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    Guys, if anyone needs any help let me know.
    Im in uni now but i did AQA chem.

    Im happy to help c:
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    OCR CHEMISTRY A GUYS! someone help in cyclohexane/organic solvents and testing for halogens? help!!
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    (Original post by Disney0702)
    I do AQA, if you do the same as me please feel free to ask me anything.

    Don't worry about your U grade. When I did the CHEM2 MOCK in April (not March sorry) there were only topics that I understood completely like Maxwell-Boltzmann and Equilibria but the rest of the content was just mumbo jumbo. It'll take time and practise to get the grade you want, you'll be fine mate

    I also do AQA we have a test on Friday on haloalkanes. So tying to remember as much as I can normally I do prefer organic chemistry though
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    (Original post by Super199)
    Anyone care to help me with a question.

    Use the data from the table above to calculate a value for the enthalpy of formation of ammonia?

    Why is it sum of bonds broken - sum of bonds made and not sum of products - sum of reactants?
    When you are GIVEN enthalpies of formation, you use PRODUCTS - REACTANTS.

    When you are GIVEN mean bond enthalpies, you use REACTANTS - PRODUCTS.

    This is sort of because all values given in mean bond enthalpies are positive, i.e. energy is required to break a bond. So in effect you are doing [positive number] - [positive number] right? (since both sums are positive since all values given are positive).

    If more energy is released, the sum of the products will be greater right? This makes sense because [small positive number] - [big positive number] = negative answer = exothermic, which makes sense if more energy was released.

    You're probably a little confused because it says "formation of ammonia" so you instantly think "must be products - reactants", however, you are GIVEN mean bond enthalpies and not GIVEN formation enthalpies, so you cannot use that formula! You use products - reactants when you are GIVEN enthalpies of formation! (Basically, focus on the values you are GIVEN rather than what reaction is actually happening!)
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    (Original post by Dylann)
    When you are GIVEN enthalpies of formation, you use PRODUCTS - REACTANTS.

    When you are GIVEN mean bond enthalpies, you use REACTANTS - PRODUCTS.

    This is sort of because all values given in mean bond enthalpies are positive, i.e. energy is required to break a bond. So in effect you are doing [positive number] - [positive number] right? (since both sums are positive since all values given are positive).

    If more energy is released, the sum of the products will be greater right? This makes sense because [small positive number] - [big positive number] = negative answer = exothermic, which makes sense if more energy was released.

    You're probably a little confused because it says "formation of ammonia" so you instantly think "must be products - reactants", however, you are GIVEN mean bond enthalpies and not GIVEN formation enthalpies, so you cannot use that formula! You use products - reactants when you are GIVEN enthalpies of formation! (Basically, focus on the values you are GIVEN rather than what reaction is actually happening!)
    Ah I see. That makes sense. Thanks
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    (Original post by Dylann)
    When you are GIVEN enthalpies of formation, you use PRODUCTS - REACTANTS.

    When you are GIVEN mean bond enthalpies, you use REACTANTS - PRODUCTS.

    This is sort of because all values given in mean bond enthalpies are positive, i.e. energy is required to break a bond. So in effect you are doing [positive number] - [positive number] right? (since both sums are positive since all values given are positive).

    If more energy is released, the sum of the products will be greater right? This makes sense because [small positive number] - [big positive number] = negative answer = exothermic, which makes sense if more energy was released.

    You're probably a little confused because it says "formation of ammonia" so you instantly think "must be products - reactants", however, you are GIVEN mean bond enthalpies and not GIVEN formation enthalpies, so you cannot use that formula! You use products - reactants when you are GIVEN enthalpies of formation! (Basically, focus on the values you are GIVEN rather than what reaction is actually happening!)
    Can someone explain part b. It doesn't seem to be in the book.
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    (Original post by KeilahDeere)
    OCR CHEMISTRY A GUYS! someone help in cyclohexane/organic solvents and testing for halogens? help!!
    Well I do CCEA so unfortunately I can't help you with the cyclohexane question as we don't do that until next year but, when testing for halogen ions (I assume that's the question) you simply add Silver Nitrate

    Chloride Ions present - Colourless solution --> white precipitate forms then dissolves when Diluted Ammonia is added
    Bromide Ions present - " "--> Cream precipitate forms which will dissolve if Concentrated Ammonia is added (not diluted)
    Iodide Ions present- " "--> Yellow precipitate which will not dissolve in the presence of Concentrated Ammonia

    Hope this helps mate!
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    (Original post by Super199)
    Can someone explain part b. It doesn't seem to be in the book.
    I'm not completely certain, but I think coke (carbon) is the cheapest AND also produces the purest sample. I think coke is the cheapest because in order to get aluminium you have to electrolyse it from it's ore Al2O3 which is very expensive. Hydrogen is also quite expensive as it's very flammable. I don't think these are sufficient reasons for why coke is the cheapest though, so check with your teacher!

    As for the purest sample of iron, carbon also produces the purest sample. This is because the products are in different states, i.e. one is a solid (iron) and the other is a gas (carbon monoxide) so the carbon monoxide doesn't really interfere with the solid at all! Al2O3 is also a solid and H2O may condense into water etc....CO has a very very low freezing point so it's the safest bet
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    Does anybody know where the January 2009as chemistry ed excel paper is?

    I have been searching high and low, probably not doing a very good job. On the ed excel website it only has the examiner report and not the actual paper, can someone help me locate it?
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    https://chemrevise.files.wordpress.c...extraction.pdf

    I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.

    It is one of the equations for method 1:

    It says Iron oxide + Carbon ---> Iron + Carbon monoxide.

    But obviously the symbol equation

    Isn't it supposed to be Carbon dioxide?
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    Could someone please explain markovnikov's rule please I'm really confused about it?
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    (Original post by AyshaU)
    I also do AQA we have a test on Friday on haloalkanes. So tying to remember as much as I can normally I do prefer organic chemistry though
    Good luck with your test

    Oh my! I hated Organic Chemistry at AS and I still hate it at A2, you must be those rare weirdos that like it, take it as a compliment mate
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    (Original post by Super199)
    https://chemrevise.files.wordpress.c...extraction.pdf

    I may be being daft but can some check this for me. If the link doesn't work it is the metal extraction notes AQA unit 2 on Chemrevise.

    It is one of the equations for method 1:

    It says Iron oxide + Carbon ---> Iron + Carbon monoxide.

    But obviously the symbol equation

    Isn't it supposed to be Carbon dioxide?
    Yes you're right it is meant to be CO2.

    But I believe the equation in the link is still right as everything is balanced and Fe has still been reduced from 3+ to 0.

    But seeing as you're doing AQA the equation that you should know for the exam is: 2Fe2O3(s) +3C(s) -> 4Fe(l) + 3CO2(g)

    I hope that helps
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    (Original post by Georgiam247)
    Hey guys,

    I was wondering, does anyone have a tutor for chemistry? I don't have any particular areas that I suck at but my teacher is so careless I feel like it'd be a good idea to get one. Anybody seen a lot of benefit from a tutor?
    Hi there!

    I'm an A2 AQA Chemsitry student and I got an A at AS.

    I didn't get a tutor, all I had was my CGP Chemistry textbook.

    It is absolutely amazing and I owe my grade to this book.

    I really think you should get it even if you do opt to get a tutor.

    AQA CGP Student book

    Sample pages
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    Hello everyone please help me!

    I have been asked to calculate enthalpy change of formation for N2O using data given.

    table says:
    C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
    C +1/2O2 -> CO -111 KJ mol-1 (again not specified)


    how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!

    and secondly,

    in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?

    another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
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    (Original post by haemo)
    There most likely is, however I don't know it, sorry.
    Hi, I a a post where you said if combustion given you draw a certain cycle and if formation given you draw a certain Hess' cycle. I have found questions where only enthalpy changes are give when combustion or formation is not specified What o I do then???
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    (Original post by Science100)
    Hello everyone please help me!

    I have been asked to calculate enthalpy change of formation for N2O using data given.

    table says:
    C + N2O -> CO +N2 -193 KJ mol-1 (not specified whether C or F)
    C +1/2O2 -> CO -111 KJ mol-1 (again not specified)


    how to do I know whether to draw a Hess' cycle where the arrows point upwards from the elements (formation) or whether to draw one where they point down to combustion products (combustion) IF DONT KNOW WHETHER THE TABLE VALUES ARE FOR COMBUSTION OR FORMATION ENTHALPY CHANGE??? PLEASE HELP ME EVEN MY TACHER COULD NOT EXPLAIN THIS TO ME!!!!!!!!!!! a clear description would help Can u tell me how to figure out if its a combustion cycle AND ALSO how to figure out if its a formation cycle! thanks!

    and secondly,

    in Q=MCdeltaT you determine the sign (-/+) by yourself by looking at whether the temperature has increased or decreased. I did a question where two solutions were mixed (NaOH and H2SO4) the temperature went from 20 degrees and rose to 29 degrees. The answer in the book said use a + sign as it's endothermic? can someone explain this?

    another question mixed solid iron powder to copper sulphate in a polystyrene cup. The sign here needed was - when the temperature went from 21 to 25 degrees? here the temperature s increased but the sign is - , why is it EXOTHERMIC NOW??? I am so confused please explain clearly thanks.
    • I can't tell you about Hess' cycle and that
    • But the endo/exo problem goes like this
    • If you are mixing NaOH and H2SO4, you're carrying out a neutralisation reaction. This is exothermic. So heat is given out into the surroundings, so if you're measuring the temperature of the solution, it will increase.
    • Exothermic reactions have a negative value for Enthalpy Change, because in an exothermic reaction, the enthalpy of the reactants is greater than that of the products.
    • Enthalpy change = enthalpy of products - enthalpy of reactants
      so if reactants have more enthalpy than products, then enthalpy change will be negative
    • so deltaH is negative for exothermic reaction
    • when you add iron powder to copper sulphate, the iron displaces the copper because it is more reactive. if the copper sulphate solution increases in temperature when this happens, you know the reaction is exothermic.
    • so this reaction will also have negative deltaH
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    Hi guys,

    Has anyone done the OCR A chemistry practical on enthalphy, I would really appreciate some help and advice on how to go about answering questions.

    Thanks
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    (Original post by afghan123)
    Hi guys,

    Has anyone done the OCR A chemistry practical on enthalphy, I would really appreciate some help and advice on how to go about answering questions.

    Thanks
    Follow instructions carefully (fairly obvious) and remember Q = mc(triangle/delta T)
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    (Original post by Aaron80)
    Well I do CCEA so unfortunately I can't help you with the cyclohexane question as we don't do that until next year but, when testing for halogen ions (I assume that's the question) you simply add Silver Nitrate

    Chloride Ions present - Colourless solution --> white precipitate forms then dissolves when Diluted Ammonia is added
    Bromide Ions present - " "--> Cream precipitate forms which will dissolve if Concentrated Ammonia is added (not diluted)
    Iodide Ions present- " "--> Yellow precipitate which will not dissolve in the presence of Concentrated Ammonia

    Hope this helps mate!
    OOOOh thank you!! this helps a lot
 
 
 
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