Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: leemw0108 on July 01, 2010, 01:26:47 AM
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Why can't catalyst affect equilibrium constant ?
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Catalysts increase (or decrease if negative catalysts) the rate of both forward and backward reactions by the same amount. i.e., they catalyze both to the same extent in the same fashion.
So, if Kf is the rate of forward reaction and Kb is the rate of backward reaction, both increase by the same factor.
K(equilibrium constant) = Kf/Kb
The ratio remains the same, hence, equilibrium constant is not altered.
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Rate depends on kinetics and thereby on the reaction path. The equilibrium constant depends on free energy and is therefore only dependent on the starting and final situation. A catalyst influences the reaction path (lowering the transition state), but as long as the initial and final compounds are the same as without the catalyst, the free energy change is the same and thereby also the equilibrium constant.
You can look at a chemical reaction as downhill skiing (John Albery). If the path downwards is smooth it will go fast, but if there is a big *Ignore me, I am impatient* in the hill you will slow down as you go over it. A catalyst helps smoothening the hill or finds another route thereby raising your speed. The free energy is just the difference between how high you started and how low you finished, but it doesn't know about any bumps along the way.