The concentrations of reactants and products play a very important role in determining not only the rates of the forward and reverse reactions but also the direction in which the net reaction proceeds—whether products or reactants are accumulating at a given time.
Consider the following reversible reaction that has reached chemical equilibrium:
If at this point we increase the concentration of one of the reactants, the rate of the forward reaction will increase and lead to increased product formation. In contrast, increasing the concentration of one of the product molecules will drive the reaction in the reverse direction, increasing the formation of reactants. The direction in which the net reaction is proceeding can also be altered by decreasing the concentration of one of the participants. Thus, decreasing the concentration of one of the products drives the net reaction in the forward direction since it decreases the rate of the reverse reaction without changing the rate of the forward reaction.
These effects of reaction and product concentrations on the direction in which the net reaction proceeds are known as the law of mass action. Mass action is often a major determining factor controlling the direction in which metabolic pathways proceed since reactions in the body seldom come to chemical equilibrium as new reactant molecules are being added and product molecules are simultaneously being removed by other reactions.
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