Energy is used to break the bonds of the reactants.

Energy is released when the bonds of the products are formed.

Since more energy is absorbed when the bonds are broken than is released when new bonds are formed, the reaction is endothermic, and the temperature goes down.

Energy is used to break the bonds of the reactants.

Energy is released when the bonds of the products are formed.

Since more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants, the reaction is exothermic, and the temperature goes up.