U6 Discussions
Posted by: intechemistry on: September 22, 2010
In this unit – which chiefly caters for the A2 part of the specification. you’re probably going to have to do the following:
a) Identify A2 organic unknowns* (carbonyls, phenol, esters, acid chlorides etc)
b) Identify inorganic unknowns* – esp transition elements
* = You should be able to do this in reverse also, i.e. be given a specific compound and identify a test that will unambiguously be able to identify it.
c) Describe preparation i.e. synthesis techniques as well as separation techniques and yield determination, as in the multi-stage synthesis you are going involving a benzene derivative. It’s very likely you will have to draw an experimental setup. Rod Beavon (an edexcel chief examiner) says “The candidate who can draw a good, working fractional distillation apparatus is still the exception“. Highly advisable then to check out his page on this issue.
d) Possibly kind of analytical technique (IR, HPLC, TLC, 1H NMR, UV, MS, GC)
e) Use a redox reaction. Quite possibly a clock reaction for kinetics or quantitative determination of some species capable of undergoing a redox reaction.
f) Give some kind of error identification and estimation.
There is quite a lot of scope in Unit 6 to talk about kinetics. Experiments that can find the Ea or order of reaction with respect to a particular reactant and methods of measuring rates etc (e.g. quenching, colorimetry etc). You may also get questions involving some kind of pH titration.
In summary. any A2 chemistry that can be ‘done’ in the lab has a chance of coming up, in addition to the almost certain identification of unknowns.
Some more about U3 in response to Draco’s question.
Dear Draco.
In the syllabus you will find the practical skills which you need to have theoretical knowledge of.
Unit 3: Lab Skills I Page 47
Unit 6: Lab Skills II Page 77
http://www.edexcel.com/migrationdocuments/GCE%20New%20GCE/UA024832%20GCE%20in%20Chemistry%20Issue%204%20250510.pdf
Despite being from the ‘old syllabus’, this will probably also be of help to you:
http://www.edexcel.com/migrationdocuments/GCE%20Curriculum%202000/264935_chemistry_session_2.pdf
and this
http://www.edexcel.com/migrationdocuments/GCE%20Curriculum%202000/264936_3B_tables.pdf
Alternatively, you can <b>buy this George Facer book</b> which covers ALL th stuff for the new syllabus (Unit 3 code = 6CH07 and Unit 6 code = 6CH08):
http://www.amazon.co.uk/Edexcel-AS-level-Chemistry-Laboratory/dp/1444108433
I can order it for you if you can’t find it in Msia. It costs (with 8 day international delivery from the UK, about £18 (around RM90)
Also, get past years papers for Unit 3 (called 3B in the old syllabus) and Unit 6 (called Unit 6B on the old syllabus)
And of course understand ALL the practicals you did in year one and year two.
I will post this in the U3 and U6 sections.
A good focused document (in English) covering A-level purification techniques (Re)Crystallisation, Vacuum filtration, Solvent Extraction, Fractional Distillation, Assessment of Purity: MELTING POINT, Optical rotation, Thin Layer Chromatography (TLC). http://www.upv.es/herme/files/01-purification-techniques.pdf
(Re)Crystallisation, Vacuum filtration, Solvent Extraction, Fractional Distillation, Assessment of Purity: MELTING POINT, Optical rotation, Thin Layer Chromatography (TLC).
Some steam distillation info: DocBrown http://www.docbrown.info/page07/equilibria8e.htm simple site: http://www.essentialoils.co.za/steam-distillation.htm and wikipedia http://en.wikipedia.org/wiki/Steam_distillation
33 Responses to "U6 Discussions"
March 6, 2011 at 7:55 pm
Chem_O.
It’s better for me, and for you, if you were to quickly write down your thoughts on this. Could you do so?
March 6, 2011 at 8:28 pm
I thought either ethanol or secondary alcohol can undergo iodoform reaction. So, I was blurr. Then, I SAW the question above says 3 carbon. Hahha, I am soo sorry for my recklessness sir.
March 6, 2011 at 8:33 pm
That’s why I like students to give a brief description of their thinking. It often means they solve their own problems. 
and if they still have a problem I can usually see the flaw in their thinking.
All methyl alcohols CH3-C(OH)… that can be oxidised (therefore excludes 3o methyl alcohols) and methyl carbonyls can undergo the iodoform test.
March 6, 2011 at 8:37 pm
Chem_O
You point out something valuable…
There are usually clues in the Questions. Remember what edexcel advise: TRQ3… Read the question THREE times. Once to get a feel of the Question. Then read it again SLOWLY to pick up on these clues. Then, after you’ve put you answer down, read it again to make sure you answered the question asked of you.
March 7, 2011 at 3:45 pm
If there’s a arrow with the label heat pointed directly at a conical flask, does it indicate heating of bunsen burner or does it just mean the heat source i.e. heating mantle, water bath.. so on ?
March 7, 2011 at 4:15 pm
I was hoping you could tell me!
Seriously though, it’s not taken to mean any specific for of heat. As it’s not specified it means ANY source of heat. So it could include a Bunsen burner, which I guess was the second error., otherwise I’d only have scored 1 mark myself.
March 7, 2011 at 7:07 pm
Was looking frantically for the next error ’cause i am not sure whether heat means Bunsen burner direct heating or it just means heat source so i just settled on talking about the whole thing being a close system and pressure building up in the apparatus. Sigh…
By the way sir, there’s also on question regarding cations. The question states that the colour in aqueous is pink (my guess is Mn2+ or Co2+). Then the next part of the question says, it turns blue when sodium hydroxide is added and is insoluble. ( Co(OH)2 i think ). Then the questions says the precipitate turns light brown in standing air. ( huh ? i though Mn(OH)2 does that. My recollection was Co(OH)2 turns pink in standing air =( ) In the end, i settled with Co2+ but pretty much doubting the answer. May i know what is the actual answer ?
March 7, 2011 at 9:13 pm
(Mn)x+ does not make a blue ppt so Co(OH)2′s your man.
If it does something in air that means an oxidation takes place. Co(OH)2(s), must be going to Cr(OH)3(s) – it’s talking about the ppte so all the species here are sill solid i.e. must not have a charge. So Co with 3 OH’s matched with a 3+ charge.
It’s usually most easily seen via the amine complex or a faster working O.A. rather than O2(g). H2O2 would do the job.
The Q does say it turns pink or light brown, so you have your pink colour and (even more) confirmed it’s Co. Some may say it looks light brown as it’s undergoing the colour transition and is mixed well.
June 2, 2011 at 7:55 pm
Hello sir,
May I know the answer to Trial examination Chemistry Unit 6B on 7/3/2011 question 3f which ask for the endpoint of titration of acidified sodium dichromate (VI) (in burette) and iron (II) ions (in conical flask), please ? I seem to get different answers from different classes =S
Thank you, sir
June 6, 2011 at 6:05 am
Hi Q
Well I may give you another answer. :s
When using a coloured solution in a burette, I’d say the end point is determined when a small excess of that solution is in the conical flask. Acidified Cr(+VI) is orange so to say then a permanent orange colour appears then it’s the end point. That orange may appear as a pale orange as it’s diluted.
I know the Fe2+/Fe3+ aqua ions have colour, but they are not usually taken as interfering (I guess the justification for that would be because their concentration is low). For a paper based exam, it has to be this way or else how could you possibly be expected to know that the mixture of colours would look like? That’s not a learning requirement.
Same for KMnO4 titrations. The end point here being permanent pale pink (slight excess from an incompletely consumed single drop from the burette, of MnO4- in the conical flask).
June 7, 2011 at 12:00 am
Thank you, sir.
I have further queries. Is it possible that another answer to be that an external indicator is needed which can react with dichromate ions. external indicator is diphenylamine where endpoint is color change from green to deep blue. I have always thought that this kind of reaction is self-indicating, so may I know under what circumstances that an external indicator is needed?
For this example of titration of iron(II) ion with dichromate(VI) ion, is it because the green color of chromium (III) ions may mask the orange color of dichromate ions? so external indicator needed?
Thanks again =)
June 12, 2011 at 2:41 pm
External indicators are needed when in the reaction there is no overall colour change. A great example is HCl(aq) with NaOH(aq). An external indicator, phenolphthalein, is needed as HCl, NaOH and NaCl(aq) and H2O all have no colour.
Sometimes an external indicator is used to make a colour change more visible. e.g. in a titration when I2 is turning into I-. The colour change goes from brown (lots of I2 present), to a pale yellow or ‘straw colour’ (only a few I2 molecules left), to colourless (all I2 molecules consumed). Going from increasingly fading pale yellow to colourless isn’t easy to spot when exactly ALL the colour has finally vanished, so in these cases, starch is added. which gives a very strong, easily seen, blue/black coloured complex with only a few I2 molecules present. Eventually the solution will go colourless (when all I2 molecules have been consumed).
I wasn’t aware diphenylamine was used with (Cr2O7)2- but to me it looks it’s the equivalent of starch with iodine, i.e. make the colour change more obvious, ‘cos dichromate titrations are usually self indication hence no need to add any other compounds.
The original question was “endpoint of titration of acidified sodium dichromate (VI)” and no mention of diphenylamine was made, so in this case, you cannot suddenly introduce it. If you had to plan a titration, then perhaps you could perhaps mention diphenylamine. You are right, the reaction is self-indicating so again, no need for anything else.
The reaction you mentioned above (Cr2O7)2- with Fe2+ to Cr3+ and Fe3+ is self indicating.
“chromium (III) ions may mask the orange color of dichromate ions? so external indicator needed?” – That ‘masking’ answer is sometimes accepted as an alternative answer. It kind of depends really on how concentrated the Cr3+ ions are in the conical flask. If the conc is high then you may get some effect whereby the slight excess of orange isn’t easily seen. In such a case addition of the diphenylamine would be necessary.
I think most of the time, for self-indicating reactions, the issue of masking will only rarely ‘come up’.
January 18, 2012 at 10:46 pm
I’ve entered unit 6B exam and a Question about 2-ethanolyaminobenzoic Acid preparation came up they used excess ehtnoly chloride and aminobenzoic Acid then reflux then cool then added water …etc
so why do we add water ??? I wrote to reomve excess unreacted ethnaoly chloride is this right ???
why do we cool ??? reaction exothermic
what do you think
January 18, 2012 at 11:03 pm
I’d agree, to destroy the remaining CH3COCl
Cooling is necessary in to prevent a ‘runaway’ (read: potentially dangerous) reaction (and too much heat would also encourage the formation of side products). Like adding a nitrating mixture to benzene. It’s cooled while the nitrating mixture is being added to benzene. After mixing, the mixture is normally heated/warmed under reflux.
January 18, 2012 at 11:15 pm
o….but Sir they already have the aminobenzoic acid so need for nitrating
g=here is the method which came insted of anyhydride my examused ethanoly chloride
http://www.google.com.sa/search?hl=ar&safe=active&rlz=1T4RNQN_enSA458SA459&biw=1024&bih=556&q=2-ethanoylaminobenzoicacid+preparation&oq=2-ethanoylaminobenzoicacid+preparation&aq=f&aqi=&aql=&gs_sm=e&gs_upl=25453l35953l0l36343l11l10l0l1l0l0l718l2062l5-2.1l3l0
the first link
January 18, 2012 at 11:49 pm
@ 17, rif January 18, 2012 at 11:15 pm.
I was just using nitration of benzene as a hopefully more familiar example of the need for heat control. I wasn’t proposing doing nitration on aminobenzoic acid. 
January 19, 2012 at 12:03 am
ahhh so water is okay by my 18 method on the link
lol……………sorry for misunderstanding you Sir
April 17, 2012 at 2:06 pm
Greetings sir…
must i know the procedures to prepare azo dyes, phenylamine, nitrobenzene, methylamine, saligenin, ethyl ethanoate ,ethanal .iodoethane ,1-bromobutane, and ethene.shld i learn up the reasons as to why each step is done?is it imp 4 u6/u5 ?
thank in advance sir.
April 17, 2012 at 10:21 pm
azo dyes – yes
phenylamine – yes
nitrobenzene – yes
methylamine- yes
saligenin – what’s that? & very probably no.
ethyl ethanoate – yes
ethyl ethanoate – yes
iodoethane – mainly by chemical equation only
1-bromobutane – yes
ethene – yes
shld i learn up the reasons as to why each step is done? – best to do so to cope with A(star) challenges.
is it imp 4 u6/u5 ? – yes
There is noting peculiar about the scope of those reactions and preparations you mentioned.
May 16, 2012 at 9:41 pm
salam sir..for the experiment A2 that we do not do this sem, we still need to read right? What about the questions? haven’t see the questions yet. If simple then ok, but for complicated one should we goggle the answer?
tq
May 17, 2012 at 1:45 am
Salam erly.
Oh yes, you MUST read the practicals you were given but very unfortunately were not given the chance to do.
You are very likely to be asked about
a) A multi-step organic synthesis and purification inc. recrystallization (which was one of the last labs you did) and probably separation / distillation and analysis (e.g. NMR, IR, Mass Spec, TLC, HPLC, GC, elemental analysis) inc 2,4-DNP derivative mpt determination.
b) Preparation of Cr(II)acetate
c) Organic unknowns identification e.g. aldehyde’s, ketone’s, 1o and 2o methyl alcohols, ethanal and methyl ketone’s, carbox acids, and diazo compounds,
d) Identification of inorganic unknowns like Transition elements
e) quantitative REDOX titrations, using (MnO4)-, Cu2+/CuI/I-/I2/(S2O3)2- or similar species.
f) Possibly stuff about Redox cell/potentials/& half-cells
g) Anything specifically mentioned in the specification that covers lab work for A2.
h) An application involving the above.
May 19, 2012 at 1:13 am
Sir, what should we study for Chemistry 6B? Do we need to study the whole A2?
May 19, 2012 at 2:57 am
SS. I think what’s written above in this whole post is quite a comprehensive guide as to what you need to know.
May 20, 2012 at 9:45 pm
sir quick Q hr as m preparin 4 physics , why are the recent past yrs ( 2011-wards has a diffrent pattern of Qs. The old 6B (b4 2009-wards I guess mostly ask about everything ,in strecth almost every single concept 4rm u1 to u5 .bt the recent ones STRICTLY FOCUS on experimental questions alone. so wat shud we expect this june ppr? …. cz syllbs change/eq???
May 20, 2012 at 10:12 pm
At a guess, I’d say the Qualifications and Curriculum Development Agency instruct the exam boards to ensure the paper changes sufficiently so that the students chemistry knowledge is tested and not whether or not they can perform well due to them spotting patterns/routines in the exams. But it’s just a total guess.
It may also be due to the general internal script writing committee’s within edxcel and shuffling of examiners plus their responsibilities.
In my estimation, it’s not that easy to make exam papers so if I was a betting man, I’d bet that the papers would run in two’s e.g. Jan and June papers will be similar and then change for the next Jan June, but I’ve not analysed the papers in that detail.
May 22, 2012 at 10:36 am
Salam Sir. For the preparation of chromium(II) acetate, why the whole experiment has to be done in an inert atmosphere (stated in the book, such as in nitrogen). Is there any significant reason?
And, why have to remove oxygen from all liquid, too?
May 22, 2012 at 10:57 am
Cr(II) is not stable in air. The air will oxidise the Cr(II) to Cr(III). Look at the relevant reduction potentials for each species and you can calculate it will have a +;ve Ecell.
May 22, 2012 at 11:50 am
Thank you, Sir.
Another one, can i just use Sn/Hcl in reaction of nitrobenzene to get phenylamine, instead of Sodium tetrahydridoborate (Pearson book stated this in their steps of that reaction).
TQ again SIr.
May 22, 2012 at 12:22 pm
I GUESS either will do. NaBH4 is a known reducing agent. I don’t know it’s reduction potential nor that of nitrobenzene, but I guess it’ll be OK.
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March 6, 2011 at 7:47 pm
sir, if you see in july 2010 6B paper Q2 ryte, how do we know it’s a propan-2-ol not propan-1-ol? thank you:)