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F334 (19/06/2013 9:00AM) - Salters Chemistry (Chemistry of Materials) Revision Thread Watch

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    (Original post by Tikara)
    Whats the difference between a condensation reaction and an elimination reaction? I thought condensation was losing water but HCl can be lost too
    Condensation is two molecules coming together and creating a small molecule (water/HCl..) but elimination is where one molecule looses water/HCl

    (I think!)
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    (Original post by Stardust95)
    Condensation is two molecules coming together and creating a small molecule (water/HCl..) but elimination is where one molecule looses water/HCl

    (I think!)
    so a small molecule is lost completely from one molecule in elimination and from both molecules in condensation?
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    of all the atoms we learn about I think Nitrogen is my favourite
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    (Original post by Tikara)
    Whats the difference between a condensation reaction and an elimination reaction? I thought condensation was losing water but HCl can be lost too
    A condensation reaction is an addition reaction followed by and elimination, whereas an elimination is a loss of a small molecule.
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    (Original post by hannah1994)
    Do Tm and Tg like go up with each other?
    What I mean is say Tm is 'higher than average' does that mean Tg is 'higher than average'?
    Usually yes. However, it really depends on the individual polymer; so I wouldn't worry about the relative effects of Tm and Tg on each other.

    Main thing is you know the properties of polymers above its Tm, and below its Tg; plus giving explanations why such properties are shown

    (Original post by Kreayshawn)
    what colours of compounds/ions etc do we need to have memorised for tomorrow? do we need to remember all the complex ion colours?
    You need to mainly know the reaction of Fe2+, Fe3+ and Cu2+ ions with OH- ions. CGP revision guide sums it up neatly on page 70.


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    (Original post by Dualcore)
    A condensation reaction is an addition reaction followed by and elimination, whereas an elimination is a loss of a small molecule.
    aaaaaaahhhhhhhhhhhhhhhhhhhhhhhhh hhhhh, thanks very much just got it
    so in terms of atom economy would an elimination reaction generally be lower
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    (Original post by Tikara)
    aaaaaaahhhhhhhhhhhhhhhhhhhhhhhhh hhhhh, thanks very much just got it
    so in terms of atom economy would an elimination reaction generally be lower
    It generally will depend on the Mr of the molecules involved, there is not hard and fast rule for the atom economies of elimination vs condensation like there is for addition and rearrangement where it is always 100%.
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    (Original post by Stardust95)
    Can someone explain the difference between secondary and tertiary structure for me? I know it's really simple but I never know how to write it... :/
    i think most of the time mark schemes define secondary structure as folding/twisting of the polypeptide/amino acid chain/primary structure, where as tertiary structure is defined as further/final folding or 3D shape/structure.

    Also secondary structures are alpha helix or beta sheet and the only intermolecular bonds are hydrogen bonds
    tertiary structures are fibrous or globular and can have idid pdpd hydrogen and ionic intermolecular bonding. (not sure if we need to know that much detail)

    hope this helps
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    why does the OH of a carboxylic acid donate a proton but not the OH of an alcohol? is it to do with the C=O making C positive or something?
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    When writing the full ionic equations for those electrodes, which equation flips?


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    (Original post by Tikara)
    why does the OH of a carboxylic acid donate a proton but not the OH of an alcohol? is it to do with the C=O making C positive or something?
    It's due to the stability of the anion formed when an acid donates a proton. In a carboxylic acid, when it donates a proton, the negative charge can spread (delocalise) across the carboxyl group. However, in an alcohol, the negative charge is solely on the oxygen atom and no such delocalisation occurs; making it very unstable.

    Page 4-5 of the A2 revision guide sums it up well

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    (Original post by abzy1234)
    It's due to the stability of the anion formed when an acid donates a proton. In a carboxylic acid, when it donates a proton, the negative charge can spread (delocalise) across the carboxyl group. However, in an alcohol, the negative charge is solely on the oxygen atom and no such delocalisation occurs; making it very unstable.

    Page 4-5 of the A2 revision guide sums it up well

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    your the best thanks makes perfect sense and now you mention it I remember those delocalisation diagrams
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    (Original post by stealth_writer)
    http://www.ocr.org.uk/Images/76549-u...s-specimen.pdf
    Q4 b(i), (ii) and (iii).


    And then the following tricky Q's please

    Jan 12 -
    http://www.ocr.org.uk/Images/79818-q...-materials.pdf

    Q1 (d) (iii) and (iv)

    I'm struggling with all the above, i have tried to understand them but i need the steps broken down in a methodical way. Please can someone help me
    thanks
    Can someone please answer those above Q's please?
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    I'm gonna cry omg is there a simple way to determine whether molecules will have perm-perm dipoles or just inst ind.? is it if there is an electronegative atom?
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    (Original post by super121)
    When writing the full ionic equations for those electrodes, which equation flips?


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    The one with the more negative/less positive electrode potential; as that's the stronger reducing agent, which thus is oxidised

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    (Original post by abzy1234)
    You need to mainly know the reaction of Fe2+, Fe3+ and Cu2+ ions with OH- ions. CGP revision guide sums it up neatly on page 70.


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    thanks!

    do we need to be able to do Rf calculations?

    edit: june 12 paper just killed my confidence.. feel alright about all the others
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    I posted this on the F335 page, but some of it will be useful for Materials tomorrow too!

    Generic Answers:

    Why does a Dye appeared coloured whereas benzene appears colourless? 5/6 marks

    1) A dye has a greater chromophore which has more delocalisation, and
    2) therefore requires less energy to excite electrons
    3) so can absorb light energy in the visible region of the electromagnetic spectrum
    4) transmitting the colour complementary to that of the frequency of light absorbed.
    5) Benzene has less delocalisation, so its electrons require more energy to excite
    6) So absorbs light in the Ultraviolet region of the electromagnetic spectrum, which appears colourless because we cannot see UV light.
    7) Delta E = hv - the frequency of light is proportional to the energy gap.

    What is meant by the circular ring in the structure of benzene? 2/3 marks

    1) The ring symbolises delocalised electrons
    2) That exist above and below the plane of benzene/the ring of carbon atoms
    3) And exist over the whole molecule (i.e they are not associated with a single atom)
    4) Each carbon atom provides 1 delocalised electron

    Explain the origin of colour in transition metal complexes, and why the colour varies with the nature of the ligand (5/6 marks)

    1) Delta E = hv - the frequency of light absorbed is proportional to that the difference in energy levels.
    2) Transition metals have partially filled d-orbitals (electrons)
    3) Ligands cause splitting of the d-orbitals into higher and lower energy levels,
    4) When the complex absorbs a frequency of visible light it promotes an electron into the higher energy level
    5) Transmitting the colour complementary to that of the frequency of light absorbed
    6) Different ligands cause different amounts of splitting = altered differences in energy levels of the d-orbitals.

    Describe how you could use a colorimeter to determine the concentraion of X in a sample 5 marks

    1) Zero the colorimeter using a cuvette containing distilled water
    2) Find a filter that is the complementary colour to that of the sample
    3) Make a range of solutions of concentrations above and below that of the sample (by dilution)
    4) Measure the absorbance of these solutions
    5) Plot a graph of absorbance vs concentration
    6) Draw a calibration curve
    7) Measure absorbance of sample and read concentration from calibration curve.

    Describe how you could purify a sample of X 4/5 marks

    1) Find a suitable solvent that a the solid is very soluble in at high temperatures, but is almost insoluble in at low temperatures
    2) Dissolve solid in a minimum quantity of hot solvent
    3) Cool the sample
    4) Filter, and retain fitrate
    5) Wash and dry to obtain pure crystals.

    Describe how you could use thin-layer chromatography to show X and Y are present in a sample 4/5 marks

    1) Draw pencil line 1cm above the bottom of the TLC plate and place sample spots on line
    2) Place TLC plate in beaker of solvent, ensuring that solvent doesnt reach the pencil line
    3) Cover beaker, and allow time for solvent to rise up TLC plate
    4) When solvent reaches the top of the TLC plate, remove plate and dry
    5) Apply locating agent (UV light, ninhydrin)
    6) Calculate rf value = Distance moved by spot/Distance moved my solvent and compare with known rf values.

    Explain the meaning of the term buffer solution and explain its use. 4/5 marks

    1) A solution that resists small changes in pH
    2) Despite addition of small amounts of acid/alkali
    3) Reservoir of [A-]
    4) Buffer solutions are useful in instances such as the body
    5) Where a required pH is required to be maintained - such as that of the environment that enzymes are found in to prevent the disruption of ionic bonds.
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    (Original post by Tikara)
    I'm gonna cry omg is there a simple way to determine whether molecules will have perm-perm dipoles or just inst ind.? is it if there is an electronegative atom?
    Look for C=O as this is the main functional group that is used to represent perm-dipole-dipoles. You are right in saying that there may be an electronegative atom present, but beware that if it is bonded to hydrogen it hydrogen bonds instead!
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    (Original post by Kreayshawn)
    thanks!

    do we need to be able to do Rf calculations?

    edit: june 12 paper just killed my confidence.. feel alright about all the others
    No you don't, they've never asked it before; but I would make sure what Rf is and why we use it. It doesn't hurt to learn its formula also, just in case, you never know with OCR!

    That paper was quite weird, as it mainly had a really different style compared to the past papers. Question 2 and the last calculation question was pretty challenging. I still found it quite doable though; I think what it mainly emphasises is that we have to be prepared for whatever they can chuck at us. Be confident in your chemistry, and you can't go wrong
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    Could someone summarise zwitterions please
 
 
 
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