destroyer chem 41 and 125

[pistolpete007]

ok on number 41 it says decreasing temp results in exothermic while increasing temp results in endothermic

looking at 126 the questions was ammonium nitrate will dissolve in water at 25C, and a decrease in temp is noted. which of the following is true?
a. -H, +S
b. -H,-S
c. +H, +S
d, +H, -S
e. +h S=0

i thought the answer would be A bc number 41 daid decreasing temp is exotherm and since solubilty increases entropy it would positive S

however in the solution it says since teh temp is decreased the rxn is endothermic, for heat has been absorbed so we see a +H....doesnt this contradict number 41?

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[happyasaclam88]

i'm not using destroyer, but i think it's sort of misleading to think about temp decreasing as exothermic and temp increasing as endothermic. this depends on what is increasing/decreasing, and dH doesn't always determine T. for the ammonia in water problem, the temperature decreasing means the solution is pulling heat from the thermometer, resulting in a temp drop for the thermometer, but the solution is taking in heat - so it's endothermic (pos dH).

 

[DRHOYA]

I just did this question today. What you have to think about is the surroundings relative to the rxn. It said the temperature decreased. They are referring to the temperature of the surroundings, which means if that temp decreased, then the rxn absorbed the heat, which would make it endothermic, which is +delta H. I know the wording can be tricky, but just step back and look at the big picture.