Pressure of gases in a container

[mikkol]

Hello,
my tutor told me the answer to this questions went beyond the scope of the course.

We are taught that according to Le Chatelier principle, a system at equilibrium compensates a changement of pressure by shifting towards products or reactants and thus changing the number of molecules in the system. So, a pressure's increase is compensated by a reduction of the number of molecules in the system, shifting the equilibrium towards more massive molecules. A reduced number of more massive molecules should exert on the container's walls the same overall pressure than a larger number of less massive molecules, so nullifying the system's shift. So why is that not the case?

Besides, we are also taught that pressure exerted by gases on a container's wall depends on the number of molecules hitting the walls. The Pascal, pressure's SI unit, is defined as N x m^-2 , so kg m^-1 s^-2. Therefore, pressure should also depend on the mass of the molecules colliding with the container's walls?

Best wishes
Nick

 

[Andrew Mason]

Hello,
my tutor told me the answer to this questions went beyond the scope of the course.

We are taught that according to Le Chatelier principle, a system at equilibrium compensates a changement of pressure by shifting towards products or reactants and thus changing the number of molecules in the system. So, a pressure's increase is compensated by a reduction of the number of molecules in the system, shifting the equilibrium towards more massive molecules. A reduced number of more massive molecules should exert on the container's walls the same overall pressure than a larger number of less massive molecules, so nullifying the system's shift. So why is that not the case?

Le Chatelier's principle is a little bit like Occam's Razor. It is a general observation but it is not a physical law.

There are various ways that an increase in pressure in a system at equilibrium will cause a system to reduce pressure. It depends on the system. For example, an increase in pressure may reduce the rate of one reaction, which may reduce the temperature or the number of gas molecules, and thereby reduce pressure.

Besides, we are also taught that pressure exerted by gases on a container's wall depends on the number of molecules hitting the walls. The Pascal, pressure's SI unit, is defined as N x m^-2 , so kg m^-1 s^-2. Therefore, pressure should also depend on the mass of the molecules colliding with the container's walls?

Not necessarily. The pressure is determined by the translational energy of the molecules. If you increase the mass of the molecules you may also increase the number of degrees of freedom of the molecules and, thereby, decrease the translational kinetic energy of the molecules at a given temperature and decrease the pressure as a result.

AM