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Effect of an Inert Gas on Equilibrium

  1. Dec 14, 2016 #1
    1. The problem statement, all variables and given/known data
    N2O4(g) + energy <---> 2NO2(g)

    How are Keq and [N2O4] affected by the addition of Ne, an inert gas, in a container at constant volume and temperature.
    Keq [N2O4]
    a) no change/no change
    b) no change/ increases
    c) increases/decreases
    d) decreases/increases

    2. Relevant equations
    n/a

    3. The attempt at a solution
    I got an answer of A, however the correct answer is apparently B. If my understanding is correct, B is the correct answer because the pressure of the system has increased, therefore increasing the chemicals to a higher concentration. Equilibrium will be shifted left to attempt to restore the original ratio of concentration to volume.

    Is this reasoning correct? I did some research on the internet, and a lot of people say the addition of an inert gas will not change concentrations if the volume is constant, just like this question. Also, Keq will not change regardless, as mathematically nothing has changed since the concentrations rise and fall in proportion with their molar ratios.
    Thank you once again~
     
  2. jcsd
  3. Dec 15, 2016 #2

    Borek

    User Avatar

    Staff: Mentor

    It depends on how you express concentration.

    If as molarity - inert gas doesn't matter, "a" is the answer. Just take a look at the molarity definition - everything remains constant.

    If as a molar fraction or a mass fraction (percentage) - then definitely concentrations go down. It doesn't matter for the equilibrium, as this is governed by partial pressures which remain constant.

    No idea why the answer "a" is not accepted as a correct one though, if the question doesn't state what concentration type to use.
     
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