Explain the effect of pressure change on an equilibrium system with suitable examples.

(N/A) The effect of pressure change on equilibrium is only observed in reactions involving gaseous components.
$1$. If the total number of moles of gaseous products and reactants are the same,pressure change has no effect on the equilibrium position.
$2$. If the total number of moles of gaseous products and reactants are different,a change in pressure will shift the equilibrium to establish a new state.
$3$. According to Le Chatelier's principle,if the pressure is increased (volume decreased),the equilibrium shifts in the direction that produces fewer moles of gas to counteract the pressure increase.
Example: $CO_{(g)} + 3H_{2(g)} \rightleftharpoons CH_{4(g)} + H_{2}O_{(g)}$
In this reaction,the reactant side has $1 + 3 = 4$ moles of gas,while the product side has $1 + 1 = 2$ moles of gas.
If the pressure is increased (volume decreased),the system shifts towards the side with fewer moles,which is the forward direction (towards products).
Using the reaction quotient $Q_c$:
$Q_c = \frac{[CH_{4}][H_{2}O]}{[CO][H_{2}]^3}$
If the volume is halved,the concentration of each species doubles. The new $Q_c$ becomes smaller than $K_c$ $(Q_c < K_c)$,causing the reaction to proceed in the forward direction until a new equilibrium is established.