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Edexcel 6CH07/01- May 8th 2013 - AS Chemistry Unit 3B

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Can u guys tell me why the mouth of the beaker cannot be stoppered? Thx!

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Because you are collecting a liquid. and not a gas!
You are condensing a gas into liquid. thats why you have a condenser

Reply 61

Relaxedexams

Original post by Lamalam

Can u guys tell me why the mouth of the beaker cannot be stoppered? Thx!

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To allow an outlet for air and to prevent build up of gas pressure. The apparatus has to be open at one end when heating. In simple distillation it is open at the receiving end.

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Reply 62

RoyalBlue7

Have we to know what pH indicator to use, like for different addition of alkali or acid to each other depending on the colour change?

Reply 63

Lamalam

there is a very interesting q, can someone explain it ? :wink:

____When C is warmed with excess aqueous potassium hydroxide the following
reaction occurs.
CH2=CH-CH2Br + OH- --> CH2=CH-CH2OH + Br-
To test for the presence of the bromide ions formed dilute nitric acid
followed by aqueous silver nitrate is added to the cooled mixture.
Why is dilute nitric acid added?

Neutralise (KOH/OH)
Ignore ‘acidify’

Reply 64

RoyalBlue7

Original post by Lamalam

there is a very interesting q, can someone explain it ? :wink:

This is from the Sample Assessment ?

The usual answer would be to make sure than any precipitate formed will be that of a silver halide and not a carbonate or hydroxide or a sulfite or sulfate or a hydrogencarbonate or hydrogensulphate etc but in this context there is NO possibility of a carbonate but surely a hydroxide may be precipitated if it was not neutralized back into water.

Always look at the products and reactants. For example,There is no S in above reactants..so no chances for a sulphate salt. So only answers will be neutralize/remove OH-/etc
Acidify is also quite a good answer but thats not the point us having an acid added. Our aim was to neutralize, so this shoud be the answer. But as a result of us adding acid it gets acidified, but this "to acidify" was not the purpose. So examiner should ignore it.

(edited 10 years ago)

Reply 65

NilFBosh

Original post by StUdEnTIGCSE

Have we to know what pH indicator to use, like for different addition of alkali or acid to each other depending on the colour change?

Phenolpthalein and methyl orange
starch is used for iodine titrations

Reply 66

NilFBosh

Original post by StUdEnTIGCSE

Have we to know what pH indicator to use, like for different addition of alkali or acid to each other depending on the colour change?

BTW. That paper that we did in CLASSSSSSS today. The first MCQ. Was it D?

Reply 67

DanielDaniels

NilFBosh u mean tip squash..? lol

Reply 68

MrNevermind

Does anyone have the mark scheme for 2013 Jan Chemistry 3B? I have done the paper but I couldn't find the answer. Please help. Thanks a lot :smile:

Reply 69

Lamalam

Original post by StUdEnTIGCSE

what's the purpose of neutralising oH- ?? isn't OH- need to attack the halogenoalkane?? why it needs to be neutralised? :confused:

Reply 70

Relaxedexams

Do we have to remember the iodine, thiosulfate ionic reaction?

Reply 71

NilFBosh

Original post by Relaxedexams

Do we have to remember the iodine, thiosulfate ionic reaction?

U dont need to. Just know that the indicator is starch

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Reply 72

Relaxedexams

Original post by Lamalam

what's the purpose of neutralising oH- ?? isn't OH- need to attack the halogenoalkane?? why it needs to be neutralised? :confused:

This is after the OH- ion has hydrolysed the C-halogen bond.

Now it has to be removed because otherwise it will form Silver oxide precipitate with silver, leading to a false result.

Therefore, dilute nitric acid is added, so that the solution is just acidic to litmus, to remove these excess OH- ions.

Reply 73

Relaxedexams

Original post by NilFBosh

U dont need to. Just know that the indicator is starch

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You are 100% sure?

My tutor keeps on insisting to learn it... :mute:

Reply 74

awesomesoccerfan

Original post by MrNevermind

Does anyone have the mark scheme for 2013 Jan Chemistry 3B? I have done the paper but I couldn't find the answer. Please help. Thanks a lot :smile:

Here http://uppit.com/17bwcy54u41g/6CHO7_Jan_2013_MS.pdf

Reply 75

awesomesoccerfan

Original post by Relaxedexams

You are 100% sure?

My tutor keeps on insisting to learn it... :mute:

I2 + 2s203(2)- ---> s4o6(2)- + 2I-

Reply 76

Relaxedexams

Original post by StUdEnTIGCSE

Have we to know what pH indicator to use, like for different addition of alkali or acid to each other depending on the colour change?

Iodine based titrations - Starch

Normal Acid/Alkali titrations - Methyl orange or phenolphthalein

We have never got a question as such yet, but is it possible that we might get a question where we have to decide on the indicator based on the colour of the solution before and at the end point?

Reply 77

awesomesoccerfan

Original post by NilFBosh

U dont need to. Just know that the indicator is starch

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and also the colour change before and after?

Reply 78

Lamalam

Original post by Relaxedexams

but the acid is added before (aq) silver nitrate, how can the acid remove oH- after the OH- ion has hydrolysed the C-halogen bond?

Reply 79

Relaxedexams

Original post by Lamalam

but the acid is added before (aq) silver nitrate, how can the acid remove oH- after the OH- ion has hydrolysed the C-halogen bond?

(1) The OH- ion hydrolyses the C-halogen bond BEFORE the acid is added.

(2) Then acid is added to remove excess OH- ions.

(3) Then aqueous AgNO3 is added to obtain a silver halide ppt.

If OH- ions were not removed in step (2), in step (3) Silver oxide ppt will form.