Chemistry Question

[dvduplex]

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Hi everyone,

Could someone show help me with this problem? I know what the answer is, but I don't understand why that is the answer. Thanks!

 

[g23armani]

Spontaneous, so G < 0

Temperature of water decreases, so endothermic, H > 0

It's a solid dissolving AB(s) --> A(aq) + B(aq) so entropy is increasing S > 0

 

[americanpierg]

Hi everyone,

Could someone show help me with this problem? I know what the answer is, but I don't understand why that is the answer. Thanks!

I think this is a very, very poorly writen question.

It WAS spontaneous, but if the temperature drops, anything could happen. if the temperature drops far enough, the compound could supersaturate and the delta G for the reaction could be positive... then again it may not have reached saturation point and delta G could still be negative



Temperature of the water decreased, so heat is given off into the surround, so and delta H < 0



Entropy is positive b/c dissolving salts leads to more stuff in the system, but a decrease in temperature also leads to a lost of heat, so q = negative and entroy = negative...s=q/T... (I THINK)

 

[Erhatstil]

I think this is a very, very poorly writen question.

It WAS spontaneous, but if the temperature drops, anything could happen. if the temperature drops far enough, the compound could supersaturate and the delta G for the reaction could be positive... then again it may not have reached saturation point and delta G could still be negative



Temperature of the water decreased, so heat is given off into the surround, so and delta H < 0



Entropy is positive b/c dissolving salts leads to more stuff in the system, but a decrease in temperature also leads to a lost of heat, so q = negative and entroy = negative...s=q/T... (I THINK)

i think delta H>0 because the water is losing heat while the surrounding is gaining it, making it an endothermic reaction

 

[pbure9]

So as others have said... since it says its spontaneous deltaG is negative.

Also, when something dissolves or becomes a gas, entropy is positive.

Finally, if the temperature of the water decreases, that means that the reaction is endothermic and is making the water feel colder since its absorbing heat

So the answer should be e.. Good question

 

[americanpierg]

i think delta H>0 because the water is losing heat while the surrounding is gaining it, making it an endothermic reaction

actually i just thought about it as i was typing cuz it was confusing me too, since endothermic reactions are cold yet they absorb heat.

after a bit of thinking, i realized that the heat goes into the reaction, not the surrounding. the water temperature goes down becuase heat is going into the reaction, not the surrounding, so the reaction is endothermic.

and i thought it said the temperature of the water IS decreaesd (by external forces) lol so ignore everything i said.

 

[dvduplex]

i think delta H>0 because the water is losing heat while the surrounding is gaining it, making it an endothermic reaction

Sorry, I am kind of dumb, but how do you know that the surrounding is gaining heat. Because the question never mention anything about the surrounding, so I am quite curious to where you got that information. Is it in the Kaplan book by any chance? Thanks!

btw, the answer is "E". I got all the choices right except for delta H.

 

[UCB05]

Sorry, I am kind of dumb, but how do you know that the surrounding is gaining heat. Because the question never mention anything about the surrounding, so I am quite curious to where you got that information. Is it in the Kaplan book by any chance? Thanks!

btw, the answer is "E". I got all the choices right except for delta H.

The system is the dissolved substance. It has an implied increase in energy because heat is taken away from the water. You have to think of just the compound itself as the reaction, and the aqueous medium as it surroundings.

 

[keliao]

[/QUOTE]
I am confused too!!...but thisis what i would come up with.

"unknown diss in water SPONTAN" saying "deltaG < 0 " so choices left are [c d e]

"water temperature DECREASES" "deltaH and deltaS < 0 " to have them both negative because it is Spontaneous at low temperature.???

i dont understand why is "e" when "deltaH and deltaS > 0"
wouldnt it be spontanous at HIGH TEMPERATUR???????

 

[UCB05]

I am confused too!!...but thisis what i would come up with.

"unknown diss in water SPONTAN" saying "deltaG < 0 " so choices left are [c d e]

"water temperature DECREASES" "deltaH and deltaS < 0 " to have them both negative because it is Spontaneous at low temperature.???

i dont understand why is "e" when "deltaH and deltaS > 0"
wouldnt it be spontanous at HIGH TEMPERATUR???????

I really dont understand any of what you're trying to say. All the information you need is given in the question. It's spontaneous, so G<0. The compound dissolves in the water, implying that S>0. The temperature of the water decreases, which means heat was transferred from the solvent (surroundings) to the solute (reaction) so the reaction itself is endothermic, so H>0.

more talk about whether reactions should be spontaneous or not or endothermic or not with certain values of G have nothing to do with the question.

 

[americanpierg]

I am confused too!!...but thisis what i would come up with.

"unknown diss in water SPONTAN" saying "deltaG < 0 " so choices left are [c d e]

"water temperature DECREASES" "deltaH and deltaS < 0 " to have them both negative because it is Spontaneous at low temperature.???

i dont understand why is "e" when "deltaH and deltaS > 0"
wouldnt it be spontanous at HIGH TEMPERATUR???????[/QUOTE]

I had this problem at first, then I read the question over.



Bascially, the question says that ::

The compound dissolves in water. This process then causes the temperature of the water to decrease.


I originally interpreted that they asked what would happen if the temperature was decreased (such as by an external force).


The compound dissolves spontaneously, so G<0, temperature of water decreases (heat is going into the reaction) so H>0, compound dissolves, leading to an increase in components and disorder, so S>0

It's not saying temperature is low (mistake i made too), its just saying the reaction causes the temperature to drop (bascially so you know what delta H is)

 

[dvduplex]

wow...I never realized that this question would be so complex. Good thing, I post it here or else I will still scratch my head over it.

After I read many posts, I think I finally understand it, but it is still hard to digest though.

btw, is this concept in the kaplan book? I want to know because my weakness is chemistry and I want to know if reading that book is suffice for preparing for the DAT. thanks!

 

[bustedgrill]

This is from Destroyer