Help Required!

[ChemSiddiqui]

Hi, i have a few questions. These appeared in an A'level paper and since i am to prepare for my next month exams, i am doing some past papers and i have a few troulbles doing these questions. Any help will be appreciated by who so ever writes in!.

 

 

1.Which reagent, when mixed and heated with ammonium sulphate, liberate ammonia?

 

A. aqueous bromine

 

B. dilute Hydrochloric acid

 

C. limewater

 

D. acidified potassium dichromate(VI).

 

Well i thought it was B, dilute hydrochloric acid but the mark schemes say that lime water will release ammonia. How come?

 

Another question is the one concerning esters with its displayed forumla given but i could not attach it because i did not know how to do it. Anyway, i would be greatful if u tell me why limewater and not dil.HCl?

[YT2095]

Lime water is calcium hydroxide, it`s not very soluble but Calcium sulphate is even Less soluble, so the Calcium will displace the Ammonium ion and form calcium sulphate as a precipitate.

 

lower Solubility will be the reaction driver.

[woelen]

YT, this answer is not correct. You indeed will obtain calcium sulfate, but that is not relevant here. The use of sodium hydroxide would equally work for liberating ammonia.

 

The real reason for expelling ammonia is that lime water contains hydroxide ions. Ammonium ion, NH4(+) is an acid (albeit a weak one). It reacts with hydroxide as follows:

 

NH4(+) + OH(-) <--->>>> NH3 + H2O

 

The equilibrium is far to the right. So, with any soluble hydroxide, you will get liberation of ammonia from the ammonium ion.

[YT2095]

I stand corrected then, Thnx

[ChemSiddiqui]

Thank for that woelen, i understood why limewater is used, but you made no mention of dil.HCl. Would not dil.HCl liberate ammonia as well, if so then why choose limewater than dil.HCl? that is what my question was.

 

thnx anyway!

[woelen]

No, dilute HCl will not liberate ammonia. It even is the other way around. If you have free ammonia, and add dilute HCl, then it will be converted to ammonium ion, according to the following reaction/equilibrium, which is far to the right:

 

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

 

In order to free ammonia from ammonium ion, you need something, which can pick up the associated H(+). Hydroxide can do that, if you add even more H(+), then ammonium ion definitely cannot be split up.

[ChemSiddiqui]

Thank woelen very much, i appreciate ur explainations!