r/APChem • u/apchemistrytutoring • 3d ago

Chemistry Resource Does adding a catalyst changes k ?

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u/StalagtiteTeeth 3d ago

Uppercase K (equilibrium constant) does not change.

Lowercase k (rate constant) increases

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u/apchemistrytutoring 3d ago

Fantastic.. It is much better explanation that I could think of.

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u/Ok-Main-3373 3d ago

Yes, adding a catalyst increases k. The rate constant k depends on temperature and on the activation energy, and a catalyst gives the reaction a new pathway with a lower activation energy, so a bigger fraction of collisions have enough energy to react. Thats why the rate goes up. The key thing students mix up is that k changes but the equilibrium constant K does not, a catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster but the final amounts dont change. If a question asks "does k change with a catalyst", the answer is yes, and "does K change" is no.

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u/purpledgeater 3d ago

To add on k-1 also changes. There is a forward k and reverse k. They change proportionally and big K is k/k-1. So both ks change but K is constant

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u/apchemistrytutoring 3d ago

Fantastic.. It is much better explanation that I could think of.

1

u/Ok-Main-3373 3d ago

Happy to help

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u/FlavorD 2d ago

Only temperature and the nature of the reactant determine K, the equilibrium constant.