Explain direction of reaction by reaction quotient - `Q_(c)`.

Explanation

Predicting the direction of a reaction
The equilibrium constant, `K_(c)` helps us in predicting the direction in which a reversible reaction will proceed at any state. A useful approach for predicting the direction of a reaction is to use the concept of 'reaction quotient' `Q_(c)`. The reaction quotient `Q_(c)` has the same mathematical form as the equilibrium constant `K_(c)` expression. `Q_(c)` is the ratio of actual concentrations of products to reactants in the reaction mixture at any point of time of measurement, instead of equilibrium concentrations.
Thus, for a reversible reaction `A+BhArrC+D`, Reaction quotient `Q_(c)=([C]_(i)[D]_(i))/([A]_(i)[B]_(i))`
whereas, equilibrium constant `K_(c)=([C]_(eq)[D]_(eq))/([A]_(eq)[B]_(eq))`,
where subscript 'i' represents the initial or concentrations at any instant of measurement before the attainment of equilibrium and `eq` represents the equilibrium concentrations.
The value of the reaction quotient is a measure of the progress of the reaction. The value of `K_(c)` will be small at the initial stages of the reaction. Its value increases with time until at equilibrium it becomes equal to `K_(c)`. i.e., `Q_(c)=K_(c)`

Thus, three conditions arise:
1. If `Q_(c)=K_(c)`, the reaction is at equilibrium.
2. If at any time `Q_(c)ltK_(c)`, the reaction will procced in the forward direction until equilibrium is reached.
3. If any time, `Q_(c)gtK_(c)`, the reaction will proceed in the backward direction (reverse reaction) until equilibrium is achieved.