What conditions must be met in order for a reaction to be considered exothermic?
An exothermic chemical reaction is one that releases energy as heat because the combined strength of the chemical bonds in the products is stronger than the bonds in the reactants.
The potential energy and kinetic energy of electrons in a strong chemical bond (like the N-N triple bond in nitrogen gas) is lower than in a weak chemical bond (like the Br-Br single bond in bromine gas). When a chemical reaction takes place that results in stronger chemical bonds in the products compared to reactants, the total energy of the electrons is lowered. Overall, energy must be conserved, so the excess energy of the products is usually released as heat. This is an exothermic reaction. A common example is combustion of gasoline with oxygen to form water and carbon dioxide (both of which have unusually strong bonds).
In less common circumstances, the reverse can happen. If the bonds in the products are weaker than in the reactants, then the reaction is endothermic, and this requires that heat be taken up from the surroundings, leaving the products at lower temperature than the reactants.