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

Poll: How pumped up are you for this exam?-(warning)-(bad jokes arene this poll!)
"Titanium-I'm not going to corrode (even at high temperatures)" (A*) (22)
16.67%
"Benzene's my middle name, give me the paper in a week and I'll ace it!" (A) (27)
20.45%
"Yeah, I'm fairly electrophillic (positively charged) about the exam" (B) (27)
20.45%
"I'm in the middle of the salt bridge, but I will pass-eventually" (C) (21)
15.91%
"I'm feeling rather electroNegative about this exam" (D) (18)
13.64%
"Benzene, what's that?" (E) (6)
4.55%
"Chemistry, what's that?" (F) (11)
8.33%
This discussion is closed.
Knoyle quiah
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#1421
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#1421
can anyone please explain solvent extraction to me clearly, cant really fully understand it from the book
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Knoyle quiah
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#1422
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#1422
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
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LeaX
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#1423
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#1423
Does anyone know of any good online resources for this unit? Chemguide doesn't seem to be working today.
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shumen9523
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#1424
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#1424
(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.
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jethacan
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#1425
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#1425
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!
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Knoyle quiah
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#1426
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#1426
how are we supposed to know answer is D?? through elimination or is it obvious?? pls help me
Attached files
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Knoyle quiah
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#1427
(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
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LeaX
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#1428
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#1428
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
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SKK94
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#1429
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#1429
(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)
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SKK94
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#1430
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#1430
(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
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LeaX
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#1431
(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. 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?
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SKK94
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#1432
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#1432
(Original post by LeaX)
Thank you so much. 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

(It looked like you accidentally wrote Cr instead of Cu? Hope you don't mind me editing it)
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Weaselmoose
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#1433
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#1433
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?
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Knoyle quiah
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#1434
(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
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LeaX
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#1435
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#1435
(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

(It looked like you accidentally wrote Cr instead of Cu? Hope you don't mind me editing it)
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?
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GeorgeL3
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#1436
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
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SKK94
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#1437
(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?
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SKK94
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(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
Amazing! Thanks
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AS01
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#1439
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#1439
(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
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posthumus
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Just went through the transition metals chapter today, and I have like a list of questions which I'd appreciate if anyone could answer Some questions may be relatively stupid or unnecessary so please excuse me :

-How do I deduce how many and what oxidation states a transition metal can be in? I mean Scandium only forms Sc3+, but why not Sc2+ since for example titanium can form Ti2+
- Since 4s and 3d energy levels require similar energy to remove an electron and is also easily compensated. Then why haven't I for example seen Fe8+ around?
- How can I deduce which is a weak/strong ligand... can some values in the data booklet suggest which one is weak or strong. Like in unit 4 you could us Ka value to deduce which acids are strong/weak.
- Cu+ has full d-orbital and is colourless, so why is it so unstable?
- "Dichromate(VI) ions are the best oxidizing agents and Cr3+ are the worst" "Cr3+ are the worst reducing agents and Cr2+ are the best" .... just wanted confirmation here. I find it odd for some reason that the weakest oxidizing agent is also the weakest reducing agent (when referring to transition metals only).
- Why do some oxidizing agents require acidic conditions? If it's unnecessary to know, is it usually when the oxidizing agent contains the oxygen element (& usually water is formed when it's reduced).

Thanks in advance
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