Kinetics :  Collision Theory Model
Collision Theory Model - Assumes that the rate of any step in a reaction depends on the frequency of collisions between the particles involved in that step.  This can be used to explain the rates of chemical reactions

The Kinetic Molecular Theory (the theory that heat is associated with the thermal motion of particles.  In the kinetic theory, heat is not conserved---an inexhaustible amount of heat can be created by doing work on a system.) assumes that the number of collisions per second depends on the number of particles per liter.

Therefore, in a single step reaction, the concentrations of the of both reactants should be directly proportional to the formation of the product.