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The ideal gas equation (or the van der Waals equation, for this matter) states that holding volume and temperature constant, increasing the number of moles of particles in the container will increase the total pressure exerted by the gas mixture.

That’s all well and good, but when it comes to looking at chemical reactions and their equilibria, Le Châtelier’s Principle states that when a gas mixture is pressurized, the equilibrium shifts to the side of the reaction that produces a lesser total number of moles. This would mean less particles, and therefore less pressure, as I see from the ideal gas equation.

What I do not understand is why the mass of each molecule does not affect the pressure exerted by the gas. If you have heavier molecules, they would each have more momentum, and would exert more force on the container’s walls—i.e. more pressure. So is this not a factor when considering Le Châtelier’s Principle? The equilibrium shifts toward the formation of more particles, but they are each lighter and exert less force. Why is this favorable over having less but heavier particles?

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    $\begingroup$ Heavier molecules would fly slower so as to exert exactly the same pressure as lighter ones. $\endgroup$
    – Ivan Neretin
    Nov 5, 2015 at 13:48
  • $\begingroup$ The equipartition principle applies to energy rather than momentum. The heavier molecules have lower values of velocity-squared (rather than simply having lower velocities if it were momentum to which that equi-partitioning were occurring.) Since most chemical reactions have both forward and backward rates that are going to be "driven" by energy as well, I would guess that other concerns will be dominant. The smaller volume taken up by bonded atoms appears to be that "other issue". $\endgroup$
    – 42-
    Feb 7, 2017 at 1:40

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The Maxwell–Boltzmann distribution is based on the principle that the distribution of kinetic energies of gas molecules is the same. So in a mixture of $\ce{N2}$, $\ce{N2}$, and $\ce{He}$, the molecules of each gas would all have the same KE distribution. However since He is much lighter, its molecules would have higher velocities.

Normally we'd think of the gas mixture as being homogeneous in a liter bottle in the laboratory. However if we consider the atmosphere then gravity has an effect. So helium will drift upwards in the atmosphere since it is less dense. In fact at the edges of the outer atmosphere a helium atom can get enough velocity to escape earth's gravity. So all helium that leaks into earth's atmosphere is ultimately lost into space.

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Root mean velocity and most probable velocity, both vary inversely with M^1/2, therefore heavier particles will move slower, keeping momentum constant.

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