Destroyer GChem, # 126, please help!

[drzakisadiq]

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the question states,

Ammonium nitrate will dissolve inw ater at 25 degrees celsius, and a decrease in temperature is noted. Which of the following is true?

I wont type out the answers but they said that it will be + delta H, and + delta S. Now I understand the entropy part because you dissolve something in water, the disorder increases. But they said there was a decrease in temperature. I assume that to be because temperature is released into the environment, making it an exothermic reaction. But they said the reaction is endothermic because a decrease in temp means heat is being absorbed, thats why there is a + delta H. Can anyone explain this to me? I'm real confused.

 

[dentalknight]

sorry i dont have much of a detailed answer for you, because i suck at these too. but according to my kaplan book, a positive enthalpy is an endothermic process. a negative enthalpy is an exothermic process.

also this is just an assumption, but if ammonium nitrate is dissolving in a solvent, then bonds are being broken. the breaking of bonds is an endothermic process which is a positive H as you said due to the increase in disorder. Maybe you had your endothermic and exothermic mixed up?

 

[smilefirst]

when something is exothermic (- delta H) it releases energy in the form of heat, so the temperature will therefore increase. Thus if something is endothermic (+ delta H) that means it requires energy which will be absorbed during the reaction, so the final temperature is lower. I think your understanding of endothermic and exothermic is reversed. Just remember: exothermic will increase a solution's final temperature and endothermic will decrease it.

 

[futuredentist21]

This question is similar to question 75 in Destroyer GChem which states: An unknown compound dissolves in H20 spontaneously. If the temperature of the water decreases, which is true?

I also kinda don't get this question either...I understand why entropy increases, since a substance is dissolving, why is the soln endothermic though? I guess also it has to do with bonds breaking

can anyone help out drzaki and I...I'll be eternally grateful 🙂 Thanxs

 

[admap]

When any question says that there is an increase in heat, that means it (being the test tube or a beaker) gets warm and that means the reaction is exothermic. When the question says it decreases, the test tube becomes cold as soon as the reaction takes place. They are not waiting for the heat to dissipate into the environment. The information they are giving you is exactly for the reaction.
Don't assume anything till they tell you to... atleast for this question!! Hope this helps...

 

[Lazerous]

I asked this same question about three days ago. Here is what I understood and it makes sense to me:

This is a System and Surroundings type of problem (both 75 and 126).

1) Think of the system as the reactants, and the surrounding as the products.
2) Now if you recall, in endothermic reactions the "+heat" is written on the reactant side and for exothermic the "+heat" is written on the product side.
3) Endo is positive delta H and exo is negative delta H

Just to recap:
Endo=system
Exo=surroundings

Temperature changes as discussed in these two questions are concerned with what happens to the surroundings, not the system. So when they say the temperature decrease then they are talking about the temperature of the surroundings. This means that in order for the temperature to decrease then the system (or reactants) must be absorbing the heat. And if the reactants are absorbing heat then they are endothermic:

System + heat --> cool surroundings

I hope this helps.