Physico-Chemical Gas Dynamics

Abstract

When two particles, atoms or polyatomic molecules, collide there is always a possibility that one outcome of the event will be a redistribution of the energy contained in the internal structure of either or both collision partners. Such molecular encounters are called inelastic collisions to distinguish them from their less complicated elastic collision companions, in which no such redistributions of internal energy take place. Elastic collisions conserve all of the external manifestations of molecular presence, namely molecular mass, molecular momentum and molecular translational energy and, in addition, they conserve the identities of the two collision partners. Inelastic collisions also preserve mass and momentum but, where energy is concerned, it is the sum of molecular translational and internal energies that is preserved. Insofar as an inelastic collision between a pair of molecules may result in the emergence from the encounter of two molecules that are chemically totally different from the original incident pair, it is clear that inelastic encounters may not preserve identity. Indeed if one molecule is identified by both its chemical type and by its internal quantum state it can be said that identity is always lost in an inelastic collision, since internal quantum state must change when internal energy is exchanged in an encounter.