Henry Eyring Theory - Lecture Notes | CHM 453, Study notes of Physical Chemistry

Download Henry Eyring Theory - Lecture Notes | CHM 453 and more Study notes Physical Chemistry in PDF only on Docsity! Henry Eyring (1901-1981) « Born in Colonia Juarez, Mexico ¢ Family moved to El Paso when Henry was 11 to avoid Mexican Revolution * Studied engineering at the Univ. of Arizona, then worked for a mining company. * Received M.S. in metallurgy at Arizona, then went to the Univ. of California, Berkeley. ¢ Obtained Ph.D in 2 years! * Worked with Polanyi in Berlin ¢ Princeton Univ. 1931-1946 * Univ. of Utah 1946-1981 Activated Complex Theory * Basic Idea: Reactions pass through an activated complex stage that can be treated with a combination of kinetic and thermodynamic approaches. * “Absolute reaction rate theory” paper sent late to the J. Chem. Phys. because Eyring was in an auto accident: “... it was as finished a paper as I have ever written--strictly by accident” * The editor, Harold Urey sent it to a reviewer who concluded that the theory was incorrect and couldn’t be true. The paper was rejected. * Hugh Taylor and Eugene Wigner persuaded Urey to reconsider and accept the paper. It was published [J. Chem. Phys. 3, 107 (1935)] and is now a classic. Transition State Theory e Consider the bimolecular reaction A + B > C+D: (AB)? 1 AGE A+B . + | AG (thermo.) C+D ¢ Assume the following mechanism: (1) A+B = (AB)? (rapid equilibrium) (2) (AB)# > C + D (slow rate-determining step) e We can express the equilibrium (1) as K* = ((AB+/[A][B] and the rate of formation of products as Rate = k [(AB)+] * Thus the reaction rate can be written Rate = k K¢ [A][B] Transition-State Theory (cont.) ° The rate expression v = k K# [A][B] is the Standard expression for the rate of a bimolecular elementary reaction. Note: an elementary reaction is a reaction at the molecular level--a molecular event. When we write an overall chemical reaction it may proceed by a number of elementary steps. This set of elementary reactions is termed the “mechanism” of the reaction. ¢ We can picture reaction (2) as occurring along some weak vibrational coordinate of the activated complex (AB)+. Instead of vibrating the molecule separates to form the products C and D. The frequency is k = v. * As noted before, we can calculate Kt using Statistical mechanics. (We'll do this later.) ¢ Acomment on the sign of AS# for the reaction A + B —> (AB)#: the activated complex in a gas-phase reaction is always more order- ed than the reactants, so AS? is negative. Recall, sj] A+B \ AS# = S((AB)#) - S(A + B) ap  Basic Ideas of the Transition-State Theory ¢ There are some basic ideas associated with the transition-state theory: (1) An equilibrium is established between the reactants A and B and the activated complex (AB)?. (2) In order to form the products P the activated complex (AB)* breaks up along a weak vibrational coordinate. ¢ We can express the equilibrium constant K+ between reactants and (AB) as K* = [(AB)*)/[A][B] ¢ We can further analyze this using statistical mechanics and partition functions q: K¥= q?# exp(-Ae,/kT) Gade where q} is the partition function for (AB)# and Ac, is the energy difference between A+ B and (AB).