Question

How is Le Chatelier’s Principle used to ensure the reaction will go to completion?

Answer 1

You haven’t specified which reaction you are studying in particular; hence, I shall provide a very commonly studied reaction, the reaction between nitrogen, N₂ and hydrogen, H₂.

Nitrogen, N₂ and hydrogen, H₂ react to form ammonia, NH₃ as per the reaction below.

N₂ (g) + 3 H₂ (g) <======> 2 NH₃ (g)

The equilibrium constant for the reaction is given as

K_c = [NH₃]²/[N₂][H₂]³ …….(1)

At a particular temperature, K_c is constant. Suppose, the reaction takes place in a closed container; the reactants and the products maintain the equilibrium and at equilibrium, the rate of formation of NH₃ equals the rate at which NH₃ decomposes to give N₂ and H₂.

Now, envision a condition where the product NH₃ is removed from the system. Therefore, the numerator in expression (1) decreases. However, in order to keep K_c constant, the denominator in (1) must decrease proportionately. This is possible only when more N₂ and H₂ react to form NH₃. Therefore, by removing the product, we can force the equilibrium reaction in the forward direction and the reaction proceeds to completion.

In general, this is an useful rule of thumb. In order to make an equilibrium reaction proceed to completion, it is an useful tactic to remove one or more of the products, thereby, forcing the reactants to react further.