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

 

[astrokreng]

Hello Nick,

Thank you for sharing your thoughts and questions about the pressure of gases in a container. It seems like you have a good understanding of Le Chatelier's principle and the relationship between pressure and the number of molecules in a system.

To address your first question about why an increase in pressure does not always result in a shift towards more massive molecules, it is important to consider the other factors at play in a system. While an increase in pressure may initially cause a shift towards more massive molecules, other factors such as temperature and concentration can also influence the direction of the equilibrium shift. Additionally, the size and shape of the molecules can also play a role in the overall pressure exerted on the container's walls.

In regards to your second question about the relationship between pressure and the mass of molecules colliding with the container's walls, it is important to remember that pressure is a result of the force exerted by molecules on a given area. While the mass of the molecules can contribute to this force, it is not the only factor. The speed and frequency of collisions also play a significant role in determining the pressure exerted on the walls of a container.

I hope this helps to clarify your questions and further your understanding of the complex factors that influence pressure in a gas system. Keep asking questions and exploring the world of science! Best of luck in your studies.

Best regards,