Question

consider the generic reaction below where a dilute aqueous solution of "A" reacts with water, the solvent. Explain why, even though water is a reactant, increasing the amount of water is likely to decrease the reaction rate

A(Aqua) +H2O (I) -------> B (Aq)

Answer 1

The reation is aqueous A reacting with water.

Here the solution of A is made in water with a particular concentration , say 1M.

Now water being the other reagent , we add more water.

But water is the solvent also, which dilutes the solution of A. Now the concentraton of A is less than 1 M.

We know from the law of mass action, the rate of a reaction depends on the concentration of the reactant.

Thus the increase in water , decreases the effective concentration and thus the rate of reaction is lowered.

Though water is a reactant, it is in such an excess (being the solvent) increase in its concentration does not change the rate of reaction.

the only effect on rate of reaction is from the concentration of A.