Molecular Reaction Dynamics (Theories of Reaction Rates)

Basic Theories: Collision theory Complex theory (transition state theory) - Molecular encounters make then excited (collisions or energy can be taken from surroundings-solutions)

Assumptions - Hard spheres - Molecules must collide - Reactions occur only if molecules are energetic - Collisions should transfer certain minimum energy

Steric factor is difficult to evaluate P: order of 10-6

One case where P is large for the reaction K+Br2 KBr + Br P= 4.8 harpoon mechanism

Reactions in solution Cage effect Activation energy, encounter pair A+ B AB rate=kd[A][B] AB A + B rate=k‘d [AB] AB P rate=ka[AB]

Simplest chemical reaction: H+H2 H2 + H

Figure adapted from Science, 331 411-412 (2011)