1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Borek

    User Avatar

    Staff: Mentor

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

    --
    methods
     
  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

    Borek

    User Avatar

    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?

    --
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Constant Pressure Calorimetry with Piston
Loading...