Solubility

[ajkby52]

Question on TPR Practice Passages Gchem 13:

Question asks what the change in temperature would do to a saturated solution of Mg(OH)2. The dissolution of it is exothermic. Therefore if you add heat, it should reform the ppt, and decrease the pH? However it is saturated, so I put no effect, however the first answer is correct? Does the solution being saturated change if you add heat so that it would push it to the reactants?

Also the graph kinda threw me off... It was basically a exponential graph that went toward the acidic side. I'd still presume that if there were even the slighest amount of base left, it would have a pH > 7. LMK. THanks.

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[loveoforganic]

Solubility of a solid into liquid increases with increasing temperature always (I believe, can't think of an exception).

 

[Will Hunting]

Question on TPR Practice Passages Gchem 13:

Question asks what the change in temperature would do to a saturated solution of Mg(OH)2. The dissolution of it is exothermic. Therefore if you add heat, it should reform the ppt, and decrease the pH? However it is saturated, so I put no effect, however the first answer is correct? Does the solution being saturated change if you add heat so that it would push it to the reactants?

Also the graph kinda threw me off... It was basically a exponential graph that went toward the acidic side. I'd still presume that if there were even the slighest amount of base left, it would have a pH > 7. LMK. THanks.

Maybe there was some passage info. However, I would disagree with the answer. If you go by le chat then the answer is correct. However, heating the solution increases the ksp which would allow more to dissolve. This is how you can get supersaturated solutions by heating and letting more dissolve and then lowering it to a standard 25 C. Also, thermodynamics only partially agrees with le chat. In an exothermic heat is a product. In solvation which is exothermic then entropy must be positive in order for it to be less favorable deltag= E-deltaS*T. However, for mcat purposes, many times they are testing le chat and go off that when in thermo terms it actually decreases.

 

[loveoforganic]

Yeah, dG = dH - TdS

when dS > dH, increasing T results in lower free energy

 

[Dr Gerrard]

So are you saying as the temperature increases, the effect the enthalpy has on the reaction matters less and less and the entropy matters more. And since a dissolved compound has a greater entropy than a precipitate, increasing the temperature will favor the forward direction even more?

So you are going against the answer they put.

The real answer would actually be that more would dissolve, not even no effect, correct? Because you would get a supersaturated solution, like Will Hunting said.

 

[loveoforganic]

No one posted the answer I don't think, so I don't know who I'm agreeing with, lol. You're making it sound like it's an exponential relationship - it's a linear one.

 

[Dr Gerrard]

In the first post, the OP mentioned that the precipitate reforming is the correct answer. He put a question mark at the end, but I assume he just meant why is that correct.

Anyways, I didn't mean to make it sound exponential, just saying that as temperature increases, the enthalpy matters less and less compared to the entropy, which is what that equation says.

 

[loveoforganic]

Oh I see it now, sorry. What you said is right, and I don't understand why that would be the answer.

 

[Will Hunting]

Question on TPR Practice Passages Gchem 13:

Question asks what the change in temperature would do to a saturated solution of Mg(OH)2. The dissolution of it is exothermic. Therefore if you add heat, it should reform the ppt, and decrease the pH? However it is saturated, so I put no effect, however the first answer is correct? Does the solution being saturated change if you add heat so that it would push it to the reactants?

Also the graph kinda threw me off... It was basically a exponential graph that went toward the acidic side. I'd still presume that if there were even the slighest amount of base left, it would have a pH > 7. LMK. THanks.

Ok, when you see exothermic, then think of heat as a product. If you add heat then it has to back the other way. Although in real science this might not have been the case. However, in the mcat they like to keep it simple. So in most cases go with le chat. I'm interested because if you heat the ksp goes up. However, there is DEFINITELY a change. The question is what so then look at the answer choices. It's either the increase graph or the decreasing one. If you have TPR they covered le chat but they don't cover thermo. HTH

 

[ajkby52]

Correct. It basically shows an exponential decay graph going toward the lower pH.

The chemical equation was

Mg(OH)2 ---> Mg + 2OH + Heat.

Kind of throws you off since it is the heat that ppt out the solution. Guess they didn't care as long as you understood lechat.