Content area
Full Text
The Royal Swedish Academy has awarded the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail (California Institute of Technology, Pasadena, CA) "for his studies of the transition states of chemical reactions using femtosecond spectroscopy". Zewail's work has taken the study of the rates and mechanisms of chemical reactions to the ultimate degree of detail-the time scale of bond making and bond breaking. How Do Chemical Reactions Take Place?
Some chemical reactions take place very slowly, as when a nail rusts. Others occur very rapidly, as when dynamite explodes. After a reaction, the atoms in the product substances are arranged differently from the way they were before the reaction. The chemical bonding and structure are different, which implies that chemical bonds are broken and/or made when a reaction takes place. Knowledge of when and how bonds are broken and formed enables us to control reaction rates and reaction products, thereby also enhancing the industrial processes that create many of the important substances and materials of everyday life.
Which bonds are formed or broken and the order in which they are formed or broken are described by a reaction mechanism. A mechanism consists of a sequence of chemical equations, one for each molecular collision. If the rates of individual steps in a mechanism can be predicted or measured, the overall rate of a process can be calculated from them. The mechanism tells us which molecules need to collide and in what order. However, it does not provide much detail about the rate at which each step can occur, about how or why specific bonds are made and broken, or about how temperature affects the reaction rate.
The rates of most single-step reactions increase as temperature increases. Higher temperatures correspond to more violent motion of molecules relative to other molecules and of atoms relative to other atoms within the same molecule. More violent motion means that molecules collide more often, and when they collide they hit each other harder. Both...