AQA a level chemistry specimen paper3 Q:22

Q: What is the final species produced when an excess of aqueous ammonia is added to aqueous aluminium chloride?
For this reaction i suppose the reaction's equation were: AlCl4 (aqueous aluminium chloride) +6NH3 (aqueous ammonia)> (as ammonia is in excess) [Al (NH3)6]3+ +2Cl2
but why is this wrong? as i suppose mostly complex ion with excess ammonia end up with [M (NH3)6]3+, so why is the answer [Al(OH)3(H2O)3]?

Reply 1

Pigster

Original post by Violet Chow

It is wrong for three reasons:
1. your suggestion involves AlCl4, which doesn't exist
2. even if you used AlCl4-, the charges on your equation do not balance
3. the product is what is says on the MS. Not all metal complexes dissolve in excess ammonia.

Reply 2

Violet Chow

o, ok, but the question only say excess ammonia, so how we suppose to know how many ammonia were placed in ( OH replace H2O)?

Reply 3

charco

Study Forum Helper

Original post by Violet Chow

o, ok, but the question only say excess ammonia, so how we suppose to know how many ammonia were placed in ( OH replace H2O)?

The question is from the inorganic qualitative identification of cations.

The tests that are carried out to identify cations involve adding:
1. A few drops of NaOH(aq) followed by excess NaOH(aq) and making observations.
2. A few drops of ammonia solution followed by excess ammonia solution.

In the event that a precipitate is observed that is not coloured, it is possible to identify the metal ion by the effect of excess of both of these reagents. For example. The white ppt formed by addition of NaOH(aq) to Al³⁺ (aq) dissolves in excess, but the white ppt formed by addition of NH₃(aq) does not dissolve in excess.

It is possible to differentiate Mg²⁺(aq), Zn²⁺(aq), Pb²⁺(aq) and Al³⁺ (aq), all of which have white precipitates in base.

If you had learned the reasons why, then you would have known the answer to the question.