Chemical kinetics is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. The principles of chemical kinetics apply to purely physical processes as well as to chemical reactions. One reason for the importance of kinetics is that it provides evidence for the mechanisms of chemical processes. Besides being of intrinsic scientific interest, knowledge of reaction mechanisms is of practical use in deciding what is the most effective way of causing a reaction to occur. Many commercial processes can take place by alternative reaction paths, and knowledge of the mechanisms makes it possible to choose reaction conditions that favour one path over others.