About the ads

  1. Check out PREMED.me!
    Offers FREE service to help premeds manage their applications and MCAT study calendars and identify schools to target.

heat of formation and combustion problem

Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 22, 2009.

  1. SDN is a nonprofit organization. Services are made possible through the generous support of SDN members and sponsors. Thank you.
  1. inaccensa

    inaccensa

    Joined:
    Sep 5, 2008
    Messages:
    510
    Status:
    Medical Student
    SDN 5+ Year Member

    SDN Members don't see this ad. (About Ads)
    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:
    Jun 11, 2007
    Messages:
    169
    Status:
    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:
    Jun 6, 2009
    Messages:
    23
    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.

Share This Page


About the ads