Bitburger - 30-7-2011 at 01:10
I would like to prepare ammonium carbonate from 12% ammonia and CO2.
CO2 has a better solubility in cold water, so I try to perform this reaction in a cold solution of ammonia.
BUT: is it possible to form ammonium carbonate without heating? How can I calculate this?
[Edited on 30-7-2011 by Bitburger]
redox - 30-7-2011 at 06:24
Well, wouldn't simply bubbling CO2 into the aqueous ammonia give you ammonium carbonate? I don't think any heating is necessary.
CO2 + H2O => H2CO3
NH3 + H2O => NH4OH
H2CO3 + 2NH4OH => NH4CO3 + 2H2O
Bitburger - 30-7-2011 at 07:25
Yes, it will form in a cold solution. Maybe, it's a equilibrium reaction and heating causes the opposite effect, because of an increase in entropy
during heating?
But, there MUST be a way to calculate gibbs free energy for this reaction, just to prove that this reaction exists, which is where science is all
about!
With lots of coffee I will try this calculation but I don't know how to start!
What values do I need in this case?
Just any help is welcome!
Bitburger - 4-8-2011 at 10:30
I calculate the Gibbs free energy of this reaction in aqueous solution:
H2O + CO2 + 2 NH3------------> 2
NH4+ + CO32-
When CO2 is dissolved, but not dissociated then delta H° = -150.78 kJ/mol
delta S° = -402.51 J/mol.K
delta G° = -30.78 kJ/mol
By lowering the temperature to 5°C, the calculated value of delta G = -38.822 kJ/mol so that it seems right to claim that the lower
the temperature is, the more favourable the reaction is.
When the CO2 is in a gas state, it will still react with the ammonia, which is proven by the crystals that where
formed on the top of the erlenmeyer flask.
delta H°= -107.53 kJ/mol
delta S°= -496.935 J/mol.K
delta G°= -22.4 kJ/mol
By lowering the temperature to 5°C, the calculated value of delta G = -32.208 kJ/mol, so that the same conclusions can be drawn.
However, if you suppose the formation of HCO3- it might be possible that following reaction take place:
2 H2O + CO2 + 2 NH3----------> 2
NH4+ + HCO3- + OH-
If that is true the calculated value would be:
delta H°= -109.665 kJ/mol
delta S°= - 232.324 J/mol.K
delta G°= 122.659 kJ/mol
Since delta G° is positive this reaction should never take place!
[Edited on 4-8-2011 by Bitburger]
[Edited on 4-8-2011 by Bitburger]
Bitburger - 11-8-2011 at 11:24
Despite all the calculations that I've done and the prediction from another member that this synthesis is favourable, no ammonium carbonate is formed
in the solution after a evaporation at room temperature of the water.
I repeat the addition of CO2 bubbeling into the ammonia solution several times, since the solubility of carbon
dioxide is low even at the low temperatures that I used.
When the solution was evaporated I observed a rather strong smell of ammonia.
What is the scientific explanation that not even a crystal is formed?
[Edited on 11-8-2011 by Bitburger]