Equilibrium Expression for Dissolution...

[Bernoull]

Can someone explain this....

I know for the Ksp expression, we exclude pure liquids and solids (I think the rationale is that mole fraction is 1, and x/1=x)

I have these questions:

1. If the solute is a gas say O2 and solvent is water do u include O2 (g) in the solubility product expression? I can't reason this out....

2. What is the definition of pure liquid/solid, on the MCAT how do u determine purity? Or do u just assume purity unless the question states compound A is a mixture of B&C.

3. If the solid is a mixture, how would you express it in the Ksp expression..

Thanks in advance..

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[Chowdder]

Perhaps someone could best double check to make sure I had this right. It has been a while since I took physical chemistry and I'll try my best at your question.

I recall learning about the equilibrium constant as
.........A1^a*B1^b....
Keq = -----------------
.........C1^c*D1^d....

The letters A, B, C, D represent the activity of individual species. A solid and a pure liquid will have an activity of 1. This is why Ksp is written as [A]^a*^b etc. The reactant (a solid) has a activity of 1, hence not important in the Keq equation.

Regarding your first question. Activity of gases in Keq is inputed as their partial pressure. This is the case when it is in the gas phase. I am not quite sure whether this applies to gases dissolved in liquid.

The way I judge whether a substance is a pure liquid and solid is by looking at the equation they provided. I'm pretty sure this is not how it is suppose to be judge, but for the purpose of the MCAT, it should suffice. Substances that has the physical phase indicator (l) or (s) are considered pure liquid. (g) are gases and (aq) are aqueous solution that will dissolve into their corresponding ion species. So a pure liquid and solid I would judge by looking at the equation and see whether it is (l) or (s).

I'm assuming you're referring to an alloy for your last question and I don't believe Keq could be applied to solids. This is because solid particles are fixed and you can't have equilibrium between different species.

 

[Bernoull]

Perhaps someone could best double check to make sure I had this right. It has been a while since I took physical chemistry and I'll try my best at your question.

I recall learning about the equilibrium constant as
.........A1^a*B1^b....
Keq = -----------------
.........C1^c*D1^d....

The letters A, B, C, D represent the activity of individual species. A solid and a pure liquid will have an activity of 1. This is why Ksp is written as [A]^a*^b etc. The reactant (a solid) has a activity of 1, hence not important in the Keq equation.

Regarding your first question. Activity of gases in Keq is inputed as their partial pressure. This is the case when it is in the gas phase. I am not quite sure whether this applies to gases dissolved in liquid.

The way I judge whether a substance is a pure liquid and solid is by looking at the equation they provided. I'm pretty sure this is not how it is suppose to be judge, but for the purpose of the MCAT, it should suffice. Substances that has the physical phase indicator (l) or (s) are considered pure liquid. (g) are gases and (aq) are aqueous solution that will dissolve into their corresponding ion species. So a pure liquid and solid I would judge by looking at the equation and see whether it is (l) or (s).

I'm assuming you're referring to an alloy for your last question and I don't believe Keq could be applied to solids. This is because solid particles are fixed and you can't have equilibrium between different species.

Thanks for the answer. To clarify by solid I mean a solid soluble salt, say NaCl. I'm trying to think of a scenario where the solid is impure and how that will affect your Keq. From what I've read, u can exclude PURE solids and liquids from Keq and that got me thinking about what if it's not pure do you include it, if so how. Say u scooped salt from the beach and it's 50% sand thus impure, how do u represent salt/sand mixture in our Ksp expression.

Can you elaborate on:
"(g) are gases and (aq) are aqueous solution that will dissolve into their corresponding ion species."

If you're dissolving O2(g) in water do u get an Keq = [O2(aq)]/[O2(g)] even when O2(g) is pure?? In other words is there a purity criterion for including gas in Keq expression similar to liquids and solids??

Also say you add "50:50 NaCl/sand mixture"(s) to pure water, what is will your Ksp look like given the impurity of the salt...

Thanks again..

 

[Chowdder]

Thanks for the answer. To clarify by solid I mean a solid soluble salt, say NaCl. I'm trying to think of a scenario where the solid is impure and how that will affect your Keq. From what I've read, u can exclude PURE solids and liquids from Keq and that got me thinking about what if it's not pure do you include it, if so how. Say u scooped salt from the beach and it's 50% sand thus impure, how do u represent salt/sand mixture in our Ksp expression.

Can you elaborate on:
"(g) are gases and (aq) are aqueous solution that will dissolve into their corresponding ion species."

If you're dissolving O2(g) in water do u get an Keq = [O2(aq)]/[O2(g)] even when O2(g) is pure?? In other words is there a purity criterion for including gas in Keq expression similar to liquids and solids??

Also say you add "50:50 NaCl/sand mixture"(s) to pure water, what is will your Ksp look like given the impurity of the salt...

Thanks again..

That's a lot of questions which I believe goes way beyond the depth required by the MCAT. In any case I'll try and sort out one by one.

If you have a solid that is impure as you've described (50/50 mixture of sand and salt). Then your Ksp will be concentration of the dissolved species. If a species do not dissolve in the solvent, then it does not contribute to the Ksp value.

As for your gas question. I am saying that Keq for species in gas phase means you use partial pressure (i.e. a haber process for making ammonia). For aq compounds, you use concentrations in the Keq of the corresponding ion species. I am not quite certain about your question pertaining to O2 in water. Like I've said, it has been a while since I've done physical chemistry and I some how vaguely recall instances where I saw gas partial pressure partaking with keq value where there are aq solution. But again this was 3-4 years ago and I can't remember whether it's something I've made up or actually real (don't have photographic memory ). I'll try and do some diggin and see if I can find a suitable answers for that.

EDIT: found something that might be of use to direct your concern. Scroll down to B. Heterogeneous reactions. That should answer your question. But also do keep in mind that the Keq for gases with liquids also depends on pressure, if your case more O2(aq) at higher pressure. Vaguely recall something about Henry's Law, but that's beyond the scope of the MCAT. You can read it up if you're curious.

 

[Bernoull]

That's a lot of questions which I believe goes way beyond the depth required by the MCAT. In any case I'll try and sort out one by one.

If you have a solid that is impure as you've described (50/50 mixture of sand and salt). Then your Ksp will be concentration of the dissolved species. If a species do not dissolve in the solvent, then it does not contribute to the Ksp value.

As for your gas question. I am saying that Keq for species in gas phase means you use partial pressure (i.e. a haber process for making ammonia). For aq compounds, you use concentrations in the Keq of the corresponding ion species. I am not quite certain about your question pertaining to O2 in water. Like I've said, it has been a while since I've done physical chemistry and I some how vaguely recall instances where I saw gas partial pressure partaking with keq value where there are aq solution. But again this was 3-4 years ago and I can't remember whether it's something I've made up or actually real (don't have photographic memory ). I'll try and do some diggin and see if I can find a suitable answers for that.

EDIT: found something that might be of use to direct your concern. Scroll down to B. Heterogeneous reactions. That should answer your question. But also do keep in mind that the Keq for gases with liquids also depends on pressure, if your case more O2(aq) at higher pressure. Vaguely recall something about Henry's Law, but that's beyond the scope of the MCAT. You can read it up if you're curious.

Greatly appreciate your help, gotta say that site is an excellent resource too!! I guess i'm overthinking this topic beyond what the MCAT requires, I know the basics and that should do.

Thanks again!!