Why do some reactions release energy while others absorb energy?

A reaction in which energy is absorbedfrom the surroundings is called an endothermic reaction. In endothermicreactions the enthalpy of the products is greater than the enthalpy of the reactants. Because reactions releaseor absorb energy, they affect the temperature of their surroundings. If reactant molecules in a particular reaction have more energy than the product molecules, then energy is released to the surroundings in the form of heat and light. And when this happens, the reaction is usually called an exothermic reaction.

On the other hand, if the product molecules have more energy than the reactants, then the reaction absorbs energy from the surroundings. And when this happens, the reaction is usually called an endothermic reaction.