Many reactions are reversible. Reversible reactions are reactions in which there are both forward and backward reactions. Consider an experiment in which two reactants have been mixed. At first, the reaction proceeds with considerable rate in the forward direction (forward reaction favored). Before the reaction goes on to completion, the backward reaction takes place. Then again the forward reaction takes place, followed by the backward reaction and so on. These uneven back-and-forth directional changes take place until the reaction mixture reaches the equilibrium. At the equilibrium, the rate will be the same for both forward and backward reactions.
The Equilibrium ConstantWe already talked about the rate of a reaction and its general expression. For reactions involving simple one-step mechanisms, we can easily write the rates of forward and backward reactions. Examine a one-step hypothetical reaction represented by the equation (balanced) given below:
The equilibrium constant of a reversible reaction is equal to the ratio of the product of the concentrations of the products raised to their corresponding coefficients, to the product of the concentrations of the reactants raised to their corresponding coefficients. Keep in mind that the coefficients are taken from the balanced equation. To further clarify this concept, take a look at the mathematical expression for equilibrium constant (Kc) shown below:
The best way to find out the equilibrium constant of a particular reaction at a given condition is to do it experimentally. Equilibrium constant depends on temperature, and hence it differs with change in temperature. You should also ask yourself this question. Does the initial concentration of the reactants dictate the equilibrium constant? The answer is no.
We can also explore the meaning of equilibrium constant in terms of forward and backward reactions. A small value (less than one) for the equilibrium constant indicates that the forward reaction is not favored. A value greater than one for the equilibrium constant indicates that the forward reaction is favored.