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    sure as soon as i find it....
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    so the enthalphy change is bonds broken- bonds formed
    If the bond energies are given then the enthalphy change = mean energy of bonds formed - mean energy of bonds broken .
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    why does a branched compound have a lower melting point than a straight chained one?

    im doing a2 and cant even do this... :eek:
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    (Original post by goliath)
    why does a branched compound have a lower melting point than a straight chained one?

    im doing a2 and cant even do this... :eek:
    Because the branches mean the compound cannot line up in parallel lines so the surface area that the compounds have to do van der Waals bonding is smaller, so the intermolecular forces in general are weaker and less energy is needed to break these bonds.
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    Hey im an a level student resitting HFHF? for Ocr, I was just wondering if ne1 could tell me

    the reaction of an acid with bases and alkalis typified by hydrochloric acid

    ive got a couple of versions and would like a clear answer cheers xxx.
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    (Original post by Dust)
    U put hot boiling concentrated NaOH in Bauxite (conc NaCl)
    this dissoves the Si2O3 and Al2O3 but does not dissolve Fe2O3
    u then filter it to remove the Fe2O3
    and u bubble CO2 in it.
    this makes Al(OH)3 percipitate
    u filter it and then heat and dry it

    hope that helps!
    how is this then converted to Al2O3 then???????
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    (Original post by lgs98jonee)
    how is this then converted to Al2O3 then???????
    You roast the Aluminium hydroxide, which leaves you with the Al2O3. Which you then dissolve in cryolite, because melting the actual AL2O3 would have to be done at like 1900*C which is insane and not economical.
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    Guys, try this exam paper [download attachment]- it has some good questions which will test understanding of organic chemistry (a chains and rings paper) . this zip file also includes the markscheme.in particular try question 3 and question 9.
    Attached Files
  1. File Type: zip ChemsitryCRmarch98.zip (41.1 KB, 51 views)
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    nice one thanx!

    i was running out of questions esp in organic...
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    I'm doing As 2morrow Edexcel does anybody know what the hell the triiodoform is and its siginificance?
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    thats on the edexcel syllubus?!

    i no wat an iodoform is... but urm triodo?
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    guys, i have a slightly different extraction of aluminium

    here it goes:

    firstly, the bauxite is crushed and heated under pressure with concentrated NaOH. Anhydrous Al2O3 dissolves and SiO2 dissolves but Fe2O3. Conditions will try to minimize how much the SiO2 dissolves. This gives sodium aluminate solution. The Al(OH)3 turns into an insoluble form, which is hydrated Al2O3. A little of this form is added to the cold, filtered sodium aluminate solution to seed it. The freshly precipitated hydrated Al2O3 is then filtered off, washed and roasted to give pure Al2O3. Some Al remains in solution but is used again for the extraction.
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    (Original post by z!D4N)
    guys, i have a slightly different extraction of aluminium

    here it goes:

    firstly, the bauxite is crushed and heated under pressure with concentrated NaOH. Anhydrous Al2O3 dissolves and SiO2 dissolves but Fe2O3. Conditions will try to minimize how much the SiO2 dissolves. This gives sodium aluminate solution. The Al(OH)3 turns into an insoluble form, which is hydrated Al2O3. A little of this form is added to the cold, filtered sodium aluminate solution to seed it. The freshly precipitated hydrated Al2O3 is then filtered off, washed and roasted to give pure Al2O3. Some Al remains in solution but is used again for the extraction.
    hmmm mines copied from the mark scheme.. but yeah i guess wat u said makes sense...
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    (Original post by Dust)
    hmmm mines copied from the mark scheme.. but yeah i guess wat u said makes sense...
    i dont know if u bubble CO2 into it or not...
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    u doin edexcel rite?
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    (Original post by Dust)
    u doin edexcel rite?
    yes
    examin 3 hours
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    3 hrs?

    its half one the exams at 8 isnt it?
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    lol just realised hong kong... so wait havent u even slept?
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    i jus woke up
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    lol ah i think I need sleep lol good luck btw.. and since u finish b4 we start a quick email of wat came up would b cool :P
    lol i;m just kiddin its all good, good luck
 
 
 
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