Berkeley Review CBT4 Solubility

[BenZq]

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I feel totally confused on this solubility question. If a compound becomes less soluble as the temperature of the solution is increased, then what are the signs or entropy and enthalpy? I said (-)S and (-)H. The explanation says that S is (+) and H is (-) because its a dissolution, which makes sense to me... but how can the affect of temperature be explained then? (Question is PS #15 on CBT4)

 

[justhanging]

I feel totally confused on this solubility question. If a compound becomes less soluble as the temperature of the solution is increased, then what are the signs or entropy and enthalpy? I said (-)S and (-)H. The explanation says that S is (+) and H is (-) because its a dissolution, which makes sense to me... but how can the affect of temperature be explained then? (Question is PS #15 on CBT4)

AB(s) <--> A(aq) + B(aq)

If the substance becomes less soluble as temperature increases then it has to be exothermic because heat causes it to shift to the right (le chatliers).

therefore
AB(s) <--> A(aq) + B(aq) + heat

Then you can say that enthalpy is negative because its exothermic.

 

[loveoforganic]

-S and -H sounds right to me

 

[BenZq]

AB(s) --> A(aq) + B(aq)

If the substance becomes less soluble as temperature increases then it has to be exothermic because heat causes it to shift to the right (le chatliers).

therefore
AB(s) --> A(aq) + B(aq) + heat

Then you can say that enthalpy is negative because its exothermic.

Thanks, that's good logic but I still don't understand what drives the reaction. I was under the impression that delta G must be - for the reaction to be favorable. Is there something wrong with my reasoning?

 

[justhanging]

Thanks, that's good logic but I still don't understand what drives the reaction. I was under the impression that delta G must be - for the reaction to be favorable. Is there something wrong with my reasoning?

No, you have good reasoning. If the salt dissolves that means dG is negative, just because it happens without us putting any work into the system. Since dissolving salts always create greater disorder, dS is positive. In this case since the reaction is exothermic both enthalpy and entropy drive the reaction (dG = dH - TdS). Sometimes enthalpy for disolution reactions is positive which means that if the salt dissolves entropy drives the reaction.

 

[BenZq]

No, you have good reasoning. If the salt dissolves that means dG is negative, just because it happens without us putting any work into the system. Since dissolving salts always create greater disorder, dS is positive. In this case since the reaction is exothermic both enthalpy and entropy drive the reaction (dG = dH - TdS). Sometimes enthalpy for disolution reactions is positive which means that if the salt dissolves entropy drives the reaction.

but wouldn't a +S and -H create a situation with -G for all temperatures... so increase in temperature would further increase solubility, not decrease it as the answer states?

 

[loveoforganic]

That's right, which is why I agreed with you. Dissolution of a salt isn't always an entropically favorable process - some ions form very ordered shells of water around them that overcome the increase in entropy of the salt dissociating

 

[BenZq]

Thanks for the feedback. I guess I'll chalk this one up to an imperfect test.