In order to determine if a reaction is endothermic or exothermic you should ask yourself if energy is being transferred from the surroundings to the system or from the system to the surroundings.

Endothermic reactions absorb energy from the surroundings. A chemical ice pack that gets cold when two components are mixed and is then used to treat an injury is an example of an endothermic reaction. It cools your injured body part because heat is being absorbed from your body by the reaction system. The energy gained by an endothermic reaction system is equal to the energy lost by the surroundings.

Exothermic reactions release energy to the surroundings in the form of heat, light or electrical energy. Combustion is a common exothermic reaction, giving off heat and light. The energy gained by the surroundings as a result of an exothermic reaction is equal to the energy lost by the system. From a mathematical perspective you can determine if a reaction is endothermic or exothermic by the enthalpy change.  This is the energy transferred in a reaction and is designated ∆H. You can determine whether a reaction is endothermic or exothermic by the sign of ∆H:

If ∆H is positive, the reaction is endothermic.

If ∆H is negative, the reaction is exothermic.