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Homework Help: Beginner pchem help needed

  1. Sep 11, 2011 #1
    i really need help with my hw if people can help lead me to the answer it would be really helpful. so far i was only able to do number 1

    1) If 2.00 mol of an ideal gas, initially at 1.53 atm and 280 K, is allowed to expand isothermally under
    a constant external pressure of 0.920 atm until the final internal pressure is 0.920 atm, what is the
    work done?
    my answer: -2368 J

    2) If the gas involved is ideal and the process is reversible, obtain an expression for w (work), using
    our ‘regular’ thermodynamic variables, P, V, n, and T.

    3) For which of these systems is the system’s energy conserved in every process: (a) a closed system;
    (b) an open system; (c) an isolated system; (d) a system enclosed in adiabatic walls? Please provide
    a brief explanation for your answer.

    5) We showed ΔH = q for a constant-pressure process. Consider a process in which P is not constant
    throughout the entire process, but for which the final and initial pressures are equal. Need ΔH be
    equal to q here? (Hint: One way to answer this is to consider a cyclic process)

    6) A sample consisting of 1.00 mol of a monatomic perfect gas (for which CV,m = 3/2R) is taken
    through the cycle shown below. (a) Determine the temperatures at 1, 2, and 3; (b) Calculate q, w,
    ΔU, and ΔH for each step and the overall cycle. If a numerical answer cannot be obtained, then
    write +, -, or ? as appropriate.


    7) 2.50 mol of an ideal gas with CV,m = 12.47 J mol-1 K-1 is expanded adiabatically against a constant
    external pressure of 1.00 bar. The initial temperature and pressure of the gas are 325 K and 2.50
    bar, respectively. The final pressure is 1.25 bar. Calculate the final temperature, q, w, ΔU, and ΔH.
    Last edited: Sep 11, 2011
  2. jcsd
  3. Sep 11, 2011 #2
    attempt on #4

    a. yes matter is not transferable
    b. no matter is transferred
    c. yes, no interaction
    d. yes adiabatic does not conduct heat
  4. Sep 12, 2011 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper
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    Sorry to tell you this, but this answer isn't correct because the process described isn't reversible.
    What's the general formula for work?
    Knowing how these various types of systems are defined would be a good start. What exactly is a closed system, etc.?
    Hint: What's special about state variables?
    According to the forum rules, you need to show some effort at solving these problems yourself.
    There is no #4 in your original post.
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