Note, however, most reactions do not run to completion. And that's because before one of the reactants is used up, equilibrium is established. Now, suppose the following reaction x plus Y react to form our product XY. And suppose this reaction achieves equilibrium before one of the reactants is used up. What that means is that the forward reaction rate is equal to the reverse reaction rate. So the rate at which our reactants react to produce our product is the same as the rate at which the product associates into our reactants.