Ka and Keq

[amgen]

So I see in my book that for an acid dissociation, the Ka = K eq [H 2 O]
and that the Keq includes H2O in the eqn. Shouldn't there be no liquids in Keq?

Keq = [H 3 O + ] [A - ]
[HA] [H 2 O]

Why is there H2O in Keq?
If I am asked to calculate Keq do I have I have to include [H2O]?

Thanks

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[Kalamazoo]

You are right. Liquids are not part of the Keq.

 

[Erythropoietin]

solids and liquids are not included; aqueous & gases are only included in K

 

[MT Headed]

While the two posters above are not technically correct (in ochem you learn to compute the acid dissociation constant of water itself, and the concentration of reactant water (55.56 mol/L) does go into the denominator to arrive at the correct result of pKa=15.7), I have NEVER seen

Ka = Keq [H2O]

Never in gen chem, never in ochem, and never in biochem. I can't think of any situation where that would be true. That's just crazy talk. Ka, Kb, Ksp, and the others are just specific instances of Keq. The closest I can come up with is "Kw = Ka [H2O]" but that basically has nothing to do with what the OP posted.

 

[cheesier]

While the two posters above are not technically correct (in ochem you learn to compute the acid dissociation constant of water itself, and the concentration of reactant water (55.56 mol/L) does go into the denominator to arrive at the correct result of pKa=15.7), I have NEVER seen

Ka = Keq [H2O]

Never in gen chem, never in ochem, and never in biochem. I can't think of any situation where that would be true. That's just crazy talk. Ka, Kb, Ksp, and the others are just specific instances of Keq. The closest I can come up with is "Kw = Ka [H2O]" but that basically has nothing to do with what the OP posted.

When would Ka = Keq [H2O] not be true? I took it a little while back, but I'm fairly certain that we were told that Ka=Keq [H2O] in my analytical chem class.

If you have an equation A + H2O --> A- + H3O+, the Keq is:

[A-][H3O+]/[A][H2O] but water is a constant so it is multiplied out of both sides, giving you the Ka. You don't usually include constants when calculating equilibrium.

I want to also add that nonreactant species aren't included, as erythropoietin said, but water is a reactant species. It's important to actually understand why you disregard solids and liquids. In fact (not that it's within the scope of the MCAT), solids can play a negligible role in the equilibrium constant. You disregard them because only the surface is exposed to the reaction mixture so very little is actually involved in the reaction. Similarly, you actually would pay attention to water in an extremely strong acid in which there is a large change in water concentration on either side of the equation.

 

[MT Headed]

So, are there multiple definitions of Keq then? It seems like there is the Keq that disregards solids and liquids, even when they are reactants in the equation. This is the Keq of genchem and the MCAT. Then there is the other Keq from analytical chem which cheesier talks about above, where liquid reactants are included in the expression.

 

[cheesier]

So, are there multiple definitions of Keq then? It seems like there is the Keq that disregards solids and liquids, even when they are reactants in the equation. This is the Keq of genchem and the MCAT. Then there is the other Keq from analytical chem which cheesier talks about above, where liquid reactants are included in the expression.

Hmm, I really hope someone can chime in and clarify this. I don't think there are multiple definitions of Keq. Like I said above, liquids and solids are disregarded because their concentrations don't vary. In analytical chem you would only pay attention to a liquid when the concentration is changing under some unusual circumstances (probably beyond the scope of the MCAT).