Acid Catalysis

In specific acid catalysis taking place in solvent S , the Reaction Rate is proportional to the Concentration of the protonated solvent molecules SH⁺. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the Chemical Equilibrium between solvent S and AH in favor of the SH⁺ species.

S + AH → SH⁺ + A^-

For example in an aqueous Buffer Solution the reaction rate for reactants R depends on the PH of the system but not on the Concentration s of different acids.

\ rate= -rac{d[R_1]}{dt}  = k[SH^+][R_1][R_2]

This type of Chemical Kinetics is observed when reactant R_'''1''' in a fast equilibrium with its conjugate base R_'''1''''''H'''^'''+''' which proceeds to react slowly with R_'''2''' to the reaction product for example in the acid catalysed Aldol Reaction .

In general acid catalysis all species capable of donating protons contribute to Reaction Rate acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example Diazonium Coupling reactions.

\ rate= -rac{d[R_1]}{dt}  = k_1[SH^+] + k_2[AH^1 + k_3[AH^2 [R_1][R_2] + ...

When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.

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Acid Catalysis