Exothermic and endothermic reactions cause energy level differences and therefore differences in enthalpy (ΔH), the sum of all potential and kinetic energies. ΔH is determined by the system, not the surrounding environment in a reaction. A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system.

A system of reactants that absorbs heat from the surroundings in an endothermic reaction has a positive  ΔH , because the enthalpy of the products is higher than the enthalpy of the reactants of the system.

We know that Gibbs Free Energy is used to determine whether a reaction is favored or disfavored. It is given by the equation: 

ΔG = ΔH – TΔS

Where ΔH is the enthalpy change, ΔS is the entropy change, and T is the temperature.

Entropy change (ΔS) – Change in entropy is the change in degree of randomness or disorder.

A system will tend towards maximum entropy.

If ΔH and ΔS are either both negative, or both positive, the thermodynamic favorability of the reaction can depend on the temperature.

Reactions can be ‘driven by enthalpy’ (where a very exothermic reaction (negative ΔH) overcomes a decrease in entropy) or ‘driven by entropy’ where an endothermic reaction occurs because of a highly positive ΔS.

Favorable Conditions – ΔH < 0 & ΔS > 0

Unfavorable Conditions – ΔH > 0 & ΔS < 0