[LHM]

When NH₄NO₃ dissolves in water, the solution becomes cold. From this you can deduce that the :delta: S° for the process is

A) negative
B) zero
C) positive
D) undeterminable from the data

I figured that since sometimes the entropy of ions can be negative because it's creating more order, you can't really figure out what the entropy is from the enthalpy. However, it says the answer is C. Why? Is it actually possible to determine what the entropy is from the enthalpy?

[sci994]

Dissolving of ammonium-nitrate in water is endotermic reaction.
The formula for entropy is   :delta: S= :delta: Q/T.
If a reaction is endotermic that means that  :delta:H>0(positive)
Conditions are ideal and temperature is const. So the change of entropy is positive. =D

If u don't get smth just say

[rabolisk]

Dissolving of ammonium-nitrate in water is endotermic reaction.
The formula for entropy is :delta: S= :delta: Q/T.
If a reaction is endotermic that means that  :delta:H>0(positive)
Conditions are ideal and temperature is const. So the change of entropy is positive. =D

If u don't get smth just say

That's not right. First, :delta: S = :delta: Q/T is not correct. There is no such thing as :delta: Q. Besides, you're implying that ALL endothermic reactions necessarily result in increase in entropy of the system (positive :delta: S), which is also not right.

The reason :delta: S is positive comes from :delta: G = :delta: H - T :delta: S. You had to know that solvation of ammonium nitrate in water was a spotaneous reaction. But whenever something is said to dissolve in water, the reason it dissolves is because it is spontaneous (The fact that a reaction occurs, by definition, must mean that the reaction is spontaneous). With :delta: H being positive, :delta: S had to be positive for the reaction to occur.

[Borek]

If u don't get smth just say

Please read forum rules.

No textese on forums, just a plain English.

[sci994]

Dissolving of NH4NO3 is Endothermic reaction.

[doc_ross]

The best way to rationalize a positive entropy change is due to the spontaneity of the reaction.  Ammonium nitrate will spontaneously dissolve in water. Spontaneous processes result in a positive entropy change for the universe.
You cannot, however, infer the change in entropy for the system from the information given.  You only know that the sum of the entropy change for the system plus the entropy change for the surroundings (which equals entropy change for the universe) is a positive number.