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Homework Help: Constant Pressure Calorimetry with Piston

  1. Nov 1, 2009 #1
    This is a conceptual question...hope you guys can explain the answers that the book gives! Its frustrating me to no end. This is actually a problem in the Enthalpy chapter, calorimetry section of section exercises of the Olmsted and Williams chemistry textbook, 3rd ed.

    Any help would be GREATLY appreciated! Is this just a poorly worded question, or do I have the incorrect concept?

    Q: Imagine a calorimeter with a sliding piston that makes it able to perform constant pressure calorimetry experiments on a mixture of gasses and liquids

    Consider burning 1.250 octane in this calorimeter, which is initially at 25 degrees celsius

    Q1: Will the calorimeter temperature rise or fall?
    A: Rise

    [note: I understand this, since burning octane is an exothermic reaction, leading to the loss of heat from the piston to the calorimeter]

    Q2: Use the ideal gas equation to determine whether the volume of the system will increase or decrease. (The temperature after burning rises about 6 degrees celcius)
    A: Decrease: The volume will decrease. Although the temp rises slightly, the moles of gas decrease

    [note: I don't understand this! Since it is exothermic, doesn't the reaction cause work to be performed on the piston, increasing the volume of the piston?]

    Q3: Is the work, negative, positive, or zero?
    A: Positive

    [note: i agree with this answer. however, it seems to contradict the last answer. also, the question is not specific as to what is experiencing the work, the piston or calorimeter]
  2. jcsd
  3. Nov 1, 2009 #2


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    Staff: Mentor

    Reaction equation will help, although I feel like second part is a little bit ambiguous.

  4. Nov 1, 2009 #3
    Hi Borek-

    No equation is provided with the problem, although I think the reaction is combustion and the liquid gas mixture they are referring to is the mixture of octane, water, and co2

    2C8H18 +25O2-----16CO2+H20

    Thanks for responding! And I agree that the question is vague, the answers given seem to be contradictory :(
  5. Nov 1, 2009 #4


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    Staff: Mentor

    18H2O, but I suppose it is a just a typo.

    Trick is - we don't know what is the final temperature. If it is below 100 deg C, water is liquid - you have started with 25 volumes of gas, you are left with 16 volumes. Does it mean increase or decrease of volume?

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