Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed)

can anyone please explain solvent extraction to me clearly, cant really fully understand it from the book

Reply 1421

can someone give the 2 reactions of zinc as a base and acid, when you write the equation with h20's(opposed to simplified version) the waters often disppearin some mark schemes, i dont understand where it goes

Reply 1422

Does anyone know of any good online resources for this unit? Chemguide doesn't seem to be working today. :frown:

Reply 1423

shumen9523

6

Original post by AS01

Did it help though?

Yep, it assured me that we probably won't be asked this particular question about zwitterions in the exam.
I'm still wondering about how chemists explain this though, but I guess it isn't required for A level, afterall.
Thanks for your answer once again.

Reply 1424

jethacan

2

does everyone know the colours of all the complex ions and the aqua OH- and NH3 ones? I don't see how I'm going to remember all of it!

Reply 1425

how are we supposed to know answer is D?? through elimination or is it obvious?? pls help me

Capture.JPG22.4KB

Reply 1426

Original post by jethacan

does everyone know the colours of all the complex ions and the aqua OH- and NH3 ones? I don't see how I'm going to remember all of it!

well theres 7 and all 7 form precipitates, 3 of them have no effect when adding anything so easy to remember these, zinc is easy to remember because colourless for everything but a white precipitate(when drops of OH), but have to know formulas for all precipitates(these all follow same pattern and easy to remember) and formulas for ONLY chromium, zinc and copper when XS OH/NH3 added, can be done in around 20 mins if you keep testing yourself

Reply 1427

which transition metals change when excess NaOH or NH3 is added? also which ones undergo ligand exchange with NH3? i can't find this anywhere

Reply 1428

Original post by LeaX

which transition metals change when excess NaOH or NH3 is added? also which ones undergo ligand exchange with NH3? i can't find this anywhere

Change with excess NaOH:
Cr 3+ : forms green solution
Zinc 2+ : forms colourless solution

Change with excess NH3:
Cr 3+ : forms green solution
Ni 2+: forms pale blue solution
Cu 2+: forms deep blue solution
Zn 2+: forms colourless solution

I think all the hydrated complexes of metals listed above undergo ligand substitution with NH3 ?
Are there any conditions for substitution to occur? (except for stability of the ligand)

Reply 1429

Original post by Knoyle quiah

how are we supposed to know answer is D?? through elimination or is it obvious?? pls help me

This is based on the principle behind the recrystallization method:
It is used to purify solids that are soluble in hot solvents, but insoluble in cold solvents (benzoic acid is an example).
- The impure solid is dissolved in hot solvent.
- Undissolved impurities are removed (by filtering with a warmed stemless funnel)
- The solution is cooled (the pure solid now forms since it's insoluble in cold solvent)
- Pure solid is filtered using a Buchner funnel
- Wash the solid on filter paper with ice-cold solvent and leave to dry
- Carefully remove the solid from the filter paper.

So the answer is D because only the pure solid is present in the cooled solution

(edited 10 years ago)

Reply 1430

Original post by SKK94

Change with excess NaOH:
Cr 3+ : forms green solution
Zinc 2+ : forms colourless solution

Change with excess NH3:
Cr 3+ : forms green solution
Ni 2+: forms pale blue solution
Cu 2+: forms deep blue solution
Zn 2+: forms colourless solution

I think all the hydrated complexes of metals listed above undergo ligand substitution with NH3 ?
Are there any conditions for substitution to occur? (except for stability of the ligand)

Thank you so much. :biggrin:

So for example with Cu²⁺, the equations are:
[Cr(H₂O)₆]²⁺ + 2NH₃- --> [Cr(OH)₂(H₂O)₄] + 2NH⁴⁺
excess ammonia: [Cr(OH)₂(H2O)₄] + 2NH₃ ---> [Cr(NH3)₂(H2O)₄]
or [Cr(OH)₂(H2O)₄] + 6NH₃ ---> [Cr(NH₃)₆]

and ammonia will never have this effect: [Cr(OH)₂(H2O)₄] + 2NH₃ ---> [Cr(OH)₄(H2O)₂]^2- like sodium hydroxide would?

Reply 1431

Original post by LeaX

Thank you so much. :biggrin:

So for example with Cu²⁺, the equations are:
[Cu(H₂O)₆]²⁺ + 2NH₃- --> [Cu(OH)₂(H₂O)₄] + 2NH⁴⁺
excess ammonia: [Cu(OH)₂(H2O)₄] + 2NH₃ ---> [Cu(NH3)₂(H2O)₄]
or [Cu(OH)₂(H2O)₄] + 6NH₃ ---> [Cu(NH₃)₆]

and ammonia will never have this effect: [Cu(OH)₂(H2O)₄] + 2NH₃ ---> [Cu(OH)₄(H2O)₂]^2- like sodium hydroxide would?

Yeah, but with excess ammonia:
[Cu(OH)₂(H2O)₄] + 4NH₃ ---> [Cu(NH₃)₄(H2O)₂]²⁺ + 2OH^- + 2H₂0

or
[Cu(H₂O)₆]²⁺ + 4NH₃ ---> [Cu(NH₃)₄(H2O)₂]²⁺+ 4H₂O

For ligands other than water, only 4 of them can form a complex with copper.

I don't think ammonia reacts like that. Not so sure though :redface:

(It looked like you accidentally wrote Cr instead of Cu? Hope you don't mind me editing it) :smile:

(edited 10 years ago)

Reply 1432

Weaselmoose

9

I'm not sure if it has been asked before, but for the past paper pack from this thread, there was a trial examination paper and I was wondering if anyone had the mark scheme to it?

Reply 1433

Original post by SKK94

This is based on the principle behind the recrystallization method:
It is used to purify solids that are soluble in hot solvents, but insoluble in cold solvents (benzoic acid is an example).
- The impure solid is dissolved in hot solvent.
- Undissolved impurities are removed (by filtering with a warmed stemless funnel)
- The solution is cooled (the pure solid now forms since it's insoluble in cold solvent)
- Pure solid is filtered using a Buchner funnel
- Wash the solid on filter paper with ice-cold solvent and leave to dry
- Carefully remove the solid from the filter paper.

So the answer is D because only the pure solid is present in the cooled solution

Thank you

Reply 1434

Notes on Chemistry unit 5 specification - George Lewis.docx807.3KB

Original post by SKK94

Yeah, but with excess ammonia:
[Cu(OH)₂(H2O)₄] + 4NH₃ ---> [Cu(NH₃)₄(H2O)₂]²⁺ + 2OH^- + 2H₂0

or
[Cu(H₂O)₆]²⁺ + 4NH₃ ---> [Cu(NH₃)₄(H2O)₂]²⁺+ 4H₂O

For ligands other than water, only 4 of them can form a complex with copper.

I don't think ammonia reacts like that. Not so sure though :redface:

(It looked like you accidentally wrote Cr instead of Cu? Hope you don't mind me editing it) :smile:

I finally understand! Thank you so much. Is there any reason why Copper can only bond with 4 ammonias whilst Ni, Zn and Cr can form [M(NH3)6] complexes?

Reply 1435

GeorgeL3

5

Hi everyone, I just finished making some notes on the specification and I've tried to include everything that they might ask. If anyone spots any mistakes or something that's missing then I'd be grateful if you could let me know :smile:

(edited 10 years ago)

Reply 1436

Original post by LeaX

I finally understand! Thank you so much. Is there any reason why Copper can only bond with 4 ammonias whilst Ni, Zn and Cr can form [M(NH3)6] complexes?

Welcome

Haha, there probably is. I tried searching chemguide, but couldn't find much info on it.
Maybe something to do with energetic favourability?

Reply 1437

Original post by GeorgeL3

Hi everyone, I just finished making some notes on the specification and I've tried to include everything that they might ask. If anyone spots any mistakes or something that's missing then I'd be grateful if you could let me know :smile:

Amazing! Thanks :biggrin:

Reply 1438

AS01

12

Original post by LeaX

which transition metals change when excess NaOH or NH3 is added? also which ones undergo ligand exchange with NH3? i can't find this anywhere

the one that undergo ligand exchange are the ions that dissolve in excess ammonia. So Ag+, Cu2+ ,Zn2+,Ni2+,Co2+
what do u mean by that? colour change?
Ligand exchange doesnt happen with NaOH :smile:

Reply 1439

posthumus