The exothermic formation of ${\mathrm{CIF}}_3$ is represented bv the equation : 

${\mathrm{Cl}}_2\left(\mathrm{g}\right)+3{\mathrm{F}}_2\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{ClF}}_3\left(\mathrm{g}\right);{\mathrm{\Delta }}_{r}H=-329\mathrm{J}$

Which of the following will increase the quantity of  in ${\mathrm{CIF}}_3$ an equilibrium mixture of ${\mathrm{Cl}}_2$ , ${\mathrm{F}}_2$ and ${\mathrm{CIF}}_3$?