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Edexcel Chemistry Unit 4 Revision Thread watch

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    ok some revision questions to start you off

    1. why does tetrachloromethane not react with water but silicon tetracloride does? (5 marks)

    2. define enthalpy of atomisation and lattice enthalpy (3 marks each)

    3. what indicator would you use for the titration of a weak acid with a strong alkali? (1 mark)

    4. How does Kp change with pressure, temperature and a catalyst? (4 marks total)

    5. organic stuff! give reagents and conditions
    (a) butan-2-one to form the iodoform molecule
    (b) propanamide to form ethanamine
    (c) butanonitrile to form butanamine
    (d) propan-1-ol to form propanoic acid
    (e) ethane to form propanoic acid (2 steps!)

    6. What feature about an amino acid causes it to have a greater melting point than expected?
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    (Original post by Heidi)
    hehe I work better under pressure as well.
    I also have to revise for bio. It's really unfair how the chem and bio exam r so close together!
    i have all the sciences so close, esp phy which is this week. i havent even thought about chem. it is really unfair (stupid exam boards) :secruity:
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    (Original post by mik1w)
    ok some revision questions to start you off

    1. why does tetrachloromethane not react with water but silicon tetracloride does? (5 marks)
    1) both are covalent and tetrahedral owing to the repulsionof the 4 bond pairs of electrons. SiCl4 is rapidly hydrolysed by water. A lone pair of electrons from the oxygen in the water forms a dative covalent bond into an empty 3d orbital in the silicon atom. The energy released is enough to overcome the activation energy barrier involved in the breaking of the Si-Cl bond.
    Carbons bonding electrons arein the 2nd shell and there are no 2d orbitals. THe empty orbitals in the 3rd shell are of too high an energy to be used in bonding. ALso, C is small so that the water molecules are prevented from reaching it by four much larger Cl atoms arranged tetrahedrally around it.

    (Original post by mik1w)
    2. define enthalpy of atomisation and lattice enthalpy (3 marks each)

    2) Enthalpy of atomisation is the enthalpy change for the production of one mole of atoms in the gas phase from the elements in its standard state.

    Lattice enthalpy is the enthalpy change when one mole of ionic solid is made from its separate gaseous ions.
    (Original post by mik1w)
    3. what indicator would you use for the titration of a weak acid with a strong alkali? (1 mark)
    3) Phenolphthalein (sp)
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    (Original post by Heidi)
    hehe I work better under pressure as well.
    I also have to revise for bio. It's really unfair how the chem and bio exam r so close together! Oh well. Which board are u with? Im with AQA?
    I'm doing OCR Bio!
    (And OCR Chemistry even though I'm on your Edexcel thread :rolleyes: )

    And I know, in 2 weeks I have:
    Mon - Maths
    Tues - 2 Bios
    Weds - NOTHING
    Thurs - 2 Chems
    Fri - Bio

    Thats 6 exams :eek:
    And then I have 3 the Tuesday after!
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    (Original post by Mathemagician)
    Lattice enthalpy is the enthalpy change when one mole of ionic solid is made from its separate gaseous ions.
    .. under standard conditions
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    (Original post by rieuwa)
    I don't really agree... It's true but (according to me might I humbly add),the reason why Mg and Cl form MgCl2 and not MgCl is to do with lattice Enthalpy. The greater the magnitude of the LE, the more stable the resulting compound. The lattice enthalpy for MgCl is much smaller (in terms of magnitude not sign) than for MgCl2 and so MgCl2 is much more energetically stable...
    It's more to do with the enthalpy of formation. For MgCl2 it's something like 5 times as exothermic as the formation of MgCl, so MgCl2 is the more energetically favourable.

    This is related to the energy you "get back" in MgCl2. The formation of ionic compounds are favoured if the amount of energy used for the ionisation and electron affinity energies are "compensated" in the lattice enthalpy. The very large lattice enthalpy for MgCl2 more than compensates for the extra ionisation energy needed to form Mg2+(g)
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    (Original post by mik1w)
    (e) ethane to form propanoic acid (2 steps!)
    i can do all the rest... but... how can you do that in 2 steps?
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    ir must be three steps

    alkane-haloalkane-grignard-carboxylic acid
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    (Original post by fairieboi)
    i can do all the rest... but... how can you do that in 2 steps?
    sorry 3 steps my mistake.
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    Btw, edexcel expect you to say "heat energy released" as opposed to "the enthalpy change" for the definition of lattice enthalpy
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    really? cos that would make positive lattice enthalpies exothermic?! silly edexcel..
    I define it as energy change when one mole of ionic solid is formed from its gaseous ions
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    (Original post by mik1w)
    really? cos that would make positive lattice enthalpies exothermic?! silly edexcel..
    I define it as energy change when one mole of ionic solid is formed from its gaseous ions
    well, when would you ever get a positive lattice enthalpy? ;-/ surely it's like combustion in that it's always exothermic...
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    exactly but if you say "heat energy released" then it will always be positive as heat energy is always released in forming a lattice
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    For question 6, I believe it's to do with amino acids in their zwitterion form exert strong intermolecular ionic forces of attraction on each other, which have to be overcome => high melting pt.
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    Edexcel are stupid I know - the term lattice enthalpy can really mean the energy change when a lattice is formed OR when the lattice is broken. However, edexcel specify that the definition of the lattice enthalpy should be learnt as "when an ionic crystal lattice is FORMED" - therefore, for the exam, lattice enthalpy is always exothermic - just another one of their stupid rules. Saying "enthalpy change" unfortunately isnt specific enough and they prefer "heat energy" as opposed to just "energy" - but I think you would just about get away with it.

    And im pretty sure you don't get endothermic formation of lattices (or its well beyond us) so I don't think this definition causes a problem in that way...
 
 
 
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