Is Ammonia absorption in water exothermic?

[pranj5]

A person working in a company that manufactures absorption refrigerator told me that when Ammonia dissolves in water, the water temperature rises. In short, Ammonia absorption is water is exothermic. If it's true, then what's the reason behind it? Is it due to the fact that the latent heat of vaporisation of the Ammonia is added to the gross enthalpy of the mixture and that's why the temperature rises? If that's true then it's true for other gas/vapour too. Those who have good knowledge about this matter kindly explain.

 

[Charles Link]

I don't have a complete answer, but a quick "google of the subject says that the exothermic reaction is due mainly to hydrogen bonding rather than to any ammonium formation. Basically, the ammonia molecules must be attracted significantly to the water molecules and form a lower energy state where ammonia will actually leave the gaseous phase to go into solution. Ammonia boils at T=-33 degrees centigrade, but at room temperature, it will become part of a water solution because being attached to liquid water is a lower energy state than the vapor phase of $NH_3$. There must be a significantly greater attraction of $NH_3$ to $H_2 O$ than there is of $NH_3$ to $NH_3$.

 

[pranj5]

Suppose there is 10.8 kg Ammonia at -34°C (6% liquid and the rest vapour) is mixed with 12 kg of water at 27°C. What can be the possible final temperature of the solution? Will it be so much heated that the water will began to boil?

 

[Charles Link]

The answer seems to be that it could generate enough heat to take the water near the boiling point. I found a number for the $\Delta H=-30 kjoules/mole=-7kcal/mole$ (approximately). Just a couple quick calculations: Any feedback from others is welcome... Let's use 10.8 g of ammonia and assume 1/3 of it=3g dissolves in 12 g of water. $(3/17)moles (7000 cal/mole)=1200$ calories. That's enough heat to raise the 12g of water to 100 degrees centigrade. Will it boil? It takes 540 cal/gram to vaporize water=a fair amount of water could make it to the vapor phase=not sure whether it would be called boiling... (Note: 17=Molecular weight of $NH_3$.)

 

[pranj5]

Do you want to mean that by dissolving, the latent heat of vaporisation of the Ammonia will be added to the water and that will raise the temperature of the solution?

 

[Charles Link]

Do you want to mean that by dissolving, the latent heat of vaporisation of the Ammonia will be added to the water and that will raise the temperature of the solution?

I don't have very precise numbers for the latent heat. The conditions of the experiment for this case as well as the numbers being used would need to be carefully defined because of the complicating factor that $NH_3$ will have a heat of vaporization of its own that will enter into any calculation. Thereby, my numbers are only approximate, but do suggest a fair amount of heat will be generated upon mixing. For the numbers that I found, I don't know if they were mixing liquid ammonia or something partly in the vapor phase...A more complete answer would take a fair amount of researching the numbers in the literature...

 

[pranj5]

I have asked that because 30 kjoules/mole is such a high amount that it can only be compared with latent heat of vaporisation. Can you give me the link where you have got this information i.e. ΔH=−30kjoules/mole=−7kcal/mole?

 

[Charles Link]

This was "Wikipedia" and they gave numbers for several ammonia compounds along with ammonia. For $NH_3$, they weren't specific on how the process was carried out. If you are interested in precise answers, I would suggest a much more thorough literature search. Perhaps other members on the website could provide more precision. I am not an expert in this area, and it would take a lengthy literature search for me as well. Quick googles were supplying rather insufficient information... Hopefully my answers were at least somewhat helpful. :)

 

[pranj5]

As per wikipedia, the latent heat of vaporisation of Ammonia is 23.35 kJoules/mole, not 30.

 

[Charles Link]

You may be looking at a different article and the difference is likely to be the conditions used in the mixing. I expect that in the literature, the numbers will vary widely, even with accurate experimentation, because the components are mixed in different forms... The latent heat of vaporization of $NH_3$ that factors into all of this, will be quite significant...