Le Chatelier and Volume

[Bernoull]

Can some explain why in

N2(g) + 3H2(g) ----> 2NH3(g) + Heat

physically reducing volume shifts equilibrium rightwards to reduce # or gaseous moles (reactant gas moles > product gas moles) BUT adding an inert gas (Ar) DOES NOT.

If you add enough inert gas, well effective volume decreases since they DO take up some volume, or is there an assumption of Ideal Gas law where gases occupy no volume so adding inert gas does not change volume.

I also know that partial pressure of reactants and products is unaffected by the introduction of the inert gas so from this perspective it makes sense why equilibrium does not shift, however from volume reduction perspective, it doesn't make sense to me considering the real attributes of gases (they do occupy space.)

Thanks

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[dr seuss]

Adding inert gas changes the total pressure of the system, but the partial pressures of the other gases do not change. It is the change in partial pressures that causes the equilibrium to shift.

 

[Bernoull]

Adding inert gas changes the total pressure of the system, but the partial pressures of the other gases do not change. It is the change in partial pressures that causes the equilibrium to shift.

Thanks for the response..

How is this different from reducing the container size hence volume and pressure. I must be missing something I don't see why increasing the pressure (via container size) changes partial pressure, I mean mole fraction of reactants or products doesn't change right? Why do u get different results from increasing pressure through container size VS introduction of inert gases

 

[Chowdder]

Adding inert gas do not change the partial pressure. Think of it like this, you add more inert gas and you increase the total pressure (just like compressing a container), however you also decrease the mole fraction (which do not occur when you compress a container). This leaves the partial pressure unchanged, hence no shift in equilibrium.

 

[Bernoull]

Adding inert gas do not change the partial pressure. Think of it like this, you add more inert gas and you increase the total pressure (just like compressing a container), however you also decrease the mole fraction (which do not occur when you compress a container). This leaves the partial pressure unchanged, hence no shift in equilibrium.

Thanks got it!! So the decrease in mole fraction negates the total pressure increase such that partial pressure does not change...Swweeeet!!!

Great explainer!!

 

[loveoforganic]

I agree. That's always confused me

 

[Mayssy]

Adding inert gas do not change the partial pressure. Think of it like this, you add more inert gas and you increase the total pressure (just like compressing a container), however you also decrease the mole fraction (which do not occur when you compress a container). This leaves the partial pressure unchanged, hence no shift in equilibrium.

Nice explanation