A + B = C,

the A and B molecules react to form the product C. For a reaction to occur, an A and a B molecule must collide in the solution. The rate at which A and B react to form C is dependent on the concentrations of A and of B. The rate at which A and B react to form C is depentdant on the concentrations of A and of B. If the concentration of A is doubled, twice as many collisions take place and the rate (speed) of the reaction will be doubled. Boubling the concentraion of B will also double the rate. The rate therefore is proportional to the product of the concentration of A and B.

rate_f = k_f[A][B]

As A and B react the concentrations decrease and the rate slows down. On the other hand the product C, which has been formed, begins to react to form A and B. This rate is given by the equation

rate_r=k_r[C]

Since the amount of C is increasing, rate_r is increasing. The decrease of rate_f and the increase of rate_r continues until the two rates become equal. When the forward rate is equal to the reverse rate, the reaction is said to be at equilibrium.

Setting the rates equal and rearranging the equations, the following expression is obtained.

k_f[A][B] = k_r[C]

k_{f = [C] = K}
k_{r [A][B]}

This ratio has been found to be a constant under constant pressure and temperature conditions for many processes. For a system of reactants and products at equilibrium, the quanity is called the equilibrium constant (K).