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Ammonia + Water


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#1 KS2010

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Posted 02 February 2010 - 05:03 PM

Hello friends,

When we dissoves ammonia gas (at 20 deg C and 1atm) into water the maximum possible concentration of ammonia into water is 32% i.e. 32kg ammonia gas will dissolve into 68kg of water to give 100kg of ammonical solution and this reaction is exothermic in nature.

My questions:

1) How my reaction will look like?

i.e. NH3 + H2O ----> NH4(+) + OH(-) OR NH3 + H2O -----> NH4OH OR NH3 + 2H2O -----> NH3.H2O + H2O

2) What is the heat of reaction in kJ/mole ? How to calculate the heat of reaction for this reaction/s?

3) As I have explained above is true i.e. 32kg of ammonia in 68kg of water then, it does NOT match with any of this reactions

Any information and help will be hightly appreciated.

Thanks and regards,
KS

#2 fatimah

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Posted 02 February 2010 - 10:48 PM

Hi there

NH4OH represent ammonium solution while NH4+ and OH- show they are in form of ions. yes of course solution is a combination of ions but usually i find NH3 + H2O --> NH4OH.
NH3.OH means the solution is dehydrated.

you mentioned;

"As I have explained above is true i.e. 32kg of ammonia in 68kg of water then, it does NOT match with any of this reactions"

The equation explains 1 mol of NH3 + 1 mol of H2O will produce 1 mol of NH4OH. i'm not sure where you get the information regarding the weight unless further increasing the temp & stirring can increase the solubility of NH3 to 100% concentration.

#3 KS2010

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Posted 03 February 2010 - 02:38 PM

Hi there

NH4OH represent ammonium solution while NH4+ and OH- show they are in form of ions. yes of course solution is a combination of ions but usually i find NH3 + H2O --> NH4OH.
NH3.OH means the solution is dehydrated.

you mentioned;

"As I have explained above is true i.e. 32kg of ammonia in 68kg of water then, it does NOT match with any of this reactions"

The equation explains 1 mol of NH3 + 1 mol of H2O will produce 1 mol of NH4OH. i'm not sure where you get the information regarding the weight unless further increasing the temp & stirring can increase the solubility of NH3 to 100% concentration.




Hello Fatimah and all,

Thanks very much for your reply.

OK, I understood your point. Let's say I am considering below reaction:

NH3 + H2O -----> NH4OH
1kmol 1kmol 1kmol
mol wt(kg/kmol) 17 18 35
weight (kg) 17 18 35
weight (kg) 32 33.88 65.88


Therefore, --------> 32kg of ammonia in 68kg of water -------> does not seem match. However 32kg is the only maximum amount of ammonia which is possible to dissolve in 68kg of water at room temperature and pressure.

If this is the case, is above reaction occurs at different temp and pressure? What is the heat of reaction [dissoving ammonia into water] ?

Thanks in advance
Regards,
KS

Edited by KS2010, 03 February 2010 - 02:40 PM.


#4 fatimah

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Posted 04 February 2010 - 12:57 AM

hi there

not sure how i can help you further. maybe i can give you idea about the equation and your mole equilibrium

http://www.inchem.or...ctionNumber:2.1

See Table 1 shows different solubility at different temp at 1 atm = 101 kPa

20 degree C 529 g/litre
30 degree C 316 g/litre
the solubility decrease in function of temp (probably due to boiling point)

i believe solubility equals to concentration. in any temp it will not satisfy the equation.

i will come back again if i got any information further.

#5 MrShorty

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Posted 04 February 2010 - 11:42 AM

I think you are confusing the concepts of stoichiometry and solubility. When we say "32kg of ammonia [dissolves] in 68kg of water" we aren't talking about the stoichiometry of the specific reaction you're using to describe that solubility. Solubility will have more to do with the equilibrium constant of that reaction -- not the stoichiometry.

As for the heat of reaction. It's been a long time since I took introductory chemistry, but it seems like you calculated heat of reaction using the heats of formation of the products and reactants (which were usually given to you in a table).

Does that help?

Edited by MrShorty, 04 February 2010 - 11:45 AM.


#6 fatimah

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Posted 04 February 2010 - 08:41 PM

Hi MrShorty

when we were at high school, we always exposed to;

A mL of 1 M HCL is diluted in B mL of 2 M NaOH will produce NaCl and H2O. the calculation is based on stoichiometry. but when it comes to KS2010, i'm quite surprised to get confused as i did very good in chemistry before. Hmm

#7 S.AHMAD

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Posted 04 February 2010 - 11:05 PM

Hello friends,

When we dissove ammonia gas (at 20 deg C and 1atm) into water the maximum possible concentration of ammonia into water is 32% i.e. 32kg ammonia gas will dissolve into 68kg of water to give 100kg of ammonical solution and this reaction is exothermic in nature.

My questions:

1) How my reaction will look like?

i.e. NH3 + H2O ----> NH4(+) + OH(-) OR NH3 + H2O -----> NH4OH OR NH3 + 2H2O -----> NH3.H2O + H2O

2) What is the heat of reaction in kJ/mole ? How to calculate the heat of reaction for this reaction/s?

3) As I have explained above is true i.e. 32kg of ammonia in 68kg of water then, it does NOT match with any of this reactions

Any information and help will be hightly appreciated.

Thanks and regards,
KS

Dear KS2010

Please do not confuse solubility and chemical reaction or dissociation into ions. Oxygen, for example dissolves in water but it does not react or dissociate in water. Similarly, ammonia dissolves in water and partly dissociates in the presence of water into ammonium ions. As we all know that water dissociates into H(+) and OH(-). The NH3 reacts with H(+)to form NH4(+). So we can write a chemical reaction:

NH3 + H(+) + OH(-) <-----> NH4(+) + OH(-)

The above reaction is an equilibrium reaction. So the amount reacts depends on the equilibrium constant which is dependent on temperature. The un-reacted ammonia remains as a dissolved gas just like oxygen in the above example. This is also true for CO2 in water where carbonic acid is produced in this case.

The heat of reaction can be determined by experiment. Use calorimeter.

I hope the above helps.

Edited by S.AHMAD, 04 February 2010 - 11:16 PM.


#8 kkala

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Posted 07 February 2010 - 04:45 PM

Although recent replies clarify the matter enough, some more "comments" may be useful.
1. NH3 gas is in equilibrium with NH3 dissolved in water. NH3 dissolved in water is in equilibrium with NH4 and OH (coming from partial NH3 dissociation).
2. Attached graph shows partial pressure of NH3 over aqueous solutions of pure water at 20 0C. For NH3 pressure of 1 Atm a, NH3 content in solution should be 53 kg NH3 per 100 kg H2O. This agrees to Inchem's solubility of 529 g/l (brought by fatimah), assuming that volume l concerns water (not the solution).
3. Above NH3 content corresponds to a solution of 53/153=34.6% NH3 by weight, which is not far from the reported 32% by weight. It is noted that NH3 is dissolved less in waters having salts (as Inchem indicates).
4. Since NH3 is a weak base, a small percentage of the dissolved NH3 is dissociated in pure water. Only NH4, OH, dissociated NH3 follow stoichiometry (neglecting H2O dissociation, as insignificant in this case).
5. Heat produced during NH3 dilution in pure water may be estimated from enthalpy concentration diagram of aqueous NH3, such as this of Perry 7th ed (Physical and Chemical data - Thermodynamic properties). It may not be so easy, the diagram is not quite legible.

Attached Files

  • Attached File  NH3.xls   17KB   87 downloads

Edited by kkala, 08 February 2010 - 03:14 AM.





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