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heat of formation and combustion problem

Discussion in 'MCAT Study Question Q&A' started by inaccensa, 08.22.09.

  1. inaccensa

    inaccensa

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    Medical Student
    SDN 5+ Year Member

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    If the heat of formation of acetylene is 226.6kJ/mol, what is the heat of combustion of acetylene?

    I can't seem to understand how to get the correct ans.

    This is what I did

    C + H = C2H2 Hf = 226kj/mol
    C2H2 + 5/2 O2 =2CO2+ H2O

    Bond dissociation energy
    C=O =805
    O-H = 464

    Delta H = Products -Reactants
    = 4*805 + 2*464 -226
    = 4148-226
    = 3922
    I know this is incorrect, since heat of combustion is an exothermic rxn
  2. knightstale4

    knightstale4

    Joined:
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    Medical Student (Accepted)
    SDN 5+ Year Member
    You have the correct formula for the combustion of acetylene. Here's how I calculate the heat of combustion:
    heat of combustion of acetylene = products- reactants
    [2 x heat of formation of CO2 (g) + heat of formation of H2O (g)] -
    [heat of formation of C2H4 (g) + 2.5 x heat of formation of O2 (g)]
    = [(2 * -393.5 kJ) + (-285.8 kJ)] - [226.6 kJ + (2.5 *0 kJ)]
    = -787 -285.8 - 226.6 = -1299.4; rounded to -1300 kJ/mol

    Hope this helps.
    If you are confused about something, send me a message :)
  3. thatguy687

    thatguy687

    Joined:
    06.06.09
    Messages:
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    Location:
    Buffalo, NY
    Status:
    Pre-Medical
    Combustion of acetylene is just the opposite of the formation of acetylene so just reverse the reaction and you get -226kj/mol.

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