Aqa chem 5 ligand question

Hi, I'm okay with the ligand stuff, but its my weakest point. Most the questions I can work out how they're done, but theres this one from the January 2012 paper that I just don't understand!

Could anyone explain how and why the answera are what they are, i can't seem to figure it out :\ How would you go about doing this question in the real exam, using your knowledge to answer it.

(edited 11 years ago)

Reply 1

JuggerJay

Which question are you concerned about? I've only looked at the first so far so shall answer it and then add the second answer on once I've done it.

4 marks: It's basically asking you to describe and explain the visible changes. So mark 1 would be stating that you start with a green solution.
mark 2: Green precipitate formed of Fe(H2o)4(OH)2
mark 3: slowly change to brown precipitate.
mark 4: Due to it being oxidised by air to Fe(OH)3

The second question...you'd treat (en) (<---that being the abbreviation for the ligand) as you would NH3. So if you were to add dilute ammonia to [Al(H2O)6]3+ you'd get [Al(OH)3] and NH4+. The same reaction goes for this:

[Al(H2O)6]3+ + 3(en) --> Al(H2O)3(OH)3 + 3(en)+

And white ppt being the observation.

(edited 11 years ago)

Reply 2

Shafski

OP

Original post by JuggerJay

Which question are you concerned about? I've only looked at the first so far so shall answer it and then add the second answer on once I've done it.

4 marks: It's basically asking you to describe and explain the visible changes. So mark 1 would be stating that you start with a green solution.
mark 2: Green precipitate formed of Fe(H2o)4(OH)2
mark 3: slowly change to brown precipitate.
mark 4: Due to it being oxidised by air to Fe(OH)3

I edited the question in, I was okay with the first part. Meant to put the 2nd and 3rd part, sorry.

But im confused with the whole ₂HN-CH₂-CH₂-NH₂ business and what the equations look like. I looked at the mark scheme and don't get why the equations are what they are. In the book theres only examples of Excess/Little OH-, NH₃ or CO₃^2- added.

Reply 3

JuggerJay

Original post by Shafski

I edited the question in, I was okay with the first part. Meant to put the 2nd and 3rd part, sorry.

But im confused with the whole ₂HN-CH₂-CH₂-NH₂ business and what the equations look like. I looked at the mark scheme and don't get why the equations are what they are. In the book theres only examples of Excess/Little OH-, NH₃ or CO₃^2- added.

It's just the normal ligand substitution. As (en) has two lone pairs, three shall replace the 6 water molecules. Does that make more sense now?