Visualizing Reaction Progress and the Geometry and Instability of the Transition State

Transition state theory (TST), also called activated complex theory, is often discussed in general chemistry and physical chemistry courses when learning kinetics. This demonstration takes the abstract concepts of transition state, activation energy, and reaction progress (reaction coordinate) and makes them more concrete and accessible. The demonstration uses a children’s toy (Hoberman Switch Pitch) to model a chemical reaction that follows a reactant through a transition state before making a product. The reaction progress, activation energy, and specific geometry of the transition state are discussed as part of a question and answer dialogue. The complex, multidimensionality of a reaction coordinate and the inherent instability of a transition state become evident. The demonstration also shows that certain reactions may “go” more readily if the energy of a reactant is in a particular type of internal mode (vibrations or rotations). The preferential partition of a given total energy into internal modes of motion versus translational collision energy may allow a system to access the transition state more easily. The demonstration may also aid in the teaching of isomerization reactions, collision theory, and state-resolved reaction dynamics.