Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Somes J on 30/06/2011 21:40:30

Title: Simple question about ammonia/water solutions
Post by: Somes J on 30/06/2011 21:40:30

When ammonia dissolves in water some of it reacts to form ammonium and hydroxide ions:

NH3 + H2O = NH4+ + OH-

http://scifun.chem.wisc.edu/chemweek/pdf/ammonia.pdf (http://scifun.chem.wisc.edu/chemweek/pdf/ammonia.pdf)

What happens if you then heat or boil the solution, driving off the ammonia? Does all the NH4+ turn back into NH3?

Just a very basic chemistry question I was wondering about, I can't seem to find the answer on Google.

Thanks.

Title: Simple question about ammonia/water solutions
Post by: Bored chemist on 30/06/2011 21:53:07

Essentially yes, the reaction is driven backwards. The proton moves back from the NH4+ onto the OH- and forms water and ammonia again.

Title: Simple question about ammonia/water solutions
Post by: CliffordK on 30/06/2011 21:59:56

I believe you get ammonia (NH₃) gas.

Cations rarely exist without their corresponding anions, and it would not be energetically favorable to release cations without anions.  And, some kind of ammonia hydrate NH₃•H₂O would be unlikely to form and remain stable as a vapor.

Title: Simple question about ammonia/water solutions
Post by: lightarrow on 05/07/2011 15:14:46

NH₃(gas over the solution) [revarrow] NH₃(gas dissolved in water) [revarrow] NH₄⁺ + OH^-.

If you reduce the partial pressure of NH₃(gas over the solution) then equilibrium shifts to the left so ammonium ions react with hydroxides ions to form dissolved ammonia which then escapes the solution. Now, this happens when there is equilibrium, but if you leave the solution in open atmosphere, you don't have because the ammonia is not present (or very little) in the air. However the reaction moves to the left only at very little speed, at room temperature. Heating the solution you increase that speed.