Question

Which of the following factor is shifted the reaction
$PCl_{3} + Cl_{2} \rightleftharpoons PCl_{5}$ at left side?

• A. Adding $PCl_5$
• B. Increase pressure
• C. Constant temperature
• D. Catalyst

Answer 1

Answer: (A)

If a chemical reaction is at equilibrium and experiences a change in pressure, temperature, or concentration of products or reactants, the equilibrium shifts in the opposite direction to offset the change.
By using the above principle, Le chatelier's principle, it is clear that adding $PCl _{5}$ will shift the reaction to the left side.
Using catalyst doesn't affect the state of equilibrium. Also by keeping the temperature constant, the equilibrium state is not affected.
If you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again - if that is possible. It can do this by favouring the reaction which produces fewer molecules. If there are the same number of molecules on each side of the equation, then a change of pressure makes no difference to the position of equilibrium. So, the equilibrium shifts to the right by increasing pressure.