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    I worked out two values for enthalpy of formation for ethane. One was -86, the other -76
    The -86 answer came from working out formation from enthalpy of combustion values given for carbon, hydrogen and ethane
    The -76 answer came from working out formation from average bond enthalpies

    The question is...
    comment on the descrepency between the two calculated enthalpy of formation values(-86 and -76)..stating with a reason which answer is likely to be more accurate??

    I think two of the answers are loss of heat and incompletion of the reaction but not 100%. Its worth 3 marks

    Thanks guys
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    (Original post by tariktarkan1907)
    I worked out two values for enthalpy of formation for ethane. One was -86, the other -76
    The -86 answer came from working out formation from enthalpy of combustion values given for carbon, hydrogen and ethane
    The -76 answer came from working out formation from average bond enthalpies

    The question is...
    comment on the descrepency between the two calculated enthalpy of formation values(-86 and -76)..stating with a reason which answer is likely to be more accurate??

    I think two of the answers are loss of heat and incompletion of the reaction but not 100%. Its worth 3 marks

    Thanks guys
    Bond enthalpies are averages of many values of the same bond type measured over lots of different molecules. They are rarely useful for accurate measurement of a specific quantity. Even methane has four very different values for the four C-H bonds that it contains, varying by about 30 kJ mol-1

    Measurements of combustion values are done using calibrated equipment that is about as close as you can get to the 'real' values.

    It's worth taking a browse through the thermodynamic data on the NIST chembook site, and you will see that the combustion enthalpy values are accepted to within about 0.3 kJ in the majority of cases. They also give information about the methods used.
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    Without looking at a mark scheme I would say that a couple marks would come from recognising that mean bond enthalpies come from an average over a range of different compounds and hence the value used could be very different from the true bond dissociation enthalpies associated with this reaction.
 
 
 
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