A chemical change is one which involves a shift in chemical properties of the reactants.
Basically the reactants reach their activation energy and then start reacting to produce products whose chemical properties differ from those of the reactants.
Examples are photosynthesis, or the making of tea,
An example of reaction involving a chemical change is
H2 + Cl2 = 2HCl
A chemical change involves changes in the internal energy, entropy, reversible work, kinetic and potential energies.
A chemical reaction may be reversible, reversibility rates are defined by the reaction constants for the reaction