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    (Original post by Lamalam)
    Attachment 213899Can 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
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    (Original post by Lamalam)
    Attachment 213899Can 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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    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?
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    there is a very interesting q, can someone explain it ?

    ____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’
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    (Original post by Lamalam)
    there is a very interesting q, can someone explain it ?

    ____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’
    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.
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    (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
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    (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?
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    NilFBosh u mean tip squash..? lol
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    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
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    (Original post by StUdEnTIGCSE)
    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.
    what's the purpose of neutralising oH- ?? isn't OH- need to attack the halogenoalkane?? why it needs to be neutralised?:confused:
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    Do we have to remember the iodine, thiosulfate ionic reaction?
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    (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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    (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.
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    (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:
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    (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
    Here http://uppit.com/17bwcy54u41g/6CHO7_Jan_2013_MS.pdf
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    (Original post by Relaxedexams)
    You are 100% sure?

    My tutor keeps on insisting to learn it...:mute:
    I2 + 2s203(2)- ---> s4o6(2)- + 2I-
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    (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?
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    (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?
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    (Original post by Relaxedexams)
    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.
    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?
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    (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.
 
 
 
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