thermo question

[cloak25]

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0.75 moles of a salt are added to 1L of water. The solution warms and some solid remains. What is the sign of standard entropy change for this process?

A. positive
B. zero
C. negative
D. cannot be determined

Answer: C
can someone explain this please?

 

[Temperature101]

The answer is C. I have seen that stuff in biochem and even now I still dont understand it. I f you have a biochem book, go the section that cover the physical properties of water...you might find the explanation there.

 

[chiddler]

0.75 moles of a salt are added to 1L of water. The solution warms and some solid remains. What is the sign of standard entropy change for this process?

A. positive
B. zero
C. negative
D. cannot be determined

Answer: C
can someone explain this please?

dang.

salt is an ordered lattice. dissolving an ordered lattice, to me, is a clear instance of increasing entropy!

wikipedia seems to agree with me:

" It is possible that an ion will dissolve even if it has a positive enthalpy value. The extra energy required comes from the increase in entropy that results when the ion dissolves."

whereas here you don't even need that extra energy because it is exothermic. and corresponding with the idea, gases should have the opposite:

"Most gases have a negative enthalpy of solution."

?

 

[pfaction]

The ****? Automatically, aqueous solutions of salts are increasing entropy. This is like saying you have a 1 gram cube of salt and smash 0.75 grams off, yeah, you'll still have 0.25 grams left, but 0.75 is now disordered. It should be A.

"Most gases have a negative enthalpy of solution."

I mean, increasing heat will decrease the solubility of gases, but I would have also pegged that as increasing entropy since you're moving towards free gases?!

When you dissolve gases in liquids you need to cool down the solution, so the sign is positive negative (from 100 degrees C to 25 degrees C)?

 

[chiddler]

The ****? Automatically, aqueous solutions of salts are increasing entropy. This is like saying you have a 1 gram cube of salt and smash 0.75 grams off, yeah, you'll still have 0.25 grams left, but 0.75 is now disordered. It should be A.



I mean, increasing heat will decrease the solubility of gases, but I would have also pegged that as increasing entropy since you're moving towards free gases?!

i'm glad i'm not crazy @_@

 

[cloak25]

I thought it was A too. This question was from mcatquestion. Here's their explanation: "We may use the equation deltaG=deltaH-TdeltaS, where G,H, and S are all at standard conditions. We know that since the solution warms, that deltaH is negative (because heat must be released). Additionally, we know that the equilibrium constant at this state is less than 1, because only 0.75 moles dissolve in 1L of water. Therefore, since deltaG is positive and deltaH is negative, it must be that deltaS is negative." Since 0.75 mole dissolved, doesn't that mean you have more products therefore Keq>1? How do you know deltaG is positive?

 

[SaintJude]

Mhh, perhaps it's because solution started entirely liquid and ended with some solid. I'm not sure.

 

[Dasypus]

It seems like it's a poorly described question. If the 'process' is the whole action, including the disassociation of the salt crystals as well as the solvation of the ions, then it seems like of course the change in entropy is positive.

On the other hand, if the 'process' is just the solvation, then the change in entropy is negative, since the water becomes a bit more organized to bring the ions into solution (with the O-sides lining up with the cations and the H-ends lining up with the anions).

I say lousy question.