Explain how conc. H2so4 catalysis the speed of the reaction between salicylic acid and acetic anhydride.

It isn't possible for us to draw mechanisms on this forum but here is one from the web.

http://www.chemguide.co.uk/physical/catalysis/esterify.html

Concentrated sulfuric acid (H2SO4) can act as a catalyst in the reaction between salicylic acid and acetic anhydride. This catalytic effect can be attributed to the acidic nature of H2SO4 and its ability to provide protons (H+) to reactants.

To understand how H2SO4 catalyzes the reaction, we need to first understand the reaction itself. The reaction between salicylic acid and acetic anhydride is known as the acetylation reaction, and it involves the formation of acetylsalicylic acid, commonly known as aspirin. This reaction is typically slow and requires a catalyst to proceed at a reasonable rate.

H2SO4 acts as a catalyst by donating protons to the reaction. It can protonate the reactants, salicylic acid, and acetic anhydride, by transferring a proton (H+) from its own structure to these reactants. The protonation makes the reactants more susceptible to nucleophilic attack, which speeds up the reaction.

In this reaction, H2SO4 protonates salicylic acid to form a resonance-stabilized intermediate. This intermediate is more reactive and readily reacts with acetic anhydride. The reaction between the protonated salicylic acid and the acetic anhydride leads to the formation of acetylsalicylic acid (aspirin) and sulfuric acid as a byproduct.

The sulfuric acid generated as a byproduct can then react with freshly added salicylic acid and acetic anhydride, continuously regenerating the catalyst and enabling it to speed up the reaction further.

To catalyze the reaction between salicylic acid and acetic anhydride using H2SO4, you would typically mix the reactants, salicylic acid and acetic anhydride, and then add a few drops of concentrated sulfuric acid. The addition of H2SO4 will enhance the reaction rate and ensure efficient conversion of the reactants to the desired product, aspirin.