Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis. Via scanning tunneling microscopy, it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the solid–gas interface are in some cases related to catalysis. Simple decomposition If a reaction occurs through these steps: A + S ⇌ AS → Products where A is the reactant and S is an adsorption site on the surface and the respective rate constants for the adsorption, desorption and reaction are k1, k−1 and k2, then the global reaction rate is: where: r is the rate, mol·m−2·s−1 is the concentration of adsorbate, mol·m−3 is the surface concentration of occupied sites, mol·m−2 is the concentration of all sites (occupied or not), mol·m−2 is the surface coverage, (i.e. ) defined as the fraction of sites which are occupied, which is dimensionless is time, s is the rate constant for the surface reaction, s−1. is the rate constant for surface adsorption, m3·mol−1·s−1 is the rate constant for surface desorption, s−1 is highly related to the total surface area of the adsorbent: the greater the surface area, the more sites and the faster the reaction. This is the reason why heterogeneous catalysts are usually chosen to have great surface areas (in the order of a hundred m2/gram) If we apply the steady state approximation to AS, then: so and The result is equivalent to the Michaelis–Menten kinetics of reactions catalyzed at a site on an enzyme. The rate equation is complex, and the reaction order is not clear. In experimental work, usually two extreme cases are looked for in order to prove the mechanism. In them, the rate-determining step can be: Limiting step: adsorption/desorption The order respect to A is 1. Examples of this mechanism are N2O on gold and HI on platinum Limiting step: reaction of adsorbed species The last expression is the Langmuir isotherm for the surface coverage. The adsorption equilibrium constant , and the numerator and denominator have each been divided by . The overall reaction rate becomes . Depending on the concentration of the reactant the rate changes: Low concentrations, then , that is to say