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Homework Help: Reaction order/constant, time dependent pressure

  1. Jun 13, 2010 #1
    This isn't a homework question per se, but it is from a (graduate) textbook. I'm not taking a course in this, but I've been trying to learn some basic chemical kinetics and couldn't find other problems like this one.

    1. The problem statement, all variables and given/known data
    The rate of the reaction

    CH5NH2(g) -> C2

    H4(g) + NH3(g) is measured by noting Ptotal as a function of time. At 500 degrees celsius, the following results were obtained:
    Time (sec): 0 60 360 600 1200 1500
    Ptotal (mm Hg): 55 60 79 89 102 105

    a: Find the order of the reaction.
    b: Find the specific rate constant.

    2. Relevant equations

    3. The attempt at a solution

    At first, I thought that this was a straightforward question asking me to use the integrated rate equations and graphing to find k, but then I noticed that, although pressure can be substituted for concentration (I think?) in this case, what was measured isn't the decrease in reactant, but rather increase in product. However, I don't understand the importance of the pressure of 55 mm Hg at t=0. I suppose I can't just use the difference between two Pts as x, the decrease in reactant. It's been a while since I've taken general chemistry, so any help is appreciated.
  2. jcsd
  3. Jun 14, 2010 #2


    User Avatar

    Staff: Mentor

    55 mmHg reflects initial amount of the substance that decomposes (there is some typo in the formula). Later observed pressure is sum of both reactants and products. You can calculate partial pressures using reaction stoichiometry (how many moles of each product per every mole of reactant decomposed?).

  4. Jun 14, 2010 #3
    Ah, thank you. I haven't done these kind of questions for a while, so thinking about calculating partial pressure didn't even occur to me until much later. So the 55 mm Hg at t=0 is the pressure of the sole reactant before the reaction starts? I'll try this again later today.
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