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Homework Help: ENTHALPY ? Concept Undestanding help

  1. Mar 1, 2010 #1
    ENTHALPY ???? Concept Undestanding help

    1. The problem statement, all variables and given/known data

    Hello, I am studying about enthalpy and I don't need any homework help but what I do desperately need is someone to explain to me how it works. I don't understand what enthalpy is and what the relationship is between heats of reactions at constant pressure, volume and temperature and work. As far as I know enthalpy is heat of a reaction under constant pressure and that no work is done under constant volume....but then I see practice questions in the text book such as:

    1. A 1.00g sample of Ne (g) at 1 atm pressure and 27 degrees Celsius is allowed to expand into an evacuated vessel of 2.50 L volume. Does the gas do work? Explain.

    2. In each of the following processes, is any work done when the reaction is carried out at constant pressure in a vessel open to the atmosphere? If so, is work done by the reacting system or on it?

    a.) Neutralization of Ba(OH)2 by HCl
    b.) Conversion of gaseous nitrogen dioxide to gaseous dinitrogen tetroxide
    c.) Decomposition of calcium carbonate to calcium oxide and carbon dioxode gas.

    3. The internal energy of a fixed quantity of an ideal gas depends only on it's temperature. A sample of an ideal gas is allowed to expand at a constant temperature...

    a.) Does the gas do work?
    b.) What happens to the temperature of the gas?
    c.) Does the gas exchange heat with it's surroundings?
    d.) What is the change in internal energy (DeltaU) for the gas?

    2. Relevant equations

    DeltaH = DeltaU + P x DeltaV

    DeltaH = q_p

    DeltaU = DeltaH - P x DeltaV

    3. The attempt at a solution

    Like I said though, I don't need the answers, I have the correct answers I just need the conceptual reasoning behind them? Any help is truly appreciated!!!!! :)
  2. jcsd
  3. Mar 2, 2010 #2


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    Re: ENTHALPY ???? Concept Undestanding help

    Have you seen a relationships like:

    W = PV ln(Vf/Vo) for dT = 0 and

    W = P dV for dP = 0 ?
  4. Mar 2, 2010 #3
    Re: ENTHALPY ???? Concept Undestanding help


    No I have not.
  5. Mar 3, 2010 #4


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    Re: ENTHALPY ???? Concept Undestanding help

    Tell us what relationships that involve work you are familiar with. You need to start there to answer questions like, "Does the gas do work?"
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