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.
Bord3r
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#1441
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#1441
(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
Thank you, this is awesome (+rep)
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jethacan
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#1442
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#1442
Is [Cr(H2O)6]3+ Green or Violet? I'm quite confused, different sources are telling me different things, for example how would I answer this?

State what you would see if aqueous sodium hydroxide was added drop by drop to a solution of [Cr(H2O)6]3+ until the sodium hydroxide was in excess.
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randyaloul
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#1443
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In June 2010 Q5, there's a multiple choice question that says which of the following successive ionization energies could have come from a transition metal? Shouldn't there be a jump from the 2nd to the 3rd ionization energy as the 3rd is removing it from an inner D orbital, as opposed to the first 2 being removed from outermost S orbital? So the answer should be B, but it is C.

Help please?
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randyaloul
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(Original post by jethacan)
Is [Cr(H2O)6]3+ Green or Violet? I'm quite confused, different sources are telling me different things, for example how would I answer this?

State what you would see if aqueous sodium hydroxide was added drop by drop to a solution of [Cr(H2O)6]3+ until the sodium hydroxide was in excess.
Violet.

You would say initially it was violet, then green ppt forms, then this dissolves to form green solution.
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luckypants
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On the top of page 263 in the facer book. It talks about the preparation of phenylamine and it states that the distillate is placed in a separating funnel and some nacl is added to reduce the solubility of phenylamine. How does that work?
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JRP
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#1446
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#1446
(Original post by jethacan)
Is [Cr(H2O)6]​3+ Green or Violet? I'm quite confused, different sources are telling me different things, for example how would I answer this?

State what you would see if aqueous sodium hydroxide was added drop by drop to a solution of [Cr(H2O)6]3+ until the sodium hydroxide was in excess.
The metal-aqua ion of Cr3+ is [Cr(H2O)6]3+ which is VIOLET.
Adding sodium hydroxide creates the precipitate of Cr(OH)3 which is GREEN.
Adding sodium hydroxide in excess causes the precipitate to dissolve to form [Cr(OH)6]3- which is GREEN.

Remember:
  • the initial metal-aqua ions always have H2O only as their ligands, and the charge is that of the ion.
  • the precipitate formed by NON-EXCESS NaOH (or NH3) has no charge, and has the same number of OH ligands as the charge on the transition metal ion
  • Chromium and Zinc are the only to react with EXCESS OH. They are also the only that show amphoteric behavior. The number of OH ligands is enough to make the initial metal-aqua ion charge the negative version, with the same magnitude. ([Cr(H2O)6]3+ -----> [Cr(OH)6]3-, [Zn(H2O)6]2+ -----> [Zn(OH)4]​2-)


However, remember that in the case of dichromate(VI) ions being reduced to form Cr3+ ions, the colour change is orange to green because of impurities forming with the Cr3+ ions, despite the metal-aqua ion actually being violet. I don't think you need to remember why, but just know that there are different colours used depending on the context. This is probably what got you confused.

Hope this helped
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AS01
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#1447
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#1447
(Original post by randyaloul)
In June 2010 Q5, there's a multiple choice question that says which of the following successive ionization energies could have come from a transition metal? Shouldn't there be a jump from the 2nd to the 3rd ionization energy as the 3rd is removing it from an inner D orbital, as opposed to the first 2 being removed from outermost S orbital? So the answer should be B, but it is C.

Help please?
D and S subshell have similar energy level. So even if u ate removing electron from D orbital energy doesnt change significantly. It will only change when when u remove from p orbital

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posthumus
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(Original post by AS01)
D and S subshell have similar energy level. So even if u ate removing electron from D orbital energy doesnt change significantly. It will only change when when u remove from p orbital
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I asked a question about this above... could you possibly answer it please If you think most of my other questions are unnecessary & stupid then please let me know Those are the only things that bothered me in the transition metals chapter
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AS01
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(Original post by luckypants)
On the top of page 263 in the facer book. It talks about the preparation of phenylamine and it states that the distillate is placed in a separating funnel and some nacl is added to reduce the solubility of phenylamine. How does that work?
In separating funnel u use water right and phenylamine is soluble in water as it can form H bond. So if it dissolves in water it is hard to separate aq and organic layer so what u do is add salt to reduce solubility in water so that its easier to separate and u dont lose ur product

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AS01
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(Original post by posthumus)
I asked a question about this above... could you possibly answer it please If you think most of my other questions are unnecessary & stupid then please let me know Those are the only things that bothered me in the transition metals chapter
Wait few minutes ya it's gonna take time to answer all of it

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LeaX
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(Original post by SKK94)
Welcome
Haha, there probably is. I tried searching chemguide, but couldn't find much info on it.
Maybe something to do with energetic favourability?
Ohh okay thank you.

(Original post by AS01)
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
The ones that after forming precipitates with a little ammonia undergo further deprotonation (which I now know isn't possible with ammonia) or ligand exchange which you've now told me so thank you.
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AS01
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(Original post by LeaX)
Ohh okay thank you.


The ones that after forming precipitates with a little ammonia undergo further deprotonation (which I now know isn't possible with ammonia) or ligand exchange which you've now told me so thank you.
yeah they undergo ligand exchange only if ppt dissolves in aq ammonia
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AS01
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(Original post by shumen9523)
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.
I have answer to your question
The thing is zwitterions doesnt exist alone. Remember there is reversible reaction between amino acid and zwitterion so peptide bond forms with amino acids only as they are also present
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AS01
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(Original post by posthumus)
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
1. Thats one thing about chem those stuff u need to know. Its like asking why Ca2+ and not Ca3+ well u can still form them as we learned in AS in ionisation energy(IE) that u can keep on removing electron till u have empty shell but the one that it forms compound with will be Ca2+ or Sc3+. Its got to do with endo IE reaction overcome by exo formation of bond
2. umm I will go with same reason as above
3. strong ligand gives more dark color but weak gives color like yellow. Dont think u need to know if its strong or weak ligand but I did memorize quite a few names from george facer coz I liked the idea how strong ligand relates to dark color
4.In an aqueous medium, Cu2+ is more stable than Cu+. This is because although energy is required to remove one electron from Cu+ to Cu2+, high hydration energy of Cu2+ compensates for it. Therefore, Cu+ion in an aqueous solution is unstable.
5.No what they doing is they comparing with different oxidation states of Cr. You decide which is best and worst oxidising and reducing agent by looking at electrode potential so probably in Cr6+ going to Cr3+ electrode potential was higly positive but when compared with Cr3+ going to Cr2+ it was worst reducing agent coz electrode potential was negative
6. I have to read over this as well looking at jan 2013 paper I am gonna read over what all u need to reduce and oxidise stuff. Usually u reduce using Zn and dil HCl

None of them were stupid question
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posthumus
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#1455
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#1455
(Original post by AS01)
1. Thats one thing about chem those stuff u need to know. Its like asking why Ca2+ and not Ca3+ well u can still form them as we learned in AS in ionisation energy(IE) that u can keep on removing electron till u have empty shell but the one that it forms compound with will be Ca2+ or Sc3+. Its got to do with endo IE reaction overcome by exo formation of bond
2. umm I will go with same reason as above
3. strong ligand gives more dark color but weak gives color like yellow. Dont think u need to know if its strong or weak ligand but I did memorize quite a few names from george facer coz I liked the idea how strong ligand relates to dark color
4.In an aqueous medium, Cu2+ is more stable than Cu+. This is because although energy is required to remove one electron from Cu+ to Cu2+, high hydration energy of Cu2+ compensates for it. Therefore, Cu+ion in an aqueous solution is unstable.
5.No what they doing is they comparing with different oxidation states of Cr. You decide which is best and worst oxidising and reducing agent by looking at electrode potential so probably in Cr6+ going to Cr3+ electrode potential was higly positive but when compared with Cr3+ going to Cr2+ it was worst reducing agent coz electrode potential was negative
6. I have to read over this as well looking at jan 2013 paper I am gonna read over what all u need to reduce and oxidise stuff. Usually u reduce using Zn and dil HCl

None of them were stupid question
Thank you so much for the response Was really helpful...

... I think I'll move onto Organic this evening now !
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AS01
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(Original post by posthumus)
Thank you so much for the response Was really helpful...

... I think I'll move onto Organic this evening now !
I am glad it helped
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AS01
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Btw guys I have this file I think this is very helpful for colors of TM which is right at the bottom
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AS01
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(Original post by posthumus)
Thank you so much for the response Was really helpful...

... I think I'll move onto Organic this evening now !
after u finish can we revise tomorrow?
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posthumus
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(Original post by AS01)
after u finish can we revise tomorrow?
Yes of-course, I'd appreciate that Need to be tested, along with past papers from tomorrow on !
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Knoyle quiah
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#1460
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#1460
Someone please help me, stuck on this, Ive got as far as 2Fe+O2+2H2O --> 2Fe2+ + 4OH-
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