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Homework Help: Thermodynamics Signs question

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A gas piston is filled with propane and oxygen at room temperature to a pressure of 1
    atm. The surroundings are 1 atm and room temperature. When triggered with a sparking
    device, the propane reacts with the oxygen to produce lots of heat. Assume the system is
    infinitely flexible (constant pressure is maintained at all times) and that the surroundings
    are infinitely large (the properties of the surroundings never change).
    C3H8(g) + O2(g) >> CO2(g) + H2O(g) + q

    Balanced I believe it becomes... C3H8(g) + 5O2(g) >> 3CO2(g) + 4H2O(g) + q

    I have to fill in a table for two Times. 1. Immediately following reaction, and 2. Long after, in thermodynamic equil. For the variables, dV, dT, dntotal, w, q, and dU.

    We are to assume the rxn takes some measurable amt of time, but does not equil with surroundings until the rxn is complete.

    2. Relevant equations

    3. The attempt at a solution
    Since its Isobaric, assuming it will expand, is a closed system, and is exothermic. For Immediately after rxn:
    dV= +
    dT= +
    dn= +
    w= -
    q= -
    dU= -

    For long after the rxn:
    dV= 0
    dT= 0
    dn= 0
    w= 0
    q= 0
    dU= 0

    thanks for anyone's help, it will be greatly appreciated.
  2. jcsd
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