Question

Predict the shift in equilibrium when the volume is decreased on the equilibrium reaction: ${\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_5\left(\mathrm{g}\right)$

Answer 1

• When there is a decrease in volume, the equilibrium will shift to favour the direction that produces fewer moles of gas.
• When there is an increase in volume, the equilibrium will shift to favour the direction that produces more moles of gas.
• In the reaction : ${\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_5\left(\mathrm{g}\right)$

number of moles of reactants $\left({\mathrm{n}}_{\mathrm{R}}\right)$ = 2, number of moles of products $\left({\mathrm{n}}_{\mathrm{P}}\right)$ = 1

${\mathrm{n}}_{\mathrm{R}}>{\mathrm{n}}_{\mathrm{P}}$

• The number of moles of products is less than that of reactants, hence when the volume is decreased, the equilibrium will shift in the forward direction.