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    Question "When ethanol burns in oxygen under standard conditions, carbon dioxide, water and 1368 kJmol-1 of energy are produced. Calculate the enthalpy of formation of ethanol, given that the enthalpies of formation of carbon dioxide and water are -393.7 and -285.9 kJmol-1 respectivel"

    Hi , Im stuck on to how I get to the answer on this question

    I understand the answer by using the mark scheme is -227.1kjmol-1

    but I cant seem to get to that , can anyone help please?
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    Are you sure the energy produced is +1368? Because I only get the final answer answer if the energy produced is (minus)1368 kJmol-1. If so, I would happily help you
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    C2H5OH + 3O2===>2CO2 + 3H2O dont forget to times the H2O and CO2 enthalpies as there is more than 1 mole of it

    2 moles of CO2 so -393.7x2= -787.4
    3 moles of H2O so -285.9x3= -857.7
    -787.4+-857.7= -1645.1

    then

    -1645.1--1368= -277.1

    answer= -277.1


    Not sure if this is any help but this is how I would get to the answer

    Edit: dont forget to make sure the equation is correctly balanced
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    (Original post by madmadmax321)
    C2H5OH + 3O2===>2CO2 + 3H2O dont forget to times the H2O and CO2 enthalpies as there is more than 1 mole of it

    2 moles of CO2 so -393.7x2= -787.4
    3 moles of H2O so -285.9x3= -857.7
    -787.4+-857.7= -1645.1

    then

    -1645.1+1368= -277.1

    answer= -277.1


    Not sure if this is any help but this is how I would get to the answer
    But for formation you have to do products minus reactants so if it is plus 1368 then your answer would be -3013.1 because + and - become -?
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    (Original post by Sameer599)
    Are you sure the energy produced is +1368? Because I only get the final answer answer if the energy produced is (minus)1368 kJmol-1. If so, I would happily help you
    The energy produced, i.e. energy released by the reaction, was +1368 kJmol-1. The enthalpy change was thus -1368 kJmol-1. The calculation will come out fine.
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    (Original post by Sameer599)
    But for formation you have to do products minus reactants so if it is plus 1368 then your answer would be -3013.1 because + and - become -?

    (Original post by Big-Daddy)
    The energy produced, i.e. energy released by the reaction, was +1368 kJmol-1. The enthalpy change was thus -1368 kJmol-1. The calculation will come out fine.
    this guys right
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    (Original post by Sameer599)
    But for formation you have to do products minus reactants so if it is plus 1368 then your answer would be -3013.1 because + and - become -?
    cheers this i definitely how to do it i confused myself by overcomplicating the question and trying to use vector diagrams.
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    Spent an hour on trying to get to the answer LOL wish I posted on here and saved the time, thanks very much for your help, I will know better next time.

    Cheers
 
 
 
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