Predicting chemical changes

Prashant Bhushan Pandey

In chemistry, a system is defined as a part of the universe which is under study. A chemical system is in equilibrium when its rate of forward reaction is equal to the rate of backward reaction. Here the equilibrium is called dynamic in nature. If any of the conditions that helps in maintaining the equilibrium are disrupted such that the change affects unequally the rates of the reaction, the equilibrium shifts. The prediction in the shift of the equilibrium is done based on a principle called Le Chatelier’s Principle.

A reversible reaction at equilibrium gets disturbed if the conditions at equilibrium are changed. Conditions which can be changed are increasing or decreasing chemical concentrations, change in pressure or temperature changes. If such a change is brought about then the reversible reaction will undergo a shift in order to re-establish its equilibrium. This is known as Le Chatelier’s Principle.

Consider a reaction, which is already in equilibrium: A + B ⇌ C + D

Here, if the concentration of the reactant, say A is increased, then the rate of forward (A + B → C + D) reaction increases and the equilibrium would be disturbed. Now, by the Le Chatelier’s principle, equilibrium will shift in a way so that the concentration of reactant A decreases. Hence the equilibrium shifts to the right and more of the product is formed. But eventually, as the concentration of A decreases, the rate of forward reaction slows down. Eventually, the rate of forward and the backward reactions becomes equal and the equilibrium re-establishes.

The author is Head of Department, Science, at Indus International School, Pune. He is a post graduate in mathematics and teaches physics, chemistry and mathematics. He can be reached at

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