The ‘<i>Yard Stick</i>’ to Interpret the Entropy of Activation in Chemical Kinetics: A Physical-Organic Chemistry Exercise

Abstract

No physical or physical-organic chemistry laboratory goes without a single instrument. To measure conductance we use conductometer, pH meter for measuring pH, colorimeter for absorbance, viscometer for viscosity, potentiometer for emf, polarimeter for angle of rotation, and several other instruments for different physical properties. But when it comes to the turn of thermodynamic or activation parameters, we don’t have any meters. The only way to evaluate all the thermodynamic or activation parameters is the use of some empirical equations available in many physical chemistry text books. Most often it is very easy to interpret the enthalpy change and free energy change in thermodynamics and the corresponding activation parameters in chemical kinetics. When it comes to interpretation of change of entropy or change of entropy of activation, more often it frightens than enlightens a new teacher while teaching and the students while learning. The classical thermodynamic entropy change is well explained by Atkins in terms of a sneeze in a busy street generates less additional disorder than the same sneeze in a quiet library (Figure 1) . The two environments are analogues of high and low temperatures, respectively. In this article making use of Eyring equation a factor usually called ‘universal factor’ is derived and made use as a ‘yard stick’ to interpreting the change in entropy of activation for physical or physical-organic chemistry senior undergraduate and graduate students’ class-room.