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Discussion in 'MCAT Discussions' started by ssh18, Mar 5, 2007.

  1. ssh18

    ssh18 Member
    5+ Year Member

    Mar 27, 2006
    Likes Received:
    I'm a bit confused...for G.Chem (gases) it says that gases expand when heated and contract when cooled, which makes sense b/c as T increases so does V at constant pressure.
    However, then in the thermodynamics section where it talks about E=q+w, it also says that as gases expand they cool and that compressing gases warm up. I'm really confused about this..can someone help me out..thanks!!
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  3. Schaden Freud

    Schaden Freud MiSanthrope II
    2+ Year Member

    Apr 23, 2006
    Likes Received:
    Medical Student
    Here are the different ways of compressing (or expanding) a gas:
    In an isothermal process, temperature doesn't change, since as V goes down, P goes up by the same amount, and vice versa. Final PV thus equals initial PV.
    In an isobaric process, pressure doesn't change. Thus, as volume goes down, PV goes down, and temperature goes down, and vice versa. (This is your Gen Chem example)
    In an adiabatic process, both pressure and volume change (by different amounts). The temperature thus also changes (goes up in a compression, down in expansion). (This is your thermodynamics example)

    Gases can also undergo an isometric process, where volume is forced to remain constant. Raising temp will therefore necessarily raise pressure in this case. This isn't an expansion/compression though.

  4. booji

    booji Member
    7+ Year Member

    Jun 23, 2005
    Likes Received:
    Nice summary!

    EMT2ER-DOC Why so Serious?????
    Physician 10+ Year Member

    Oct 16, 2003
    Likes Received:
    Attending Physician
    Remember that in a gas, the molecules are very far apart and they rarely bang into each other and they are travelling very quickly so they have a lot of kinetic energy. As you compress the gas, the atoms and molecules will bang into each other more and more often. This is an elastic collision with energy being transferred from one atom to the next. But at the same time, some heat is lost during these collisions.

    As you keep on compressing the gas, the rate of collisions increases significantly, which means there is a lot more heat lost during those collisions. There are also more collisions against the walls of the container, which is why the pressure increases as the container volume decreases.

    This is why as you compress a gas, more heat is generated. The opposite is true. As you decompress a gas, you get less collisions, less heat generated from those collisions, and therefor the gas cools.

    Remember P1V1/T1=P2V2/T2

    As volume goes down, temperature goes up and as volume goes up, temperature goes down.

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