In the reaction SO2Cl2 double arrows SO2 + Cl2 heat is evolved. What happens when chlorine (Cl2) is added to the equilibrium mixture at constant volume?

The chemical reaction is:

SO2Cl2 ⇔ SO2 + Cl2

This shows that the forward reaction the compound is decomposed to smaller compounds and the backward reaction the larger compound is produced. Adding more chlorine to the mixture when in equilibrium, will shift the equilibrium to the left forming the larger compound.