms of a change in phase, or a change in
composition. The change alters the inter-molecular and intra-molecular
forces.

The total energy that substances have which
includes kinetic energy that decides the physical state they are in and the potential
energy which is present as inter-molecular and intra-molecular forces is called the
enthalpy. If the reaction results in the reactants having a lower enthalpy than the
products the reaction is endothermic. An example of this would be the change of ice to
water. This requires heat from the environment as water has a greater enthalpy than ice.
Another example is the conversion of nitrogen and oxygen to nitrogen
oxide.

On the other hand, if the products have a lower
enthalpy than the reactants, the reaction is exothermic. For example, when hydrogen
burns, the hydrogen and oxygen which combine to give water have a higher enthalpy than
water. The decrease in enthalpy is the heat released during the
reaction.