The effect of pressure changes on chemical equilibrium is an important consideration in gaseous reactions. According to Le Chatelier's Principle, if a system at equilibrium is disturbed, the system will adjust to counteract the change and restore a new equilibrium state.
- In a gaseous system, decreasing the volume increases the pressure.
- The system responds by shifting the equilibrium towards the side with fewer moles of gas, which helps to decrease the pressure.
For instance, in the reaction i. \(\mathrm{Pcl}_{3(g)}+\mathrm{Cl}_{2(g)} \rightleftarrows \mathrm{PCl}_{5(g)}\): - There are more moles on the left (reactants) side. When the volume decreases, the equilibrium shifts to the right, towards fewer moles on the product side. However, if the moles of gas do not differ on both sides of the reaction, as in ii. \(\mathrm{N}_{2(g)}+\mathrm{O}_{2(g)} \rightleftarrows 2\mathrm{NO}_{(g)}\): - There is no shift, since moles are equal on both sides, and no pressure imbalance to correct.