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Edexcel - Chemistry Unit 2 - 4 June 2013

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Avatar for Katy1704
Katy1704
Original post by bhowland1994
No but you will be required to discuss the uses of halogenoalkanes, eg as fire retardants and
modern refrigerants.


I thought so :smile: thank you !
Original post by SophieL1996
with displacement reactions of halogens with k halides, why can you shake it with an organic solvent? what does it do and why does it happen??


You shake it with an organic solvent as that brings out more vibrant colours in the top layer of the solution (The organic solvent layer) allowing you to see what halogen is present. Simple as :biggrin:
Reply 802
Avatar for ramanan
ramanan
Original post by charlieejobson
why does magnesium react diffrently with steam? do we need to know why? also does anyone know if we need to know about back titrations?- what are they?


well we had an M.C.Q qus 13 in 2013 jan, Why Mg reacts with steam cause the fraction of pariticle with energy grater than the activation energy is higher in the reaction with steam,
thats the thing we might need to knw.

what to u mean by back titration ?
like titrating with acid in burette or otherway around Base inside the burette ?
http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/june2010-qp/6CH02_01_que_20100607.pdf Q19 b ii
Original post by ramanan
well we had an M.C.Q qus 13 in 2013 jan, Why Mg reacts with steam cause the fraction of pariticle with energy grater than the activation energy is higher in the reaction with steam,
thats the thing we might need to knw.

what to u mean by back titration ?
like titrating with acid in burette or otherway around Base inside the burette ?


Yes, I too want to know what a back titration is .... anyone know ? :s-smilie:
Original post by SophieL1996


There is no part Q19 b) ii .... :s
Reply 806
Avatar for ramanan
ramanan
Original post by bhowland1994
Yes, I too want to know what a back titration is .... anyone know ? :s-smilie:

[h="3"]Back titration
Back titration is a titration done in reverse; instead of titrating the original sample, a known excess of standard reagent is added to the solution, and the excess is titrated. A back titration is useful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration, as with precipitation reactions. Back titrations are also useful if the reaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid.[28[/h]
Original post by bhowland1994
There is no part Q19 b) ii .... :s


oh sorry its supposed to be jan 2010... my bad :smile: http://www.freeexampapers.com/index.php/?location=A+Level%2FChemistry%2FEdexcel#A Level/Chemistry/Edexcel/2010 Jan
Reply 808
Avatar for ramanan
ramanan
Original post by bhowland1994
Yes, I too want to know what a back titration is .... anyone know ? :s-smilie:


iodine-thiosulfate titrations what we do is back titration :smile: how sily-me not knowing this the whole time :biggrin:


http://www.thestudentroom.co.uk/showthread.php?t=983259 look at post one
(edited 10 years ago)
Original post by bhowland1994
Yes, I too want to know what a back titration is .... anyone know ? :s-smilie:


Example: I want the concentration of this I2 solution. I've got any Na2S2O3 solution, BUT - I don't know the concentration of that either. So what I do is, titrate the Na2S2O3 against something of known concentration to find the concentration of Na2S2O3; and then I titrate the I2 with the sodium thiosulphate, and can get the concentration of the I2. It can be more complicated but this is one example. They won't ask for the definition, just do the problem in front of you when it comes.

Which experimental set-ups do we need to be able to draw out? Heat under reflux, distil in situ, alcohol dehydration, anything else?
http://www.freeexampapers.com/index.php/?location=A+Level%2FChemistry%2FEdexcel#A Level/Chemistry/Edexcel/2010 Jan Q19 bii please! :biggrin: :biggrin:
Reply 811
Avatar for posthumus
posthumus
OP
20
Original post by SophieL1996


Just copy & paste, or print screen the question
Original post by SophieL1996
oh sorry its supposed to be jan 2010... my bad :smile: http://www.freeexampapers.com/index.php/?location=A+Level%2FChemistry%2FEdexcel#A Level/Chemistry/Edexcel/2010 Jan


Figured it out MS was a bit crap telling you on that one, Right: Moles of Sodium Thiosulfate = (vol x con) / 1000 -as the volume is in cm^3- this comes out at 1x10^-3 moles We then look at the original equation, the molar ratio of I2 to Sodium thiosulfate is 1:2 so we therefore divide our original answer by two to get 6.3x10^-4
Original post by posthumus
Just copy & paste, or print screen the question


9 The concentration of iodine in solution can be measured by titration with

sodium thiosulfate solution.

I2(aq) + 2S2O3

2–(aq) → 2I–(aq) + S4O6

2–(aq)

(a) Name a suitable indicator which could be used for this titration.

(1)

...............................................................................................................................................................................................................................................

(b) The amount of sulfur dioxide in the atmosphere can be measured by passing a known

volume of air through iodine solution. Sulfur dioxide converts iodine to iodide ions.

SO2(g) + I2(aq) + 2H2O(l) → SO4

2–(aq) + 4H+(aq) + 2I–(aq)

In an experiment, 100 m3 of air were passed through 100 cm3 of iodine,

concentration 0.0100 mol dm–3. The remaining iodine was titrated with sodium

thiosulfate solution and reacted with 12.60 cm3 of sodium thiosulfate, concentration

0.100 mol dm–3.

(i) How many moles of iodine were present in the solution of the iodine at the start

of the experiment?

(1)

(ii) How many moles of iodine remained in the solution at the end of the

experiment?

(2)

(iii) Calculate the number of moles of iodine which reacted with the sulfur dioxide,

and hence the number of moles of sulfur dioxide in 100 m3 of air.

(2)

17 *N35692A01724* Turn over

(iv) The European Commission recommend exposure to sulfur dioxide in air should

be less than 350 micrograms (350 × 10–6 g) per cubic metre.

Calculate whether the sulfur dioxide in this sample of air was within this limit.

One mole of sulfur dioxide has mass 64.1 g.

(2)

(c) Explain whether the changes below would or would not improve the experimental

procedure for measuring the concentration of sulfur dioxide in air used in (b).

(i) The 100 cm3 of iodine was divided into 25 cm3 samples before titration.

(1)

...............................................................................................................................................................................................................................................

...............................................................................................................................................................................................................................................

(ii) The concentration of sodium thiosulfate used to titrate the iodine was changed

from 0.100 mol dm–3 to 0.050 mol dm–3.

(2)

...............................................................................................................................................................................................................................................

...............................................................................................................................................................................................................................................

...............................................................................................................................................................................................................................................

(iii) 150 m3 of air was passed through the iodine. The solutions used were of the

same concentrations as in the original experiment.

(2)

...............................................................................................................................................................................................................................................

...............................................................................................................................................................................................................................................

...............................................................................................................................................................................................................................................

(Total for Question 19 = 13 marks)
Original post by bhowland1994
Figured it out MS was a bit crap telling you on that one, Right: Moles of Sodium Thiosulfate = (vol x con) / 1000 -as the volume is in cm^3- this comes out at 1x10^-3 moles We then look at the original equation, the molar ratio of I2 to Sodium thiosulfate is 1:2 so we therefore divide our original answer by two to get 6.3x10^-4


ah thanks how do you do the next part please??
Reply 815
Avatar for Katy1704
Katy1704
Hello :smile: does anyone know what we need to know about drying agents(and which ones)? I can remember there was a question on them once but I don't know which paper. Any help is much appreciated, thank you.
Original post by Katy1704
Hello :smile: does anyone know what we need to know about drying agents(and which ones)? I can remember there was a question on them once but I don't know which paper. Any help is much appreciated, thank you.


anhydrous... mgso4, cacl2, cao
Reply 817
Avatar for Katy1704
Katy1704
Original post by SophieL1996
anhydrous... mgso4, cacl2, cao


Thank you :smile:
Does anyone have an executive list of apparatus we might be asked to draw?
Guys, I'd really appreciate it if someone could do a list on what apparatus to draw for which experiments? E.G Alcohol to Aldehyde= Distillation.

That would be a massive help. I get confused about what to draw when collecting bromoalkanes etc etc

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