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OCR Chemistry F322~ 4th June 2013~ AS Chemistry Watch

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    Hard A= 72
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    It was okay A= 76
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    Easy A= 79
    75
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    Very Easy A=82
    60
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    (Original post by TheFootyKing19)
    Yup that's all you need to know. Easy 3 marks always!
    No but for that question on the Jan 12 paper, people are saying you have to draw a Hess's law cycle?


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    (Original post by Jimmy20002012)
    No but for that question on the Jan 12 paper, people are saying you have to draw a Hess's law cycle?


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    Yeah some people do. Basics maths. But all questions are same
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    For enthalpy of combust if you get a positive answer, you only get 2/3 marks so rearrange your equation to get negative

    This does not apply for formation or reaction
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    (Original post by TheFootyKing19)
    Yeah some people do. Basics maths. But all questions are same
    So how would you do a question like the one is the attachment using your method?

    Thanks


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    (Original post by Jimmy20002012)
    Still don't get it, hate that question I cannot do Hess law cycles, my teacher only said you need to only know for combustion it's reactants - products and for formation you do Products - Reactants. Can out help me out please????


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    yea tbh the main ones are combustion and formation. the other one ive only seen it come up twice.. i'm not sure how else to explain it, just that the equations in the table match the overall equation. so draw a line coming down the the products and reactants for the equations that go with each one ie. whichever one has at least one of the reactants the same
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    q = mct will be up

    Hasn't been on for 2 papers so make sure you know it, calculating KJ, moles and DeltaH

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    Does cracking ALWAYS produce an alkene and alkane?
    Not alkene + alkene or alkane alkane?
    Can there be more than two different peoducts?

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    (Original post by needtosucceed=))
    yea tbh the main ones are combustion and formation. the other one ive only seen it come up twice.. i'm not sure how else to explain it, just that the equations in the table match the overall equation. so draw a line coming down the the products and reactants for the equations that go with each one ie. whichever one has at least one of the reactants the same
    I feel so stupid, I still don't get it? Ahhhh


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    (Original post by Jimmy20002012)
    I feel so stupid, I still don't get it? Ahhhh


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    ah sorry, I know my explanation sucks lol. maybe you could email one of your teachers, or post the question in the chemistry forum where people might be able to help
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    3H2+CO2 = CH3OH + H2O
    49kj of energy is released when 3moles of h2 react completely. This enthalpy is called the enthalpy change of reaction. Calculate the energy released when 1000dm^3 of hydrogen, measure at room temp and pressure, react completely with carbon dioxide.
    anyone know how to do that question from jan 13 paper.
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    (Original post by needtosucceed=))
    ah sorry, I know my explanation sucks lol. maybe you could email one of your teachers, or post the question in the chemistry forum where people might be able to help
    Its just clicked how you do it, you don't have to do an enthalpy cycle, you just have to do Hess law concepts for the first equation you are getting the enthalpy change of CO2 and N2 which is not what you want, you want to produce N2O, so you flip the equation around, so you have a reverse sign of the enthalpy change of the reaction. For the second equation you don't even have NO2 so you leave it the same,, the carbons cancel as they are on both sides:

    So you have -111 + 193 = 84KJ

    Finally!!!




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    (Original post by wndms)
    Does cracking ALWAYS produce an alkene and alkane?
    Not alkene + alkene or alkane alkane?
    Can there be more than two different peoducts?

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    Yeah always
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    I really need help on hess's law
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    (Original post by Sara_A)
    I really need help on hess's law
    i'll help you, what aspect of it do you not understand?
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    Hey guys I am applying for a summer residential at Pembroke and St. Catherine's colleges and need to fill in a section called "why have you applied to the summer residential" any ideas on what to write?

    The subject is Mathematics for the natural Sciences

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    (Original post by Mikoto)
    i'll help you, what aspect of it do you not understand?
    I dont understand constructing the cycle because i always get it wrong. In one of the question if got the right number but the wrong sign because if done the cycle wrong
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    (Original post by Jimmy20002012)
    Its just clicked how you do it, you don't have to do an enthalpy cycle, you just have to do Hess law concepts for the first equation you are getting the enthalpy change of CO2 and N2 which is not what you want, you want to produce N2O, so you flip the equation around, so you have a reverse sign of the enthalpy change of the reaction. For the second equation you don't even have NO2 so you leave it the same,, the carbons cancel as they are on both sides:

    So you have -111 + 193 = 84KJ

    Finally!!!




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    yay! glad you've understood it
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    (Original post by needtosucceed=))
    yay! glad you've understood it
    Didn't even need to go a cycle, did you use a enthalpy cycle to wok it out?


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    Is there anybody than can never do the enthalpy questions, drawing alcohols, mechanisms, all those equations and guessing the compound from a spectrum? How do you retain all of it?

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    why don't you include the methyl group in naming the alkene??

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