Question

Explain how will you predict the direction of an equilibrium reaction.

Answer 1

From the knowledge of the equilibrium constant, it is possible to predict the direction in which the net reaction is taking place for a given concentration or partial pressure of reactants and products.

Consider a general homogeneous reversible reaction,

xA + yB ⇌ lC + mD

For the above reaction under non-equilibrium conditions, reaction quotient ‘Q’ is defined as the ratio of the product of active masses of reaction products raised to the respective stoichiometric coefficients in the balanced chemical equation to that of the reactants.

Under non – equilibrium conditions, the reaction quotient Q can be calculated using the following expression.

Q = `(["C"]^1["D"]^"m")/(["A"]^"x"["B"]^"y")`

As the reaction proceeds, there is a continuous change in the concentration of reactants and products and also the Q value until the reaction reaches equilibrium. At equilibrium, Q is equal to K_c at a particular temperature. Once the equilibrium is attained, there is no change in the Q value. By knowing the Q value, we can predict the direction of the reaction by comparing it with K_c.

If Q = K_c, the reaction is in an equilibrium state.

If Q > K_c, the reaction will proceed in the reverse direction, i.e., the formation of reactants.

If Q < K_c, the reaction will proceed in the forward direction i.e., formation of products.

Predicting the direction of a reaction