Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Entropy of Dissolving Ammonium Nitrate  (Read 5328 times)

0 Members and 1 Guest are viewing this topic.

LHM

  • Full Member
  • ****
  • Mole Snacks: +1/-3
  • Offline Offline
  • Posts: 144
Entropy of Dissolving Ammonium Nitrate
« on: January 21, 2011, 09:28:10 AM »

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?
Logged

sci994

  • Regular Member
  • ***
  • Mole Snacks: +0/-3
  • Offline Offline
  • Posts: 25
Re: Entropy of Dissolving Ammonium Nitrate
« Reply #1 on: January 21, 2011, 11:54:46 AM »

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
Logged

rabolisk

  • Chemist
  • Full Member
  • *
  • Mole Snacks: +45/-25
  • Offline Offline
  • Posts: 494
Re: Entropy of Dissolving Ammonium Nitrate
« Reply #2 on: January 21, 2011, 02:21:38 PM »

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.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1367/-369
  • Offline Offline
  • Gender: Male
  • Posts: 20912
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Entropy of Dissolving Ammonium Nitrate
« Reply #3 on: January 21, 2011, 11:44:18 PM »

If u don't get smth just say

Please read forum rules.

No textese on forums, just a plain English.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

sci994

  • Regular Member
  • ***
  • Mole Snacks: +0/-3
  • Offline Offline
  • Posts: 25
Re: Entropy of Dissolving Ammonium Nitrate
« Reply #4 on: January 22, 2011, 10:18:12 AM »

Dissolving of NH4NO3 is Endothermic reaction.
Logged

doc_ross

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 3
Re: Entropy of Dissolving Ammonium Nitrate
« Reply #5 on: March 12, 2012, 08:12:43 AM »

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.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.058 seconds with 23 queries.