Factors Affecting Equilibriums

Le Chatelier’s Principle:
It states that when a system in equilibrium is disturbed by a change in concentration, pressure or temperature, a 'net' change occurs in it in a direction that tends to decrease the disturbing factor. This principle is applicable to all physical and chemical equilibrium systems.

Effect of Concentration Change

Consider the state of equilibrium for the formation of ammonia from nitrogen and hydrogen.

The concentration of nitrogen, hydrogen and ammonia become constant at the point of equilibrium. Now if any amount of reactants or ammonia is added or removed their concentration will change and the equilibrium will get disturbed.

(i) Increase concentration of reactant : When the concentration of either nitrogen or hydrogen is increased; a net forward reaction will take place which consumes the added reactant.

(ii) Increase in the concentration of any product : If the concentration of product ammonia is increased, a net backward reaction would take place to utilise the added ammonia

Effect of Pressure Change

Change in pressure affects equilibrium involving gaseous phase either in a homogeneous or hetrogeneons system.

Le Chatelier'sprinicple for systems involving gases can be studied as follows :

(i) When the number of moles of products is more than the total number of moles of reactants as in the following system

Increase in total pressure keeping the temperature constant, will cause a decrease in volume. This means that the number of moles per unit volume will increase. A net change will take place in the equilibrium in the direction where the number of moles decrease i.e. backward direction.

(ii) When the number of moles of products is less than reactants. As in the following case

According to Le Chatelier's principle increase in total pressure will bring a net change to the equilibrium in the direction where the total number of moles is decreasing i.e. to the product side as Δn_g = 2. Decrease in total pressure will bring the net change to equilibrium in the direction where the total number of moles is increasing i.e. backward direction

(iii) When there is no change in the total number of moles of reactant and product as in the following state of equilibrium.

There is no net change in equilibrium state when pressure is changed.

Effect of Inert Gas Addition

If an inert gas such as argon is added and the volume kept constant this does not take any part in the reaction. Thus, the equilibrium remains undisturbed. Because the addition of inert gas does not change the partial pressures or the molar concentrations of substances involved in the reaction at constant volume.

Effect of Temperature Change

According to Le Chatelier'sprinicple when the temperature is changed (increased or decreased) the equilibrium system reacts to nullify the change in heat content. However, the net change in equilibrium is directed by the exothermic or endothermic nature of reaction.

(i) Exothermic equilibrium : For the following system of equilibrium of exothermic nature :

according to Le Chatelier'sprinciple increase in temperature brings a net change in the equilibrium state in that direction where this extra heat is consumed. The net change is in the backward direction and some ammonia will decompose producing nitrogen and hydrogen. Similarly if the temperature is decreased the equilibrium shifts to the forward direction.

(ii) Endothermic equilibrium

If the temperature is increased the added heat will be absorbed by the reactant and the net change takes place to the equilibrium in the forward direction. If the temperature in decreased it will bring a 'net' change to equilibrium in the backward direction i.e. direction in which it is exothermic.

Effect of a Catalyst

The chemical substance increases the rate of a chemical reaction by making available a new low-energy pathway for the conversion of reactants to products.
A catalyst increases the rate of forwarding and reverses reactions that pass through the transition state which does not affect the equilibrium.

For example

contact process
2SO₂(g)+O₂(g)⇌2SO₃(g); Kc=1.7×10²⁶

The oxidation of sulphur dioxide to sulphur trioxide is partially very slow.
So, Platinum or divanidiumpentaoxide (V₂O₅) is used as a catalyst to increase the rate of reaction.