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# 13 - Chemistry 2000 Lecture 13 Chemical equilibrium Marc R...

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Chemistry 2000 Lecture 13: Chemical equilibrium Marc R. Roussel

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Thermodynamic criterion for equilibrium I Recall that under given reaction conditions (concentrations of reactants and products), Δ r G m = Δ r G m + RT ln Q I If Δ r G m < 0, the reaction is thermodynamically allowed as written. I If Δ r G m > 0, the reverse of the reaction is thermodynamically allowed. I What happens if Δ 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, Δ r G m = Δ r G m + RT ln Q = 0 Δ 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, Δ 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 .

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I If we know K , we can calculate the standard free energy change for a reaction by Δ r G m = - RT ln K I We can also rearrange this equation to calculate K from Δ r G m : ln K = - Δ r G m RT K = exp - Δ r G m RT [exp( x ) = e x ]
Δ r G m = - RT ln K Important: K is related to the standard free energy change Δ r G m (obtained from tables of standard free energies of formation), not to Δ r G m , which is zero at equilibrium. I Both Q and Δ 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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13 - Chemistry 2000 Lecture 13 Chemical equilibrium Marc R...

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