For the reaction, ${\text{PCl}}_3\left(\text{g}\right)+{\text{Cl}}_2\left(\text{g}\right)\rightleftharpoons {\text{PCl}}_5\left(\text{g}\right)$ the position of equilibrium can be shifted to the right by

  ${\text{PCl}}_3\left(\text{g}\right)+{\text{Cl}}_2\left(\text{g}\right)\rightleftharpoons {\text{PCl}}_5\left(\text{g}\right)$

The position of equilibrium will shift towards right by the addition of ${\text{Cl}}_2$ at constant volume. If increase the concentration of reactants forward reaction  favors more.