In chemistry, chemical reaction is a change in which one or more chemical elements, molecules, species, or compounds (the reactants) form new molecules, species, or compounds (the products).  In short, a chemical reaction is a process that results in the interconversion of chemical species. 
Why chemical reactions occur?
In 1854 and 1864, chemists Julius Thomsen and Marcellin Berthelot, respectively, which stated, via their Berthelot-Thompson principle, that the release of heat is what explains why chemical reactions go, the greater the heat release, the stronger the reaction; this is known as the thermal theory of affinity.
The Berthelot-Thompson “heat as the driving force” theory of chemical reactions, however, was eventually shown to be incorrect, specifically at all temperatures above absolute zero, being that entropy, otherwise known as the transformation content of a reaction process, aka the second law of thermodynamics, plays a significant role chemical reactions. It was German physicist Hermann Helmholtz’ famous 1882 paper “On the Thermodynamics of Chemical Processes” that showed that heat production is not the reason chemical reactions occur, but rather the value of the free energy, which determines in what sense the affinities between the reactants are active and will react:
“Given the unlimited validity of Clausius' law it would then be the value of the free energy, not that of the total energy resulting from heat production, which determines in which sense the chemical affinity can be active.”
“All reactions advance only in the sense of a diminution of free energy, i.e. only in the sense of the affinity.”
“A chemical transformation will occur if in the occurrence work will be done. That is ‘why’ it occurs, and if no work can be done by the chemical substances by reacting with each other they will not react of themselves.”
“A chemical reaction may thus be considered as a ‘spontaneous’ irreversible process driven toward equilibrium by its thermodynamic affinity, i.e. by the difference between the chemical potentials of the products and the reactants.”— Alfonso Liquori (1980), “Cell Growth as an Autocatalytic Relaxation Process” (Ѻ)(Ѻ)
“Chemical reactions weren’t supposed to write letters.”— Katie M. John (c.2010), Publication (Ѻ)
“If love is a chemical reaction, then you are what I need to balance the chemical equation.”— Edewede Oriwoh (c.2012) (Ѻ)