Key notes for class 12 Chemistry chapter 04 Chemical Kinetics. it includes revision notes such as It is the branch of chemistry that deals with the study of reaction rates and their mechanisms. It is the change in concentration of reactant (or product) in unit time. It is the expression which relates the rate of reaction with concentration of the reactants. The constant of proportionality ‘k’ is known as rate constant. When the concentration of reactants is unity, then the rate of reaction is known as rate constant. It is also called specific reaction rate. The constant of proportionality ‘k’ is known as rate constant.