Kneecoal

10+ Year Member
Mar 2, 2009
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Long Island, NY
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i get it wrong every time. destroyer gc, #142.

ammonium nitrate will dissolve in water at 25C, and a decrease in temp is noted. which of the following is true?

they say pos. deltaH, pos. deltaS - WHY?

the way i reason it's NOT because the reaction appears spontaneous from the way they write it, but positive deltaH and S are only spontaneous at high temp. 25C is not a high temp! or is "high temp" a relative term?

i guess i can see that a decrease in temp = endothermic and so deltaH would be positive, but it's confusing.
 

bennijai

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May 23, 2006
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When you're dealing that question, just read it and jot down what's going on. The decrease in the temperature of the water means the reaction is absorbing heat, causing water temperature to decrease, thus dH is positive. (If the water temperature was increasing that means the reaction that is dissolving, would be releasing heat into the water). Then we have a solid that is dissolving which is an increase in entropy , thus dS is positive.

If it is dissolving it is spontaneous, thus we can assume the temperature is high enough in this reaction to be spontaneous.