Branch of chemistry dealing with study of reaction rates

Chemical kinematics is the branch of chemistry that deals with the study of reaction rates or even speed of reaction and their mechanics. The kinetic studies not only help us to determine the reaction rates but also does describe the conditions by which the reaction rates can be altered. Besides this the mechanism governing the reactions could also be predicted on the basis of logical kinetic data.

During the course of study we all come across a wide variety of chemical reactions having different rates. In thermodynamics we studied that the decrease in free energy is quite helpful in predicting the feasibility of chemical reactions. However, knowledge of free energy change of reaction does not give us any idea about the speed or rate of the reaction.

So, depending upon the reaction rates, various reactions can be categorized into three types as very fast, very slow or moderately slow. Very fast reaction’s are those which occur almost instantaneously like precipitation of silver chloride which occur instantaneously by simply adding aqueous solution of sodium chloride to aqueous solution of silver nitrate. The rates of such reactions cannot be determined easily. The obvious reason for a very fast rate of such reaction is that no chemical bonds are to be broken among the reactants.

Very slow reactions comprises reactions which occur at a very slow rate. These reactions may require month or even years together for their completion like rusting of iron. The rates of such reactions are hardly of any physical importance. Moderately slow reactions refer to reactions in between very fast type and very slow type reactions. These reactions proceed at moderate speed which can be easily measured. Inversion of cane sugar and hydrolysis of starch are common examples of this type of reactions.

The rate of a chemical reaction can also be determined as the speed or velocity with which a reaction takes place. It can be expressed qualitatively as well as quantitatively. The qualitative rate is based on certain visual parameters like disappearance of reactants, color change, effervescence, etc. the rate expressed as such provides arbitrary conclusions about the rate of a reaction. Quantitative rate of a chemical reaction provides with much better information about the rate data and the accurate rate of a reaction. Quantitatively, the rate of chemical reaction may be expressed as the rate of decrease in concentration of any one of the reactants or the rate of increase in concentration of any one of the products. Thus the reactions may  be involving same stoichiometric coefficients of all the reactants and products like in the case of reaction between nitrogen dioxide and carbon monoxide, as the reactants and products appear in the same stoichiometric proportions. So the rate of the reaction may be expressed in terms of rate of disappearance of nitrogen dioxide or carbon monoxide or alternatively, by the rate of appearance of carbon dioxide or nitric oxide which ever may be convenient. In this case the rate of reaction can be followed easily by simply measuring the change in intensity of color.