[kkrizka]

Hi there,

I am currently doing equilibrium in my Chem AP class at school and I got stuck at one point. It says that the equilibrium constant has no values because it is divided by a reference value which in case of gases and solvents is 1 atm or M. But in case of pure liquids and solids it is the same as the mass/volume. I don't really understand why this is needed as the book dosn't go much into detail.

[Mitch]

equilibrium constant is just a fancy way of saying that the ratio of products:reactants is always the same value.

[kkrizka]

So why then aren't pure solids and liquids included in the equilibrium-constant equation? They are part of the products/reactants too.

[Mitch]

give example, write out the chemical equation.

[Donaldson Tan]

the concentration of a pure liquid or a pure solid is effectively the density.

the densities of solid and liquid vary negligibly with pressure and temperature. this means the concentration of the pure liquid/solid is effectively constant, and in no way will affect the concentration of other species in the system.

[Borek]

the concentration of a pure liquid or a pure solid is effectively the density.

And not reciprocal of density? The denser the substance, the higher its amount in the volume unit, thus the higher the concentration.

[kkrizka]

And not reciprocal of density? The denser the substance, the higher its amount in the volume unit, thus the higher the concentration.

Yeah, but then solids have the same denisty so it's concentration will be always the same. (same with liquids)

I kinda used a wrong example to ask my question. A more specific example would be "Why dosn't the eq constant have any units? For exampole (book) N2O4 <=> 2NO2 has an equation (pressures) keq=(NO2)^2/(N2O4) so (atm)^2/(atm) which results in units of atm. The book says that this unit is canceled out because each item is divided by a "reference value". Where exactly did they get this value from? "

Also since it dosn't have any units, can't we use the mass of liquid/solids instead of concentration?

[Donaldson Tan]

what "reference-value"?

the unit of equilibrium constants varies from reactions to reactions. There is no fixed unit. For gaseous reactions, we use partial pressure to denote concentration, thus the unit of the equilibrium constant for "N2O4(g) <-> 2 NO2(g)" is bar (or any other unit of pressure, eg. atm).

What is the unit of equilibrium constant for the Bon Haber Process?
N2(g) + 3H2 (g) <-> 2NH3 (g)

[K] = [bar]²/[bar]⁴ = [bar]^-2

[kkrizka]

Hmm, that's interesting. Because the book we have has an entire half a page on how the Keq has no units. This is "Chemistry: The Central Science; 9th edition" by T. Brown, H. LeMay B. Bursten and J. Burge.

Reference value is some value they use to cancelout the unit. So for the haber Process it would be:

(NH3/NH3ref)^2
-------------------
(N2/N2ref)(H2/H2ref)^3

and the reference value in all of them is 1atm.

[Borek]

From the termodynamical point of view you should use not concentrations but activities - which are dimensionless. Thus whole discussion should be pointless. But - as activities are seldom used - equilibrium constant has dimension.

After posting this question on some other forum (and after reading answers posted there) I suppose that your book uses these reference values just as a trick to make "equilibrium quotient" dimensionless - which is important especially when you have to take a log out of it.

Note that dividing all pressures or concentrations in the quotient by the same "reference value" equal to 1 unitofyourchoice you will remove dimension, not changing the value of the equilibrium constant.