Endothermic proceses
Energy is absorbed by the substances from the environmentThe entalphy change is positiveThe energy of the products is higher than the energy of the reactants.The temperature of the surroundings decreases.More energy is required to break the chemical bonds of the reactants than what is released from the formation of the chemical bonds of the products.
Exothermic proceses
Energy is released from the substances to the environmentThe entalphy change is negativeThe energy of the products is lower than the energy of the reactants.The temperature of the surroundings increase..More energy is released when the chemical bonds of the products are formed than what is needed to break the chemical bonds of the reactants.
Explanation:
By definition an exothermic processes are those in which the system releases energy to the enviroment, while endothermic processes are those in which the system absorbs energy from the environment.
That change in energy is generally measured as heat.
Then an exothermic process releases heat and and endothermic process absorbs heat.
From that, as a first consequence, in an exothermic process the final substances (products in the case of a chemical reaction) ends with lower energy and lower temperature than the initial substances (reactants in the case of a chemical reaction).
The heat content of the substances is usually measured as enthalpy, then, for a chemical reaction you can write th is equation for the enthalpy change:
Δ H rxn = ΔH products - ΔH reactants
Thus, for an exothermic reaction ΔH products < ΔH reactants ⇒ ΔH < 0, whilced for an endothermic reaction ΔH products > ΔH reactants ⇒ ΔH > 0.
The difference in energy between the products and the reactants is a result of the chemical potential energy of the substances, which is the energy stored in the chemical bonds. Then, in an endothermic reaction more energy is needed to break the chemical bonds of the reactants than what is released from the formation of the chemical bonds of the products. Of course the opposite is true: in an exothermic reaction, energy is released because it is required less energy for the formation of the new bonds of the products than what is needed to break the bonds of the reactants.
