Does the number of moles of reaction products increase, decrease or remain same when each of the following equilibria is subjected to a decrease in pressure by increasing the volume?
(a) PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
(b) CaO (s) + CO2 (g) ⇌ CaCO3 (s)
(c) 3Fe (s) + 4H2O (g) ⇌ Fe3O4 (s) + 4H2 (g)
When the pressure on the system is increased, the volume decreases proportionately. According to Le Chatelier’s principle, the equilibrium will shift in the direction in which there is a decrease in the number of moles, i.e., towards the direction in which there is a decrease in the volume. In general, an increase in pressure applied to the system at equilibrium favours the reaction in the direction which takes place with a decrease in a total number of moles and a decrease in the pressure favours the reaction in the direction which takes place with an increase in total number of moles. If there is no change in the number of moles of gases in a reaction, a pressure change does not affect the equilibrium.
Hence, according to Le Chatelier’s principle, if the pressure on the system is decreased, then the equilibrium shifts in the direction in which the number of moles of gases is more.
a) The number of moles of reaction products will increase. In the given reaction,
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
The number of moles of gaseous products is more than that of gaseous reactants. Thus, the reaction will proceed in the forward reaction. As a result, the number of moles of reaction products will increase.
b) The number of moles of reaction products will decrease. In the given reaction,
CaO (s) + CO2 (g) ⇌ CaCO3 (s)
The number of moles of gaseous products is less than that of
gaseous reactants. Thus, the reaction will proceed in the backward
reaction. As a result, the number of moles of reaction products will
decrease.
c) The number of moles of reaction products remains same.
In the given reaction,
3Fe (s) + 4H2O (g) ⇌ Fe3O4 (s) + 4H2 (g)
The number of moles of gaseous products equal to gaseous
reactants. Thus, the decreasing of pressure does not affect the
equilibrium. As a result, the number of moles of reaction products
will remain same.
Note: Le Chatelier’s principle states that “if a system at equilibrium is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in a direction that tends to undo the effect of the change”