[uclabruin]

Can someone explain, in dummy terms, why changing the temperature causes equilibrium to shift AND changes the numerical value of the equilibrium constant keq, while changing the pressure and/or concentration causes equilibrium to shift but DOESN'T change the keq?

I'm trying to teach myself chemistry, and this is something that has confused me while reading about chemical equilibria and La Chatelier's Principle.

[Borek]

why changing the temperature causes equilibrium to shift AND changes the numerical value of the equilibrium constant keq

Changing the temperature changes the numerical value of the equilibrium constant Keq - period. Equilibrium shifts because Keq has changed, it is not an additional effect based on something else.

When we speak about the equilibrium shift according to La Chatelier's principle basically we refer to the change in the equilibrium constant.

changing the pressure and/or concentration causes equilibrium to shift but DOESN'T change the keq?

Simple - Keq is a function of the temperature only.

[uclabruin]

Thanks for your reply.  It sounds like you are saying that "equilibrium shift" and "changing value of keq" are synonymous, and can't possibly be separated.

If that's the case, then pressure and concentration changes must not cause "equilibrium shift," because they don't change the value of keq.

Then what DO pressure and concentration changes do to equilibrium systems?  (And how is it different from "equilibrium shift"?)

[Borek]

Thanks for your reply.  It sounds like you are saying that "equilibrium shift" and "changing value of keq" are synonymous, and can't possibly be separated.

No, that's not what I tried to say.

Equilibrium shift means change in concentrations of products/reactants. It can happen without change of Keq, for example when you keep the temperature constant, but change the pressure.

Changing value of Keq (for example due to the temperature change) will be typically followed by the equilibrium shift. It doesn't mean they are synonyms, the latter is the effect of the former.

[uclabruin]

Equilibrium shift means change in concentrations of products/reactants. It can happen without change of Keq, for example when you keep the temperature constant, but change the pressure.

Thank you for clarifying.  I think my real confusion is this: since Keq = [products]/[reactants], how can the equilibrium ever shift without changing Keq?  You can't change the numerator or the denominator without changing the ratio, right?

Equilibrium shift means change in concentrations of products/reactants

Just to be 100% clear: equilibrium shift is not just a change in concentrations of products/reactants from what they were before you stressed the system, but a change in what the concentrations of products/reactants will be at equilibrium.  In other words, equilibrium shift means that there is a new equilibrium point, characterized by different relative concentrations than there were at the old equilibrium point. Right?


Thanks for your patience, I'm self-teaching on this stuff and don't have an instructor.

[Borek]

I think my real confusion is this: since Keq = [products]/[reactants], how can the equilibrium ever shift without changing Keq?  You can't change the numerator or the denominator without changing the ratio, right?

It is not just [products]/[reactants]. General form closely follows the reaction equation, for example:

aA + bB cC + dD

Keq is

[tex]K_{eq} = \frac{[C]^c[D]^d}{[A]^a[ B]^b}[/tex]

You are right that for some combinations of exponents changing pressure won't produce equilibrium shift (basically when there is no change in volume during the reaction).

Just to be 100% clear: equilibrium shift is not just a change in concentrations of products/reactants from what they were before you stressed the system, but a change in what the concentrations of products/reactants will be at equilibrium.  In other words, equilibrium shift means that there is a new equilibrium point, characterized by different relative concentrations than there were at the old equilibrium point. Right?

There is some subtle problem with what you wrote (not that it is wrong, rather I feel like what you wrote suggests some misunderstanding). For the equilibrium to shift system has to be at the equilibrium before we stress it, so I don't see why the equilibrium shift is not "just a change in concentrations of products/reactants from what they were before you stressed the system".