Spontaneity is determined by Gibbs free energy. When Gibbs free energy is less than zero, the reaction is considered exergonic and will occur spontaneously. When Gibbs free energy is greater than zero, the reaction is considered endergonic and will not occur spontaneously.

Exothermic reactions cause a release of enthalpy (heat) from the system and endothermic reactions require and input of energy to initiate the reaction. Gibbs free energy is determined by enthalpy, entropy, and temperature. A negative enthalpy, high temperature, and high entropy will cause the reaction to be more spontaneous, but must all come together to contribute. Simply because a reaction is exothermic does not meant that is increases entropy enough to be spontaneous.

An exothermic reaction releases energy into the environment. In contrast, endothermic reactions require an input of energy to initiate the reaction.

Forming a bond is always an exothermic reaction because it releases energy. Breaking a bond always requires energy, and is thus an endothermic process. Synthesis, decomposition, and single-replacement reactions can be either exothermic or endothermic, and cannot be determined without more information.