Chemistry and the Pendulum – What Have They to do With Each Other?

Physicists have known for some time that pendulum motion is a useful analogy for other physical processes. Chemists have played with the idea from time to time but the strength of the analogy between pendulum motion and chemical processes has only received prominent published recognition since about 1980, although there are details of the analogy that still remain to be explored. This paper suggests that thinking of the pendulum as a type of energy converter can help students understand the energy conversions involved in molecular collisions associated with a chemical reaction. In particular, the relationship between kinetic and potential energy becomes vital in understanding the process of bond breaking, bond making, and enthalpy change in a chemical reaction. As a result, the principles behind transition state theory become somewhat easier to grasp. However, the use of the pendulum as an analogy for a reaction approaching chemical equilibrium can lead to misconceptions. The paper also discusses the nature of catalytic feedback, periodicity and non-periodicity in oscillating chemical reactions and the extent to which the action of a pendulum might elucidate these phenomena. Identifying the limitations as well as the strengths of an analogy is an important consideration when an analogy is applied to a teaching and learning situation. While one is tempted to think that pendulum action is probably more pertinent to the study of physics, there are important applications in the field of chemistry.