The acid serves to protonate one double bonded oxygen on the acetic anhydride this makes the reaction with the salicylic acid faster. The proton is returned to the acid in the end so it is just a catalyst. read more
the balanced equation for this reaction is C7H6O3 + C4H6O3 --> C9H8O4 + CH3COOH sulfuric acid is used as a catalyst for the reaction. On the wikipedia article, it says that phosphoric acid can be used also. For this reaction to occur you don't need sulfuric acid, but it will help the reaction proceed quicker. read more