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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%
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    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
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    anyone have a definition for vdws/London forces
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    lol I just noticed that there's nothing on "mass spectra and IR absorption" in the poll.
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    (Original post by HarryMWilliams)




    I don't think you need to know the Ecell stuff, that is Unit 5 in A2. You are just expected to have a basic understanding of electrical conductivity - so, when lots of ions are floating about, the conductivity will likely increase.
    Ooh, so for example if a substance (e.g Ba(OH)2 ) (that will react with whats already in the mixture) is added in excess the electrical conductivity will decrease (because the are less ions floating about due to them reacting with the mixture forming a product) but then as it is in excess the electrical conduc. Will increase ( assuming that the substance added will dissociate into its ions?)

    Is that right? :x


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    (Original post by Londrinna)
    Ooh, so for example if a substance (e.g Ba(OH)2 ) (that will react with whats already in the mixture) is added in excess the electrical conductivity will decrease (because the are less ions floating about due to them reacting with the mixture forming a product) but then as it is in excess the electrical conduc. Will increase ( assuming that the substance added will dissociate into its ions?)

    Is that right? :x


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    Effectively, initially the conductivity will likely decrease because ions are reacting, but as something is added in excess and then dissociates into it's constituent ions, the conductivity will likely increase.
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    (Original post by Kurraiyo)
    lol I just noticed that there's nothing on "mass spectra and IR absorption" in the poll.
    Woops sorry I could only add a certain amount of topics... what do you find it difficult? If so what part...

    ... I never really consider it difficult at all & never usually have issues with it.
    But I was totally clueless of the question they gave us in January where we had to figure out some formula using spectrum data
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    (Original post by Priya08)
    anyone have a definition for vdws/London forces
    London forces, otherwise known as dispersion forces are formed due to the uncontrolled movement of electrons. At a random point in time, the majority of electrons may be on one side of a molecule, hence it forms a instantaneous dipole.
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    (Original post by HarryMWilliams)
    Effectively, initially the conductivity will likely decrease because ions are reacting, but as something is added in excess and then dissociates into it's constituent ions, the conductivity will likely increase.
    Thank You!!!


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    (Original post by felicity95)
    Ok just read in the edexcel blue book that the boiling temperature of HCl is greater than that of HBr or HI due to greater dipole-dipole interactions, yet on the graph HCl has a bpt of -90 whilst HBr has a bpt of -70 and Hi of -40. Surely the bpts of HBr and HI are higher, because as you go from HCl to HI the size of the electron cloud increases and hence the van der waals forces increase. is it a mistake in the book?
    Quoting from this book: "You can see that the H-Cl bond will be much more polar than the H-Br or H-I bond so the dipole-dipole interactions will be greater and this will explain the higher boiling temperature." By "higher boiling temperature" they didn't mean higher than the boiling point of HBr or HI, rather they meant that the boiling point of HCl is higher than you might expect if you just extended the graph to predict the boiling point of HCl.
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    (Original post by posthumus)
    Woops sorry I could only add a certain amount of topics... what do you find it difficult? If so what part...

    ... I never really consider it difficult at all & never usually have issues with it.
    But I was totally clueless of the question they gave us in January where we had to figure out some formula using spectrum data
    Haha no problem Same here it's usually very simple but I hated the Jan question too D: came out of nowhere.
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    (Original post by Londrinna)
    Thank You!!!


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    No problemo.
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    Can someone tell me how to identify London forces and permanent dipoles between molecules please?
    Our teacher for this bit was really really not good.
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    Could someone please help me with the colour changes for the indicators in acid base titrations. All my textbooks give slightly different data and there seems to be a difference between what it is in acid, what it is in alkali and what it is at the endpoint. Also there seems to be a difference in answer depending on whether you add strong/weak acid to strong/weak alkali or vice versa. Please could someone tell me which indicator should be used in which situation and what colour change should be expected.
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    Everything will have london forces.
    If a molecule has polar bonds and is not symmetrical it will have a net dipole. Polar bonds occur when there is a significant difference in electronegativity between two atoms in a covalent bond. i.e. carbon-halide bonds are a typical example of polar bonds.
    Any molecule with a dipole will align itself with other molecule's dipole (opposite charge end) and then form electrostatic forces of attraction it the other molecule on account of there dipoles.
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    (Original post by Goods)
    Everything will have london forces.
    If a molecule has polar bonds and is not symmetrical it will have a net dipole. Polar bonds occur when there is a significant difference in electronegativity between two atoms in a covalent bond. i.e. carbon-halide bonds are a typical example of polar bonds.
    Any molecule with a dipole will align itself with other molecule's dipole (opposite charge end) and then form electrostatic forces of attraction it the other molecule on account of there dipoles.
    Excellent, thanks a bunch
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    (Original post by sidthekoala)
    Could someone please help me with the colour changes for the indicators in acid base titrations. All my textbooks give slightly different data and there seems to be a difference between what it is in acid, what it is in alkali and what it is at the endpoint. Also there seems to be a difference in answer depending on whether you add strong/weak acid to strong/weak alkali or vice versa. Please could someone tell me which indicator should be used in which situation and what colour change should be expected.
    phenolphthalein. acid- colourless neutral-pink alkali-pink/purple
    methyl orange. acid-red neutral-orange alkali-yellow
    starch(iodine) iodine present blue black no iodine colourless
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    Also, in one of those funny ionic equations, is the balancing order:

    balance water
    H+ ions to cancel excess Hs from above
    lastly electrons?

    How does electron balancing work exactly?
    Thanks
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    Why is there no hydrogen bonding between molecules of ethoxyethane? (Jan 2009, MC Q8)

    CH3CH2OCH2CH3
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    (Original post by amber206)
    Also, in one of those funny ionic equations, is the balancing order:

    balance water
    H+ ions to cancel excess Hs from above
    lastly electrons?

    How does electron balancing work exactly?
    Thanks
    screw ionic equations i always get them wrong :ashamed:

    (i think thats how you do it provided it's in acidic conditions)
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    (Original post by Goods)
    phenolphthalein. acid- colourless neutral-pink alkali-pink/purple
    methyl orange. acid-red neutral-orange alkali-yellow
    starch(iodine) iodine present blue black no iodine colourless
    I have seen it say that the colour change is from pink to colourless in the mark scheme for phenolphthalein and in the data book it says that it is red in alkali.
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    (Original post by sidthekoala)
    I have seen it say that the colour change is from pink to colourless in the mark scheme for phenolphthalein and in the data book it says that it is red in alkali.
    the colour change is dependant on the titration and if you are adding acid or alkali. the data book it wrong. although probabally you miss read it as methyl orange is red in alkali as it universal indicator
 
 
 
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