According to Le Chatelier's principle, if sulfur trioxide (SO3) is added to the reaction mixture, the equilibrium will shift to the left, toward the reactants (SO2 and O2) side, in order to consume the excess product (SO3). This will result in an increase in the concentrations of reactants and a decrease in the concentration of product to establish a new equilibrium. Therefore, the reaction will shift in the direction that opposes the change made to the system, and the concentrations of SO2 and O2 will increase to react with the added sulfur trioxide.
