(a) How does the rate of reaction depend upon the concentration of reactants ?

Explanation

(1) A chemical reaction takes place due to collisions of the reactant molecules. Higher the concentrations of the reactants more will be the frequency of collisions and faster will be the rate of the reaction.
(2) In the reaction of dil. HCI and ` CaCO_(3) , CaCO_(3)` disappears slowly and ` CO_(2)` also liberates slowly. On the other hand th reaction with concnetrated HCI takes place rapidly and `CaCO_(3)` disappears fast.
(3) Concentrated acid reacts faster than dilute acid, that means the rate of a reaction is proportional to the concentration of reactants.
Slow reaction.
` CaCO_(3).+ "dil". 2HCI to CaCI_(2) + CO_(2) + H_(2)O`
Fast reaction :
` CaCO_(3) + underset("conc.")(2HCI) to CaCI_(2) + CO_(2) +H_(2)O`