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Unformatted text preview: Chemistry 2000 Lecture 13: Chemical equilibrium Marc R. Roussel Thermodynamic criterion for equilibrium I Recall that under given reaction conditions (concentrations of reactants and products), r G m = r G m + RT ln Q I If r G m < 0, the reaction is thermodynamically allowed as written. I If r G m > 0, the reverse of the reaction is thermodynamically allowed. I What happens if r G m = 0? I Neither the forward nor reverse direction of the reaction is thermodynamically allowed, so the reaction is at equilibrium . The equilibrium constant I At equilibrium, r G m = r G m + RT ln Q = 0 r G m = RT ln Q I This last equation implies that there is a fixed value of the reaction quotient Q when we reach equilibrium. I We call this value the equilibrium constant K . Thus, r G m = RT ln K I For a given reaction, K is a number which depends only on the temperature. I At equilibrium, Q = K . I If we know K , we can calculate the standard free energy change for a reaction by r G m = RT ln K I We can also rearrange this equation to calculate K from r G m : ln K = r G m RT K = exp r G m RT [exp( x ) = e x ] r G m = RT ln K Important: K is related to the standard free energy change r G m (obtained from tables of standard free energies of formation), not to r G m , which is zero at equilibrium. I Both Q and r G m (and therefore the numerical value of K ) depend on the standard state. I The activities that go into Q depend on the standard concentration, pressure, etc....
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This note was uploaded on 03/03/2012 for the course CHEM 2000 taught by Professor Roussel during the Fall '06 term at Lethbridge College.
 Fall '06
 Roussel
 Equilibrium

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