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Edexcel - Chemistry Unit 2 - 4 June 2013 Watch

  • View Poll Results: Which topic(s) are you finding most difficult?
    Shapes of molecules and ions
    11.66%
    Intermediate bonding and bond polarity
    15.95%
    Intermolecular forces
    9.82%
    Redox
    17.79%
    Group 2 & 7
    40.49%
    Kinetics & Equilibria
    14.11%
    Organic Chemistry - Alcohols and Halogenoalkanes
    39.26%
    Mechanisms
    26.38%
    Green Chemistry
    28.83%

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    My teacher told me Green chem are often big marker questions at the end that people just do quickly because they don't have enough time he told us to just learn the basics concepts in the text book (ozone layer,carbon neutral,greenhouse gases..) and the rest is just common sense, often they give a small paragraph to read and you have to use your knowledge to answer the question. Practising with papers should help.

    Also I did the structured part first in the Unit 1 exam. I think it is really helpfull beacause in the worst cases if you are out of time you can quickly tick boxes, and often people are more efficient in the first part of the exam and more marks are available in the structured part.
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    (Original post by maryam1996)
    Hi, Can you please tell me what sn1, sn2 means. Thank you,
    Yh sure
    S - substitution
    N - nucleophile
    'no.' - how many species involved in first step

    Well usually associate the Sn2 mechanism with primary halogenoalkanes (for this particular unit anyway).
    2 species are involved in the initial step... the nucleophile & the halogenoalkane. The delta - element in the nucleophile moves towards the delta + carbon in the halogenoalkane.
    While it does this, The C-Br is in the process of breaking (electrons given to bromine).
    So a bond is forming & breaking at the same time... this is called the transition state.
    Here I've found an example of Sn2 reaction (ignore equilibria signs):
    Moving onto Sn1 ...

    In tertiary halogenoalkanes.... there are 3 methyl groups, these pushed electrons towards the carbon atom and make it less delta positive. This makes the C-X bond weak therefore it break. No nucleophile is required to approach it. The first step only contains one species... the halogenoalkane, hence the one in Sn1.
    Here's an example of an Sn1 mechanism:


    (Nu = nucleophile)

    Mechanisms for secondary alcohols never comes up I believe... but they can undergo both Sn1 & Sn2. Though Sn1 is the most likely mechanism.

    Hope that helps
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    What is the colour of Bromine in water and the colour of it in hexane? I keep thinking it is yellow and orange respectively but I don't know.
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    (Original post by posthumus)
    Yh sure
    S - substitution
    N - nucleophile
    'no.' - how many species involved in first step

    Well usually associate the Sn2 mechanism with primary halogenoalkanes (for this particular unit anyway).
    2 species are involved in the initial step... the nucleophile & the halogenoalkane. The delta - element in the nucleophile moves towards the delta + carbon in the halogenoalkane.
    While it does this, The C-Br is in the process of breaking (electrons given to bromine).
    So a bond is forming & breaking at the same time... this is called the transition state.
    Here I've found an example of Sn2 reaction (ignore equilibria signs):
    Moving onto Sn1 ...

    In tertiary halogenoalkanes.... there are 3 methyl groups, these pushed electrons towards the carbon atom and make it less delta positive. This makes the C-X bond weak therefore it break. No nucleophile is required to approach it. The first step only contains one species... the halogenoalkane, hence the one in Sn1.
    Here's an example of an Sn1 mechanism:


    (Nu = nucleophile)

    Mechanisms for secondary alcohols never comes up I believe... but they can undergo both Sn1 & Sn2. Though Sn1 is the most likely mechanism.

    Hope that helps
    Thank you so much this was very helpful
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    (Original post by Jayqwe)
    What is the colour of Bromine in water and the colour of it in hexane? I keep thinking it is yellow and orange respectively but I don't know.
    Brown-red in water. Orange in a hydrocarbon solvent.
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    In the January 2012 paper multiple choice (question 16) the phrase "when equilibrium is restored" is used in context to a chatelier principle q. The questions seems to just be a normal applying the principle q but does it mean something significant? Do we need to know about before equilibrium is restored? x
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    (Original post by Juliajuliajulia)
    In the January 2012 paper multiple choice (question 16) the phrase "when equilibrium is restored" is used in context to a chatelier principle q. The questions seems to just be a normal applying the principle q but does it mean something significant? Do we need to know about before equilibrium is restored? x
    Nope, it's a trick question and all it requires is knowledge of Le Chatelier's principle.
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    (Original post by James A)
    Nope, it's a trick question and all it requires is knowledge of Le Chatelier's principle.
    Thanks! Oh edxecel how it does love to trick us... And throw in a few geography and home economics questions.
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    (Original post by Juliajuliajulia)
    Thanks! Oh edxecel how it does love to trick us... And throw in a few geography and home economics questions.
    :')
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    (Original post by Katy1704)
    I don't know if the files attached successfully (I'm new to this HaHa)
    yep they were great thank you! ;D
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    drying agent for ethanol and how should it be used?
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    What topic do you guys Section C will be on?

    I REALLY hope it's not CFC's and a lot on Green Chemistry. Hopefully equilibrium or rate of reactions.
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    how come grade boundaries for june 09 say it is out of 60?
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    (Original post by geor)
    I really think it will be on green chemistry, unfortunately
    Having said that, they haven't asked much on kinetics really, nor the structures of carbon...? Could possibly be on one of them.
    I agree also with you on the allotropes of carbon, it has rarely turned up :/ although they do like the alcohol reactions and mechanisms as they are tricky, but just need learning.
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    Apart from flame colours.... what other colors do I need to memorize ?

    Any indicators??? If so which ones

    (I know of litmus paper tests & NH4Cl = white smoke.... & that's about it )

    Thanks in advance !
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    (Original post by posthumus)
    Apart from flame colours.... what other colors do I need to memorize ?

    Any indicators??? If so which ones

    (I know of litmus paper tests & NH4Cl = white smoke.... & that's about it )

    Thanks in advance !
    -White solids of carbonates and surfaces.-
    -Brown smoke of no2 when nitrates decompose.
    -Phenolphthalein colourless in acid pink in alkali
    -Methyl orange light yellow in alkali, permanent pink in acid
    -starch add when solution is light-yellow it goes blue black, end point is colourless

    Halide colour in water, and hexane

    Water
    F virtually colourless
    Cl virtually colourless
    Br orange
    I brown

    Hexane
    F-
    Cl-yellow
    Br -orange
    I- purple

    Halide colours in silver nitrate
    F-
    Cl- white precipitate, redissolves in ammonia
    Br- cream precipitate, redissolves in conc ammonia
    Iodine- yellow precipitate, doesn't redissolves in conc ammonia

    Standard states of halides
    F- yellow gas
    Cl-green gas
    Br-blood red liquid
    I- grey solid


    There may be a few more, but is all I can recall now
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    The grade boundaries for Jan 2013 are pretty high compared to previous years, do you guys reckon it is a trend that will continue for this summer exam?
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    (Original post by posthumus)
    Apart from flame colours.... what other colors do I need to memorize ?

    Any indicators??? If so which ones

    (I know of litmus paper tests & NH4Cl = white smoke.... & that's about it )

    Thanks in advance !
    Methyl orange
    Phenpthyline
    Nh4cl is a solid suspended in a gas hence white smoke
    Barium chloride forms white ppte with sulfate
    Starch iodine indicator
    Lead ethanoate paper as a test for h2s to show iodide is a stronger oxidising agent
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    (Original post by own)
    x
    (Original post by Goods)
    x
    Thank-you both !
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    Is there a particular structure for damp red phosphorous like P4 or is it just 2P +3I2=2PI3 for the mixture of a 3(halogenoalkane) with PI3=alkane(I)+H3PO3?
 
 
 
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