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equilibrium constant

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ksr985:
I thought of something while studying eqbm, havent managed to locate the answer yet. please see if you can help:

If i have a reversible reaction of the type:

A(s) gives B(g)

The eqbm constt. Kp for this reaction will be equal to the partial pressure of B at eqbm.

Now if i take 100 atm of A, and at eqbm, i have 5 atm of B, then Kp is 5.

Suppose i take only 2 atm of A. then the maximum amount of B that can ever be present is will have pressure 2 atms. So, will the eqbm never be reached? and if it is, what is the correct value of Kp for the rxn? If eqbm is not reached, but A gets exhausted, how will i know that this is not the eqbm point in the lab, considering the amount of B will be constant thereafter?

Borek:
To be at equilibrium all substances involved must be present. If there is only one, there is no equilibrium.

ksr985:
My point exactly. BUT, all reversible reactions continue till eqbm is reached. How will I reach eqbm? How can the terminate before eqbm. In general all components must be present, because the Keq equation invoves their concs/ pressures, but what about this example?

Borek:
If there is not enough reagents reaction ends before the equilibrium state is reached.

ksr985:
BUT, the reaction cant end until eqbm is reached. there is no end to a reversible reaction. there is simply eqbm., the minimum in the free energy curve.the eqbm is dynamic, with the backward and forward reaction happening simultaneously. do you see why this is so weird?

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