Energy is conserved in chemical reactions, so the total amount of energy in the universe at the end of a reaction is the same as it was before the reaction.
When a chemical reaction happens, energy is transferred to or from the surroundings. When energy is transferred to the surroundings, this is called an exothermic reaction, and the temperature of the surroundings increases. Examples of exothermic reactions include:
Everyday uses of exothermic reactions include self-heating cans and hand warmers.
When energy is taken in from the surroundings, this is called an endothermic reaction and the temperature of the surroundings decreases. Examples of endothermic reactions include:
Everyday uses of endothermic reactions include instant ice packs which can be used to treat sports injuries.
The slideshow describes an exothermic reaction between dilute sodium hydroxide and hydrochloric acid and an endothermic reaction between sodium carbonate and ethanoic acid.
In endothermic reactions, energy enters. In exothermic reactions, energy exits.