Well consider the reaction $A + B ⇌ C$ Adding inert gas to a container where this reaction is taking place will increase the pressure of the system. In accordance with Le Chatelier's Principle, the equilibrium should shift in the direction which opposes this change in pressure, i.e. in the direction where fewer number of molecules are created. In this specific reaction, it happens to be in the forward reaction. However, I have been taught that adding an inert gas at constant volume has no effect on the equilibrium. Why is this?

  • $\begingroup$ In the case of, say, the equilibrium partial pressure of water vapor over water, the pressure of inert gas over the water is ignored. This is because water is (pretty much) incompressible, so the inert gas pressure has no effect and Le Chatelier's principle results in no shift in equilibrium for any reasonable lab pressures. In many intro courses simple absolutes are presented - in the context of the introductory material they are fine, but the principles break down eventually, which is then the basis of the next courses... $\endgroup$ – Jon Custer Jan 20 '15 at 17:23
  • $\begingroup$ @JonCuster Well that may be applicable for the certain example that you have provided, but definitely not for all reactions. There are many equilibrium reactions which contain all reactants and products in gas phase. $\endgroup$ – Gummy bears Jan 21 '15 at 15:40

At constant temperature, only changes to the concentration (technically activity) of a reactant or product, shift the equilibrium.

Adding inert gas does not change the concentration of any reactant or product, so it does not affect equilibrium.

  • $\begingroup$ Yes but changing the pressure doesn't have any affect on the concentration either. However, it does affect the equilibrium. $\endgroup$ – Gummy bears Jan 21 '15 at 15:40
  • $\begingroup$ Changing pressure by decreasing volume changes concentration and affects equilibrium. Adding inert gas at constant volume increases pressure but does not change concentration and does not affect equilibrium. $\endgroup$ – DavePhD Jan 21 '15 at 15:48
  • $\begingroup$ Is there no other way to change the pressure? So basically it is that when you are changing the pressure, you are changing the volume. $\endgroup$ – Gummy bears Jan 21 '15 at 15:51
  • $\begingroup$ pressure can change by changing volume, temperature, or amount of one or more substances in the system. $\endgroup$ – DavePhD Jan 21 '15 at 16:02
  • $\begingroup$ Okay. So let's take that we increase the pressure by increasing the temperature. Consider the forward reaction to be exothermic, while the products have less molecules than reactants. In which direction will equilibrium be shifted? $\endgroup$ – Gummy bears Jan 21 '15 at 16:18

The gas added works in both directions and the fraction of pressure is the same becaussed the total pressure increses but the inert gas do not affect the features of the others and exerts the same effect. If the number of molecules do not change, the concentration remains constant, and the principle of Le chatelier sais that in case of increase the pressure the reaction take the direction where there are less molecules because the pressure force them to agregate.


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