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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
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    Redox
    17.79%
    Group 2 & 7
    40.49%
    Kinetics & Equilibria
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    Organic Chemistry - Alcohols and Halogenoalkanes
    39.26%
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    (Original post by posthumus)
    Thanks you too !

    Erm not so much, I was confident - but January exams went very badly ! At the moment I am on B in both Math & Physics and D in Chemistry You ?




    I need roughly 120 UMS (for a B grade) from my 3 retakes + Unit 5

    I got 70 & 68 UMS in unit 2 & 4 ... I hope to get at least 50 UMS from those to be honest

    Unit 4 I worked bloody hard knew everything inside out and just blanked out in the exam ! ergh

    how about yourself ?
    Ah no What do you need to get? I'm hoping I'll get it, but the evil AQA examiners decided to make U5 biology esssay based, and I cannot do timed essays AT ALL.

    I need 124 UMS total for 2 and 5 for a B, or 184 for an A. Don't think I'll get the A though :/
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    (Original post by Gnome :))
    Ah no What do you need to get? I'm hoping I'll get it, but the evil AQA examiners decided to make U5 biology esssay based, and I cannot do timed essays AT ALL.

    I need 124 UMS total for 2 and 5 for a B, or 184 for an A. Don't think I'll get the A though :/
    Ah yh, I dropped biology the first month of AS! God knows how much worse my case would have been had I not

    I need about the same if not more... because I've assumed I'll get 80/120 UMS for my practical assessments!

    Which unit do you feel overlaps the most with unit 5 ?
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    (Original post by posthumus)
    Ah yh, I dropped biology the first month of AS! God knows how much worse my case would have been had I not

    I need about the same if not more... because I've assumed I'll get 80/120 UMS for my practical assessments!

    Which unit do you feel overlaps the most with unit 5 ?
    Have to say that I really enjoy bio, and find it relatively easy in terms of content... It's just the essay!

    Guess we just have to push through it and get on with revising then I think units 2 and 5 fit together quite nicely with the organics and all that jazz, but everything's meging into one now
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    (Original post by Gnome :))
    Have to say that I really enjoy bio, and find it relatively easy in terms of content... It's just the essay!

    Guess we just have to push through it and get on with revising then I think units 2 and 5 fit together quite nicely with the organics and all that jazz, but everything's meging into one now
    I guess bio was just never for me I find some aspects interesting but thought there was too much content And I really hate questions where you have to write more than 2 lines!

    Ah yh, I am finding it so much more difficult and tedious this time round to study! I keep thinking I will forget all this anyway, so what is the point I have to say these forums are really really useful!

    I'm going to get some sleep now, so speak to ya soon !
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    (Original post by geor)
    Could someone take me through the steps of purifying a halogenoalkane after the intial distillation?
    Always get a bit confused!

    What I don't get is why the grade boundaries are higher for this exam than unit 1, when unit 2 is so much harder? :confused:
    Okay so you firstly add the halogenoalkane into a separating funnel with water.... you will get 2 layer - the aqueous layer on top of the halogenoalkane would have most of the impurities - so get rid of that layer

    There may still be some acid impurities in the solution so we add a very strong base - sodium carbonate (Na2CO3) or sodium hydrogen carbonate (NaHCO3), here's the formula if your interested :

    Na2CO3 + 2H+ ------> Na+ + H2O + CO2
    or
    NaHCO3 + H+ -------> Na+ + H2O + CO2

    Transfer the halogenoalkane again into a seperating funnel with water and remove the top layer again.... your halogenoalkane should be even more pure now !

    Now in dry test tube add a drying agent (such as anhydrous calcium chloride) this will remove any water... once complete you should have some very dry halogenoalkane....

    .... transfer this into a fractional distillation apparatus and distill


    I'd take a guess and say that there are more entries for unit 1 & that quite a few people may drop the subject after that, but who knows Also because it's January there are many A2 students who sit the exams (like myself ).
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    (Original post by geor)
    Could someone take me through the steps of purifying a halogenoalkane after the intial distillation?
    Always get a bit confused!

    What I don't get is why the grade boundaries are higher for this exam than unit 1, when unit 2 is so much harder? :confused:
    The AS Unit 2 isn't that bad, but there is a lot to remember. Although it would be nice if the grade boundaries were a little nicer.
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    Does anyone have an easy way of remembering which functional groups get absorbed at a specific wavelength in a IR spectrum?
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    (Original post by MSamuel)
    Does anyone have an easy way of remembering which functional groups get absorbed at a specific wavelength in a IR spectrum?
    You don't need to remember that do you ?

    All that information is in the data booklet provided
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    (Original post by posthumus)
    You don't need to remember that do you ?

    All that information is in the data booklet provided
    Oh good, I got worried there. Thank you!
    Any idea of what I can expect in the unit 2 exam?
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    And also does can anyone explain SN1 and SN2 mechanisms to me.. CGP doesn't even mention it and im totally lost.
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    (Original post by MSamuel)
    And also does can anyone explain SN1 and SN2 mechanisms to me.. CGP doesn't even mention it and im totally lost.
    The Sn1 mechanism will give rise to a carbocation; there is effectively two/three steps:
    1. The original substrate, splits into the carbocation and the leaving group.
    2. The carbocation is then attacked by an arriving ground (nucleophile) which results in a new substrate.

    Whereas the Sn2 mechanism is effectively a one-step mechanism with a transition state (both leaving and arriving groups are 'attached'at once - http://upload.wikimedia.org/wikipedi...tion-small.png
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    (Original post by HarryMWilliams)
    The Sn1 mechanism will give rise to a carbocation; there is effectively two/three steps:
    1. The original substrate, splits into the carbocation and the leaving group.
    2. The carbocation is then attacked by an arriving ground (nucleophile) which results in a new substrate.

    Whereas the Sn2 mechanism is effectively a one-step mechanism with a transition state (both leaving and arriving groups are 'attached'at once - http://upload.wikimedia.org/wikipedi...tion-small.png

    In the exam, is it likely that we'll get a question asking us to draw one of the 2 mechanisms do you think?
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    (Original post by MSamuel)
    In the exam, is it likely that we'll get a question asking us to draw one of the 2 mechanisms do you think?
    If I remember correctly, some questions will ask you to compare the two separate mechanisms or identify the key features of each mechanism.
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    (Original post by MSamuel)
    Oh good, I got worried there. Thank you!
    Any idea of what I can expect in the unit 2 exam?
    Wish I did have an idea Anything mentioned in the spec I guess... when you get onto past papers then you will get a better idea of what to expect in general


    (Original post by MSamuel)
    In the exam, is it likely that we'll get a question asking us to draw one of the 2 mechanisms do you think?
    Yup that's a very likely question
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    My problem is with shapes of molecules and ions. I need to know if there is a definite list of what to know or is there some way of knowing the shapes. I know the shapes of molecules which don't have lone pairs. But molecules with lone pairs is the problem for me how do you how many lone pairs are there in molecules like SO2? I don't know how to draw dot and cross diagrams for those types of molecules. Please help someone!


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    (Original post by NilFBosh)
    My problem is with shapes of molecules and ions. I need to know if there is a definite list of what to know or is there some way of knowing the shapes. I know the shapes of molecules which don't have lone pairs. But molecules with lone pairs is the problem for me how do you how many lone pairs are there in molecules like SO2? I don't know how to draw dot and cross diagrams for those types of molecules. Please help someone!


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    You just draw the dots and cross diagrams for a molecule to see how many bond pairs and lone pairs a molecule has
    like, for SO2 for example, the Sulphur (S) atom is in group 6, indicating, that it has 6 atoms in its outer shell/last shell/ valence shell.
    and the oxygen atom is also a group 6 element but since our CENTRAL atom is SULPHUR, we'll only be focusing on the bond pairs and lone pairs of THAT atom
    so over here , Sulphur makes 2 bonded pairs with each oxygen atom and ITSELF has A lone pair as well.
    You might be familiar with the idea that if 2 bond pairs are in the same position, we'll regard them as just one bond pair.
    So anyway, the sulphur atom has one lone pair and 2 bond pairs
    hence, the shape will be V-shaped angular

    I hope I make sense but here's a link which I found incredibly helpful

    http://www.adichemistry.com/general/...pr-theory.html

    and anyone who reads this and finds my information wrong please correct me
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    My problem is with shapes of molecules and ions. I need to know if there is a definite list of what to know or is there some way of knowing the shapes. I know the shapes of molecules which don't have lone pairs. But molecules with lone pairs is the problem for me how do you how many lone pairs are there in molecules like SO2? I don't know how to draw dot and cross diagrams for those types of molecules. Please help someone!!!
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    (Original post by NilFBosh)
    My problem is with shapes of molecules and ions. I need to know if there is a definite list of what to know or is there some way of knowing the shapes. I know the shapes of molecules which don't have lone pairs. But molecules with lone pairs is the problem for me how do you how many lone pairs are there in molecules like SO2? I don't know how to draw dot and cross diagrams for those types of molecules. Please help someone!!!
    I am also really struggling with this so any help would be appreciated. Also how do you work out the shape of ions. I am struggling to work out how many lone pairs and bonding pairs there are. For example in SO42- how would you work out that it is say tetrahedral or that CH3+ is trigonal planar? Thanks.
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    Just concentrate on the number of bonding pairs and lone pairs on the CENTRAL atom
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    I wrote this really long detailed post it never showed why -_-'

    Anyways I wrote down a few general properties that give a molecule/ion that certain shape

    2 bond pairs - Linear
    3 bond pairs - Trigonal Planar
    4 bond pairs - Tetrahedral
    5 bond pairs - Trigonal bipyramidal
    6 bond pairs - octahedral
    3 bond pairs , 1 lone pair - Trigonal pyramidal
    2 bond pairs , 2 lone pairs - V-shaped angular

    hope this helps. & if Im wrong do correct me!
 
 
 
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