Edexcel Chemistry Unit5 Revision Thread Watch

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hadi
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#1
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#1
Listen guys , this is a thread to discuss all sorts of questions and misconceptions concerning UNIT 5 and 6(Synoptic) CHEMISTRY ONLY
There are other threads for Units 1,2,3,4

Markschemes available from :

http://ecr.150m.com/downloads.htm
http://www.chemistry.thornburycottage.com/
http://www.emailed.org.uk/

and very few sample answers from http://www.passmyexams.com/pastpape..._chemistry.html

OLD syllabus questions compiled by KOH
check : http://www.thestudentroom.co.uk/t116063.html
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hadi
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It seems no body has quesitons , ok , let me ask you a question and see some answers
Redox Quesiton

Q.Explain why , despite the difficulties in reading the scale , KMnO4 is placed in the burette and not in the conical flask ?
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khil
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#3
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(Original post by hadi)
It seems no body has quesitons , ok , let me ask you a question and see some answers
Redox Quesiton

Q.Explain why , despite the difficulties in reading the scale , KMnO4 is placed in the burette and not in the conical flask ?
Why on earth is that important at A-Level Chemistry!? (I don't do Edexcel, have heard it's a bit crap - is it true?)

It's placed in the burette - surely because you need to adjust the tap so you can add it drop by drop for titrations!? - considering titration is supposed to be accurate to within 0.1 from the actual value. In a conical flask - well firstly erm the solution wouldn't be tipped the right way for it to flow downwards for titrations! And why would putting it in a conical flask make a difference to reading the scales? And conical flasks aren't even accurate! - that's why KMnO4 is put into a burette.

What sorta a question IS that!? If this is the sort of weird questions you get in Edxecel, it does sound a bit crap.

Although here's my question: Anyone remember the titration equation for KMnO4? I *think* it's:
MnO4- + 14H+ + ?Fe2+ ----> Mn2+ + 7H2O + ?Fe3+

I can't remember how many Fe's there are? :confused: It's 5 or 6...
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[ lee ]
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#4
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#4
(Original post by irisng)
Why on earth is that important at A-Level Chemistry!? (I don't do Edexcel, have heard it's a bit crap - is it true?)

It's placed in the burette - surely because you need to adjust the tap so you can add it drop by drop for titrations!? - considering titration is supposed to be accurate to within 0.1 from the actual value. In a conical flask - well firstly erm the solution wouldn't be tipped the right way for it to flow downwards for titrations! And why would putting it in a conical flask make a difference to reading the scales? And conical flasks aren't even accurate! - that's why KMnO4 is put into a burette.

What sorta a question IS that!? If this is the sort of weird questions you get in Edxecel, it does sound a bit crap.

Although here's my question: Anyone remember the titration equation for KMnO4? I *think* it's:
MnO4- + 14H+ + ?Fe2+ ----> Mn2+ + 7H2O + ?Fe3+

I can't remember how many Fe's there are? :confused: It's 5 or 6...
For God Sake, don't blame Edexcel for it... That question may come in Practicals but surely not in A2 Exams...
BTW, why put such a useless question (Practicals r over,Hadi dear)...?
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khil
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#5
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(Original post by [ lee ])
For God Sake, don't blame Edexcel for it... That question may come in Practicals but surely not in A2 Exams...
BTW, why put such a useless question (Practicals r over,Hadi dear)...?
Lol sorry don't be offended!

If not blame Edxecel, who should I blame...? :confused:

Yeh I got the impression it was what was gonna come in an Edxecel Chem exam paper, which really would be rather crap. Mind you, OCR Salters sometimes isn't that much better (the board I do) - at AS I heard people got a question that asked: What is the state of water at room temperature? :wtf?:

Mind you questions like that are actually good for passing exams

Someone test me on Chemistry I'm not confident enough
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hadi
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This question came in Edexcel Unit 5 but i don't remember which year.

The answer to the question comes in 2 views , one practical and the other theoratical.

For the practical one : The eye is more susceptible to colour appearance than colour disappearance , now what does this mean ?
Near the end point , the colour in the flask turns from colourless to very pale pink caused by the excess drop , it was shown that , while carrying out the titration , the eye can notice this colour change (colourless------>pink) quicker than colour disappearance (pink--->colourless) which occurs when the KMnO4 in the conical flask.

The theoratical point is : when considering any redox rxn , think of 2 things
1)the electron source 2)the electron sink(receiver)

in this case the electron source will be iron (II) ions and the sink MnO4- ions.
if KMnO4 is placed in the conical flask , then the electron source (iron II drop from burette) will be small compared to the electron sink (KMnO4 in conical flask) therefore the reduction will be slow causing the Mn(+7) to turn to (+6) then +5 then +4 which is (MnO2) being stable , cannot continue reduction to Mn(+2) in other words , the titre would be different.
Comparing it with the case when KMnO4 in the burette , the electron sink (KMnO4 drop) will be small compared to electron source (iron II) therefore the reduction will be rapid ) Mn(+7) to +2 reduction is efficient --------->correct titre

I hope u understood , don't worry it's not a typical question of edexcel

someone mentioned that the conical flask is inaccurate , the KMnO4 will be transferred using a pipette , therefore it is not an answer
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kpg
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#7
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I don't understand the manganate in a burette/conical flask question :confused: The exercise is to estimate the concentration of a reducing agent of which you have a known quantity [in a conical flask] which is titrated against potassium manganate of which you don't know how much will be needed for the full reaction. How else would it be done :confused:
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muppetman
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#8
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#8
i agree with kpg
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hadi
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#9
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#9
The question is u want to determine the conc. of a soln ( ex. ironII sulphate) now , as u mentioned , carry out titration till end point..................
For any given titration , i will give u solns in a bottle and tell u : take , and determine concentration . u would put one soln in the burette and other in the flask , or the reverse u will get the same result .
But for this question ,u have to put KMnO4 in the burette.
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