Liquids in Keq Expressions

[justadream]

I think generally you only put aq and gaseous things in the Keq.

The reasoning behind not including liquids is that they are often the solvent (thus, their concentration stays about the same).

But what about liquids that are NOT the solvent. Do you still leave them out?

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[type12]

I think generally you only put aq and gaseous things in the Keq.

The reasoning behind not including liquids is that they are often the solvent (thus, their concentration stays about the same).

But what about liquids that are NOT the solvent. Do you still leave them out?

Technically, all reactants and solvents are included in the equation: solids and liquids just have a value of 1 because they do not affect equilibrium. Influence of equilibrium is caused by electrochemical interactions, and if they are pure, they are not interacting or working on the system despite being necessary for the reaction to occur. It's not really highlighted why things affect equilibrium for the MCAT, so you don't need to know it.

 

[Teleologist]

Technically, all reactants and solvents are included in the equation: solids and liquids just have a value of 1 because they do not affect equilibrium.

A value of one? I think you mean an idealized activity coefficient of 1. I know that for a fact all solids and liquids don't have concentrations of "1."

 

[type12]

A value of one? I think you mean an idealized activity coefficient of 1. I know that for a fact all solids and liquids don't have concentrations of "1."

Sure, but the value you put into the equation is still one. I don't see it helpful to bombard someone struggling with a topic with new concepts.

 

[Teleologist]

Sure, but the value you put into the equation is still one.

Not necessarily. I said it was a coefficient. I recall from algebra class that we don't just plug coefficients into equations. [H2O] definitely isn't 1 when evaluating Ka(H2O).



Ka(H2O) = [H3O+][HO-]/[H2O]
Ka(H2O)*[H2O] = [H3O+][HO-] = Kw

Only if Ka(H2O) = 1.8*10^-16 and [H2O] = 55.3 does this equality 1) make sense and 2) reflect chemical fact (i.e. there are 55.3 moles of water in a liter of water).

I don't see it helpful to bombard someone struggling with a topic with new concepts.

I don't see it helpful to pass along falsehoods. If you don't know what you're talking about, it's best to remain silent than to say something false.

 

[type12]

Not necessarily. I said it was a coefficient. I recall from algebra class that we don't just plug coefficients into equations. [H2O] definitely isn't 1 when evaluating Ka(H2O).

I don't see it helpful to pass along falsehoods. If you don't know what you're talking about, it's best to remain silent than to say something false.

You stopped making sense, lol. Please, show me where I said the concentration is 1, otherwise, STFU and learn a little more before you spout off incoherently.

EDIT: OP, hope I answered your question. Please excuse me while I put this fellow SDNer in his place, the rest of this thread will not be relevant to the MCAT.

 

[Teleologist]

You stopped making sense, lol. Please, show me where I said the concentration is 1, otherwise, STFU and learn a little more before you spout off incoherently.

You said the value you plug in for pure liquids and solids is 1.

solids and liquids just have a value of 1 because they do not affect equilibrium.

 

[type12]

You said the value you plug in for pure liquids and solids is 1.

Wow, so are you saying that is not the VALUE you put in (didn't say that's their concentration)?

 

[Teleologist]

Wow, so are you saying that is not the VALUE you put in (didn't say that's their concentration)?

You try using Ka(H2O) to derive Kw using your "rule" that for any pure liquid/solid you plug 1 into the equation and see if you can actually get Kw.

And no I didn't plug one into Ka(H2O) to solve for Kw because that wouldn't have worked.

 

[type12]

Not necessarily. I said it was a coefficient. I recall from algebra class that we don't just plug coefficients into equations. [H2O] definitely isn't 1 when evaluating Ka(H2O).

Ka(H2O) = [H3O+][HO-]/[H2O]
Ka(H2O)*[H2O] = [H3O+][HO-] = Kw

Only if Ka(H2O) = 1.8*10^-16 and [H2O] = 55.3 does this equality 1) make sense and 2) reflect chemical fact (i.e. there are 55.3 moles of water in a liter of water).

I'm sorry, but your statement is false, as there are other values that make sense, and your statement only reflects a chemical fact at one specific temperature and pressure. You can see how pointless pointing out technicalities are.

You try using Ka(H2O) to derive Kw using your "rule" that for any pure liquid/solid you plug 1 into the equation and see if you can actually get Kw.

And no I didn't plug one into Ka(H2O) to solve for Kw because that wouldn't have worked.

Hey, I'm very happy you learned about the concentration of water and everything and want to share it, but again, beyond the scope of the MCAT: there's never a case (unless a passage says otherwise) where you will not plug in a value of 1 for pure liquids and solids.

 

[Teleologist]

I'm sorry, but your statement is false, as there are other values that make sense, and your statement only reflects a chemical fact at one specific temperature and pressure. You can see how pointless pointing out technicalities are.

Wait, so it's false, but it also "reflects a chemical fact"?

your statement only reflects a chemical fact at one specific temperature and pressure. You can see how pointless pointing out technicalities are.

Yes, my statement only reflects a chemical fact at a certain temperature. No, thermodynamic K constants do not necessarily vary with pressure (for our purposes; of course, you may consider fugacity). You might want to flush this from your head before your MCAT. Also, note that every single K value given to you on the MCAT only reflects fact at a certain temperature (most likely 298 K). I don't see why you wanted to point this out; the MCAT isn't going to give you the Ka of some acid at 298 K and expect you to calculate pH at 398 K.

 

[type12]

Wait, so it's false, but it also "reflects a chemical fact"?

No, it's 100% false since you said "only if," because it is a chemical fact at a particular temp and pressure.

Yes, my statement only reflects a chemical fact at a certain temperature. No, thermodynamic K constants do not necessarily vary with pressure (for our purposes; of course, you may consider fugacity). You might want to flush this from your head before your MCAT. Also, note that every single K value given to you on the MCAT only reflects fact at a certain temperature (most likely 298 K). I don't see why you wanted to point this out; the MCAT isn't going to give you the Ka of some acid at 298 K and expect you to calculate pH at 398 K.

You're making my point for me. Thank you.

EDIT: last nitpick, MCAT assumes real gases unless stated otherwise, so you're wrong as far as the MCAT is concerned (please see what forum you're on).

 

[Teleologist]

Bruh, you might want to go back and remove the sentence in which you said you plug in 1 for liquids and solids, if you're so concerned about getting the facts right.

Or are you just nit picking my facts?

 

[Teleologist]

EDIT: last nitpick, MCAT assumes real gases unless stated otherwise, so you're wrong as far as the MCAT is concerned (please see what forum you're on).

No, the MCAT considers ideal gases. Otherwise you wouldn't be able to use PV=nRT on the exam all the time.

 

[type12]

Bruh, you might want to go back and remove the sentence in which you said you plug in 1 for liquids and solids, if you're so concerned about getting the facts right.

Or are you just nit picking my facts?

Nah, brah, I'm good. You might want to edit your statements where you incorrectly accuse me of saying the concentration of things is 1, though.

I got ur back, dawg.

 

[type12]

No, the MCAT considers ideal gases. Otherwise you wouldn't be able to use PV=nRT on the exam all the time.

Yep, that's what I meant, that's a typo, but yeah, it makes your point on fugacity in this thread (and forum) pointless.

 

[Teleologist]

Nah, brah, I'm good. You might want to edit your statements where you incorrectly accuse me of saying the concentration of things is 1, though.

I got ur back, dawg.

You said plug in 1 ... What do we plug into a K expression other than concentrations?