(C6H5)2CHCl + F- in liq SO3 ?

Rate question ....

Since the rate depends on the concentrations of the reactants, as the reaction proceeds, and the reactants are "used up", the concentrations decrease, and the rate of the reaction decreases.

Are you sure that is liquid SO3? Even at low temperature, that would result in multiple sulfonation of the benzene rings. Much more likely is liquid SO2 which is an ionising solvent and you could get nucleophilic substitution of the Cl by F. In that case the reaction rate would decrease as reactants became used up but that could depend on circumstances. It is always a good idea to paste in the full question as it was presented to you. That way there is a good chance that we have all the information required.