delta H and S question

[joonkimdds]

Ammonium nitrate will dissolve in water at 25 degree celcius, and a decrease in temperature is noted. Which of the following is true?

answer = +delta H and +delta S

S increase in mixture. But it decreases if temperature goes down.
In this question, both of them apply so how do I know which one counteracts on the other?

delta H is final H - Initial H.
since final temp is lower than initial, shouldn't it be negative?
and also, to decrease the temperature, heat should be released meaning it's exothermic and -delta H is used for exothermic

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[Mstoothlady2012]

the question gives us 2 piece of information. Look at them separately, don't mix them.

Ammonium Nitrate dissolves - which tells us that entropy increases, so delta S is positive

Temperature decreases - which tells us that the reaction is endothermic & it takes up heat, so delta H is positive

 

[250rsavage]

When I read these type of questions, if it says a "decrease in temp" you know it is endothermic so +H and anytime something is going into solution or turning into a gas it is +S. So this rxn is spontaneous at high temps. Anyone correct me if I am wrong

 

[joonkimdds]

hm...isn't endothermic supposed to absorb heat and thus the temp should increase?

 

[Mstoothlady2012]

hm...isn't endothermic supposed to absorb heat and thus the temp should increase?

it absorbs heat, so the temperature of the environment (water here) is decreased.

 

[joonkimdds]

i see...but what about temp vs. S?
doesn't decrease in temp also decrease S?

 

[Mstoothlady2012]

i see...but what about temp vs. S?
doesn't decrease in temp also decrease S?

I dont know about that. I have never correlated entropy with temperatue.

 

[250rsavage]

Temp would play a part in Entropy, since at a certain degree you get a freezing point. But in an endothermic rxn, you are increasing the amount on heat in the rxn but decreasing the "universe" temp. so you are actually increasing the kinetic energy of the solution which increases entropy

Now if it has exothermic, (-H) the solution would lose energy to the "universe" but the "universe" temp would increase.
When you make ice the overall rxn is exothermic, it gives off heat, and you see an decrease in entropy

 

[joonkimdds]

Now if it has exothermic, (-H) the solution would lose energy to the "universe" but the "universe" temp would increase.
When you make ice the overall rxn is exothermic, it gives off heat, and you see an decrease in entropy

making ice = giving off heat outside so that water itself can be cooled and become an ice = exothermic(-delta H) = ice = solid = decrease in randomness = decrease in entropy = (-delta S)


the problem stated that the temperature of solution decreased. If we are keep on decreasing the temperature, it will soon become solid so shouldn't this have (-delta H) and (-delta S)? we are trying to make an ice with the above logic.


Lowering temperature of solution = giving off heat = exothermic(-delta H) = if we keep on decreasing the temperature, it will become solid = (-delta S)



According to wikipedia
"Some examples of endothermic processes are: Melting of ice"

we are not melting ice. We are making ice (decreasing temperature) so we should call this exothermic process which is (-delta H).

 

[250rsavage]

the temp of the solution decreased so where did the heat go, into the rxn giving it more KE (+S)

I think you are thinking way to far into this question, if the question stated "a decrease in temp until it froze that is one thing" but they just just said a decrease so just assume the solution is above its FP

 

[csulapredental]

the question gives us 2 piece of information. Look at them separately, don't mix them.

Ammonium Nitrate dissolves - which tells us that entropy increases, so delta S is positive

Temperature decreases - which tells us that the reaction is endothermic & it takes up heat, so delta H is positive

ugh this explanation made my day.
plain and simple.
learn ppl!!!!!! <3 ily