As an instance of adsorption, chemisorption follows the adsorption process.  The first stage is for the adsorbate particle to come into contact with the surface.  The particle needs to be trapped onto the surface by not possessing enough energy to leave the gas-surface potential well.  If it elastically collides with the surface, then it would return to the bulk gas.  If it loses enough momentum through an inelastic collision, then it "sticks" onto the surface, forming a precursor state bonded to the surface by weak forces, similar to physisorption.  The particle diffuses on the surface until it finds a deep chemisorption potential well.  Then it reacts with the surface or simply desorbs after enough energy and time.
The reaction with the surface is dependent on the chemical species involved.  Applying the Gibbs energy equation for reactions:
General thermodynamics states that for spontaneous reactions at constant temperature and pressure, the change in free energy should be negative.  Since a free particle is restrained to a surface, and unless the surface atom is highly mobile, entropy is lowered.  This means that the enthalpy term must be negative, implying an exothermic reaction.
Physisorption is given as a Lennard-Jones potential and chemisorption is given as a Morse potential.  There exists a point of crossover between the physisorption and chemisorption, meaning a point of transfer.  It can occur above or below the zero-energy line (with a difference in the Morse potential, a), representing an activation energy requirement or lack of.  Most simple gases on clean metal surfaces lack the activation energy requirement.