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Chemistry AS Enthalpy Change Question Help!! watch

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    (Original post by yorobun)
    Thankyou! i remember my teacher saying it doesn't matter if you cant do it but I just saw a question on forming the cycle so I will have to talk to him tomorrow.
    It helps to draw it out, as you can actually see whats happening! Hated Hess cycles at first but once you start getting into them you'll be picking up 3/4 easy marks each time. Born-Haber at A2 is a similar one, having to draw stuff out is a pain but once you can *see* what's happening it'll all come together and you pick up easy marks.
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    (Original post by RME11)
    Attachment 510737

    Here is how I worked it out. Made notes in red to help you understand! 1 is -1398 and 2 is octane more exothermic by 18kJg-1
    Hi, just wondering why did you divide q by their mass ? What does that work out because normally I do q / n to work out the enthalpy change. Thanks.
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    To get the enthalpy change per gram.

    You can do it either way, the reason I worked out my way is that enthalpy of combustion is by definition the E.change when 1 mole of a substance is totally combusted in oxygen. Therefore I just used the mass in g of one mole of either to work it out the kjmol-1, in the grand spec of things it doesn't really make a difference as you will come out with the same answer either way.

    I'm not going to advocate which is a better method, because the best method is the method that you understand the most and find comes more naturally (Unless they are specifically asking for a certain method).
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    I was wondering since our upcoming chemistry exams are coming up if people wanted to be part of a chemistry whatsapp revision group. If you are interested either pm me or responded to this message





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