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Phys. Chem. (Chemical Equilibria)

  1. Oct 22, 2011 #1
    1. The problem statement, all variables and given/known data
    From Atkins Phys. Chem 9th Ed, Ex6.5a:

    Dinitrogen Tetroxide is 18.46% Dissociated at 25 deg Celcius and 1 bar in the equilibrium N2O4(g) <-> 2NO2(g). Calculate K (equilibrium concentration) at a)25 deg. C. b) 100 deg. C. given that ΔH(standard of reaction) = +56.2 kJmol^-1 over the temperature range

    Note: The answer appears in the back of the textbook,
    A) 0.141
    B) 13.5


    2. Relevant equations

    Temperature dependance of Equilibrium constant derived from Van't Hoff:
    ln(K2)-ln(K1)=-(ΔrH/R)(1/T2-1/T1)

    ΔG=-RT ln(K)

    3. The attempt at a solution
    Solution was attempted using two separate methods:

    Assuming 1 bar of N2O4 present:

    K = activity of (NO2)^2 / activity of (N2O4)^2
    Using the percentage of dissociation:
    K=(0.1846*2)^2/(1-0.1846) = 0.167 (Wrong answer)

    Another Attempt:
    Using ΔG=-RT*ln(k), ΔG standard for NO2 = 51.31kJmol^-2, for N2O4 = 97.89 kJmol^-1
    ΔG for reaction = 2*ΔG(NO2)-ΔG(N2O4) = 4730 J
    K=e^(-ΔG/RT) using 8.314 JKmol^-1 for R and 298K for T
    K= 0.148 (2nd wrong answer)

    I am unable to find the correct answer of .141 using either methods and am unable to proceed to part B)
     
  2. jcsd
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