solubility of a salt with an exothermic dissolution

[BenZq]

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This question derives from Berkeley Review CBT# 4 PS#12. If the temperature of water increases when a salt is dissolved, how does an increase in water temperature affect solubility. I originally thought that since it was exothermic an Increase in water temperature would Decrease solubility, but BR says that's incorrect. If someone can explain this I would greatly appreciate it. 🙂

 

[MedChallenge]

This question derives from Berkeley Review CBT# 4 PS#12. If the temperature of water increases when a salt is dissolved, how does an increase in water temperature affect solubility. I originally thought that since it was exothermic an Increase in water temperature would Decrease solubility, but BR says that's incorrect. If someone can explain this I would greatly appreciate it. 🙂

I answered it by comparing the graph and table. Since in the table it states that at 32 degrees it is at 75g/100mL H2O and the line for KBr increases pretty much throughout, it would have to be greater than 75. Does it state it's exothermic somewhere or is that something we're supposed to know?

And my follow-up question is, why are some solvations exothermic and some endothermic? Aren't you breaking bonds when you're dissolving a salt? Doesn't bond-breaking=endothermic and bond-forming=exothermic?

 

[BenZq]

Does it state it's exothermic somewhere or is that something we're supposed to know?

The first paragraph states that as she adds salt the temperature increases... So I reason that its an exothermic reaction

why are some solvations exothermic and some endothermic? Aren't you breaking bonds when you're dissolving a salt? Doesn't bond-breaking=endothermic and bond-forming=exothermic?

yes bonds are broken, but new bonds are formed as well. If the new bonds formed are stronger than the bonds broken then the delta H is - or exothermic, and if their weaker than the broken ones its +. (deltaH rxn = bonds broken - bonds formed)

 

[MedChallenge]

The first paragraph states that as she adds salt the temperature increases... So I reason that its an exothermic reaction



yes bonds are broken, but new bonds are formed as well. If the new bonds formed are stronger than the bonds broken then the delta H is - or exothermic, and if their weaker than the broken ones its +. (deltaH rxn = bonds broken - bonds formed)

Oh, gotcha...thanks! And yeah, I definitely see where the confusion is coming from. Hmm, I don't know why increasing water temp would increase solubility then. Like you said, increasing the water temperature would seem to decrease solubility, but adding the salt would push it towards the product. So now I feel like maybe those two would balance out, B? I'm confused about why the answer is A, too, besides for the table/graph in the passage.

 

[apex954]

This question derives from Berkeley Review CBT# 4 PS#12. If the temperature of water increases when a salt is dissolved, how does an increase in water temperature affect solubility. I originally thought that since it was exothermic an Increase in water temperature would Decrease solubility, but BR says that's incorrect. If someone can explain this I would greatly appreciate it. 🙂

I dont have the exact question, table or graph. I'll assume the temperature of the solution increases as a result of the dissolution of salt. Because there is a difference between the temperature increasing as a result of the solvation i.e., (exothermin rxn) and increasing the temperature of an already exothermic rxn (i.e., adding more product (heat) to an exothermic), solubility can go either way. So an increase in water temperature due to the exothermic nature of the rxn without any external addition of heat & all things held constant is an indication of a favorable solubility of the substance. I hope i didnt confuse further....