# Effect of catalyst

Does a catalyst affect the equilibrium constant of a reaction too? I believe it affects both the equilibrium constant and rate constant.

Borek
Mentor
Does a catalyst affect the equilibrium constant of a reaction too? I believe it affects both the equilibrium constant and rate constant.

Why do you think it affects equilibrium constant?

It helps in attaining the equilibrium faster. So equilibrium constant should be increased.

Ygggdrasil
Gold Member
2021 Award
The equilibrium constant depends on the free energy difference between the products and reactants. In order to affect the equilibrium constant of a reaction, one must change the free energies of the products or reactants. Will a catalyst do this?

Borek
Mentor
It helps in attaining the equilibrium faster. So equilibrium constant should be increased.

Fast car helps me to get to the other end of the city in the shorter time. Does it mean when I use faster car distance gets shorter?

Fast car helps me to get to the other end of the city in the shorter time. Does it mean when I use faster car distance gets shorter?

That's a really good analogy. I like it.

epenguin
Homework Helper
Gold Member
If it helps, the catalyst affects the rate constant, but it affects the rate constant in each direction by the same amount and so does not affect the equilibrium constant.

An exercise is to work out how, if it did affect the equilibrium constant, you could use the fact to devise a perpetual motion machine. The exercise is academic. But not useless.

If it helps, the catalyst affects the rate constant, but it affects the rate constant in each direction by the same amount and so does not affect the equilibrium constant.

Can you have catalysts that only affect, say the forward reaction? Or a catalyst which affects the rate of the forward more than the reverse?

I apologise in advance if I have broken any laws of the universe :/

SpectraCat
Can you have catalysts that only affect, say the forward reaction? Or a catalyst which affects the rate of the forward more than the reverse?

I apologise in advance if I have broken any laws of the universe :/

Well, you sort of have broken a law of the universe, that being the principle of microscopic reversibility. Basically, the reason catalysts work is that they modify the potential energy surface of the reaction, which is just a fancy way of saying that they don't just lower the activation barrier at a single point, but rather that they effect the way the reactants interact with one another along the entire trajectory of the reaction. Since that is the case, it is not possible to change the forward barrier without also changing the reverse barrier.

However, in practice you can often manipulate the circumstances of the reaction so that you do not need to worry about the reverse reaction. For example, in heterogeneous catalysis of gas phase reactants over solid catalysts, the gas phase products can be continuously removed from the reaction volume, for example by using a flow reactor, so that there is less chance that the product molecules will encounter the catalysts and undergo a reverse reaction. Another example might be the formation of an insoluble product from soluble reactants in a homogeneous catalysis reaction.

I think all a catalyst does is create a new pathway with lower activation energy. I don't believe it affects any constants.