Ignoring solids and pure liquids in equilibrium problems, only for gases??

[johnwandering]

I noticed that we ignore pure solids and liquids from equilibrium equations.

Why do we do this? And is it Only for gas a reactions? Or also for liquids?

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[milski]

Because the amount of activity that happens does not depend on the amount of solid/liquid, only on its presence. When gasses interact, their concentration changes the probability for two molecules to collide and participate in a reaction. For solids/liquids in this case there is not anything that changes - they are either there or not.

Same applies for solids in liquids as well. I should be similar for non-soluble liquids too but I'm not completely clear on the liquid/liquid case.

 

[rjosh33]

I noticed that we ignore pure solids and liquids from equilibrium equations.

Why do we do this? And is it Only for gas a reactions? Or also for liquids?

You don't need to know why for the MCAT, but if you're curious...

The equation K = [A]/[C][D] actually has each species (A, B, C, and D) as a ratio of the concentration of that species divided by its respective standard state. That may sound complicated, but it explains why the terms are unitless and why the equilibrium constant itself is unitless. For aqueous species, the standard state is 1 M, and for gases, 1 bar. Solid and liquids are already in their standard state, so they are always going to be 1/1 = 1 (a term called unity). Since they always reduce to 1 (or unity), solids and liquids can and should be omitted from the equation entirely.

So, for example, using the equation above, let's say A and B are both liquids, C is an aqueous solute (somehow) and D is a gas. The equation reduces to K = 1/ ([C]/1M x Pd/1 bar), where Pd is the pressure of D in bars. It actually gets a little more complicated than this and involves activity coefficients for each term (as well as stoichiometric coefficients as exponents), but I think I've probably been confusing enough as it is.

 

[Dasypus]

You don't need to know why for the MCAT, but if you're curious...

The equation K = [A]/[C][D] actually has each species (A, B, C, and D) as a ratio of the concentration of that species divided by its respective standard state. That may sound complicated, but it explains why the terms are unitless and why the equilibrium constant itself is unitless. For aqueous species, the standard state is 1 M, and for gases, 1 bar. Solid and liquids are already in their standard state, so they are always going to be 1/1 = 1 (a term called unity). Since they always reduce to 1 (or unity), solids and liquids can and should be omitted from the equation entirely.

So, for example, using the equation above, let's say A and B are both liquids, C is an aqueous solute (somehow) and D is a gas. The equation reduces to K = 1/ ([C]/1M x Pd/1 bar), where Pd is the pressure of D in bars. It actually gets a little more complicated than this and involves activity coefficients for each term (as well as stoichiometric coefficients as exponents), but I think I've probably been confusing enough as it is.

Thank you rjosh. I found that really useful!