tag:blogger.com,1999:blog-9633767.post162134201415319315..comments2020-06-29T22:49:50.976-07:00Comments on The Curious Wavefunction: Why conduct reactions at low temperature?Wavefunctionhttp://www.blogger.com/profile/14993805391653267639noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-9633767.post-88241699312330463502011-08-17T09:30:26.138-07:002011-08-17T09:30:26.138-07:00Your posts are always appreciated and thank you fo...Your posts are always appreciated and thank you for them. In the latest, however, there seems to be some confusion about thermodynamic and kinetic drivers. The Arrhenius equation <br /><br />k = A exp(-Ea/RT)<br /><br />expresses a *rate* constant in terms of an experimentally determined activation energy (Ea) and temperature (T). A small difference in Ea can result in significant rate differences, which are in turn amplified at lower temperatures. On the other hand <br /><br />G = H - TS <br /><br />is a thermodynamic expression (add necessary deltas) yielding an *equilibrium* constant, <br /><br />K = exp(-[delta]G/RT). <br /><br />(The latter has no direct rate component, and is the end result of a reaction run for a long enough period. Time-to-equilibrium expressions can be rather complex and are a function of stoichiometry.)Davenoreply@blogger.comtag:blogger.com,1999:blog-9633767.post-64514671189332536892011-08-16T10:18:21.408-07:002011-08-16T10:18:21.408-07:00Indeed, my advisor had a table of ∆G vs K (eq.) in...Indeed, my advisor had a table of ∆G vs K (eq.) in his office.Wavefunctionhttps://www.blogger.com/profile/14993805391653267639noreply@blogger.comtag:blogger.com,1999:blog-9633767.post-50705565402354043942011-08-15T15:02:50.887-07:002011-08-15T15:02:50.887-07:00My Ph.D. supervisor had a graph of delta G vs. T i...My Ph.D. supervisor had a graph of delta G vs. T inside his office door, just to remind us about this effect.Jameshttp://masterorganicchemistry.comnoreply@blogger.comtag:blogger.com,1999:blog-9633767.post-65396747866410779562011-08-15T04:26:42.507-07:002011-08-15T04:26:42.507-07:00Doh indeed! Need. More. Coffee.
Bryan: True, we a...Doh indeed! Need. More. Coffee.<br /><br />Bryan: True, we are talking about the free energy of activation in the Arrhenius formula<br />∆G†† = ∆H†† - T∆S††Wavefunctionhttps://www.blogger.com/profile/14993805391653267639noreply@blogger.comtag:blogger.com,1999:blog-9633767.post-61286599696747970072011-08-14T22:18:02.902-07:002011-08-14T22:18:02.902-07:00If you're exploiting the difference in the tra...If you're exploiting the difference in the transition state energies between the two products, then your reaction is under kinetic control not thermodynamic control, and the relevant equation should be the Arrhenius rate equation not the equation for ΔG.Bryanhttp://biophysicalchemistry.wordpress.comnoreply@blogger.comtag:blogger.com,1999:blog-9633767.post-435249120711895682011-08-14T18:58:05.946-07:002011-08-14T18:58:05.946-07:00"But this phenomenon becomes even more pronou...<i>"But this phenomenon becomes even more pronounced at -78 degrees celsius which is the common standard for reactions cooled to liquid nitrogen temperature."</i><br /><br />Doh! -78 C is the temperature of a dry ice/acetone bath, not a liquid N2 bath (which is at -196 C).John Spevacekhttp://www.rheothing.comnoreply@blogger.com