[Araconan]

So when calculating the equilibrium expression for heterogeneous equilibrium reactions, my textbook states that any solid or pure liquid reactants or products, are omitted from the expression.
The reason for this it said, was because that from the perspective of the activity of the solid or liquid, the reference state of the solid or liquid is the solid/liquid itself. Why is this so? Wouldn't the reference state instead be 1g (or some other mass unit) for solids, and 1L for liquids? (Just like how the reference state for pressure is 1atm)

Also, other textbooks stated that the reason was that the concentration of liquids and solids cannot change. Why is this so? And even if the concentration doesn't change, shouldn't it still be considered in the expression? For example, if I have 0.5M of N2 gas and 1M of H2 gas creating NH3 gas in one container, and 0.5M of N2 gas and 1.5M of H2 gas creating NH3 gas in another container, the equilibrium expression for both containers would still be the same, [NH3]^2/[N2][H2]^3, where the concentration of N2 is kept even though it's concentration didn't change. What's the difference between this example and any reactions involving solids/liquids?

Thank you in advance!

[fledarmus]

Concentration is in terms of moles per unit volume (or some other term per unit volume). If your solid is reacting, there may be less total volume, but the number of molecules in any portion of the remaining volume is still exactly the same. The molecules are fixed in a crystal lattice and, barring some extreme changes in pressure that change the form of your solid, will not change their distance from any other molecules. Compare this to a solution - in a solution, the distance between molecules of a solute depends on the total number of molecules in the solution (all other things being equal), and as you add more solute, you reduce the distance between individual molecules of the solute. This is not the case for a solid - the distance between molecules of the solid, as noted above, do not depend on how much solid you add.

The same applies for a pure liquid - the concentration (molecules, moles, grams, or whatever other measure you like per unit volume does not change as you increase the volume. The molecules in the pure liquid are the same distance from each other, regardless of the volume of the liquid.

[Araconan]

I'm still a little confused, if I have 5 moles of a solid reacting in a 1 litre container, until equilibrium occurs at which there's only 2 moles of the solid left, then didn't the concentration just change from 5M to 2M? So in other words, doesn't the distance between the molecules have no effect on whether the concentration changes or not, and instead, isn't the concentration affected by the amount of molecules per litre, and since solids, like liquids and other compounds decrease in moles as the reaction progresses, wouldn't its concentration also change?

[fledarmus]

At this point, you have to look behind the definitions to the assumptions that underlie them.

The "concentration" of your solid in a 1 litre container is neither 5M nor 1M, because "concentration" assumes a homogeneous mixture. To say that the concentration is 5M implies not only that every litre of material in the mixture will contain 5 moles, but also that every fraction of a litre will contain that same fraction of 5 moles. The mixture you are describing is mostly empty space, and no reaction will take place in that empty space because there is no material there to react. Any reaction that occurs will occur only in the solid, and the concentration in that portion of the reaction vessel depends not on its size but on its density.