Pressure aspect of Le Chatelier's Principle

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Megna Patel 1I
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Joined: Fri Sep 25, 2015 3:00 am

Pressure aspect of Le Chatelier's Principle

Postby Megna Patel 1I » Sun Nov 29, 2015 12:32 am

Can somebody please explain the pressure aspect of Le Chatelier's Principle. I can't wrap my head around why it's a change in pressure due to a change in volume of a system instead of a change in pressure that drives the system to a new equilibrium.

Deanna Necula 3B
Posts: 62
Joined: Fri Sep 25, 2015 3:00 am

Re: Pressure aspect of Le Chatelier's Principle

Postby Deanna Necula 3B » Sun Nov 29, 2015 10:16 am

The equilibrium constant K for a certain chemical reaction is the "goal" of that particular reaction, because it describes a favorable state where reactants and products are constantly being replenished and the direction of the reaction is not skewed towards any one direction. Any changes to the reaction that alter the concentration of a component will force that reaction to exit equilibrium. Now, the reaction is described by Q instead of K. Because K is described as a function of the concentrations of each component, the concentrations must change in order to return from Q to K and restore the favorable equilibrium state. Temperature is the only change to the system that will result in a different K, but any other change, including pressure and concentration changes, will rely on tweaking the concentration of components in order to restore equilibrium. Therefore, if your stress does not result in a concentration change, then the system has no necessity to alter the reaction direction because it is already at K. Again, concentration is moles/V, so just pumping in a gas won't change the concentration of the components, IF that gas is not already a part of the reaction. If hydrogen is a reactant of the system and you pump in hydrogen gas, the concentration will change because you are altering the "moles" component of concentration, so the system must work to counteract that and therefore the reaction will change direction. By the same token, if you decrease or increase volume, you are changing the "volume" portion of concentration, so again you have new concentrations and the system must shift. However, if you pump in a gas that has no relevance whatsoever to the reaction (i.e. if you pump in helium, and helium is not a reactant or product of the system), then the concentration DOES NOT CHANGE, because you are changing neither how much of a certain component you have, nor the volume of the container. Therefore, the reaction won't experience any shift, because all your concentrations have not changed so you are already at K.

Does that make sense?

Jana Sandhu 3J
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Joined: Fri Sep 25, 2015 3:00 am

Re: Pressure aspect of Le Chatelier's Principle

Postby Jana Sandhu 3J » Sun Nov 29, 2015 9:15 pm

Change in pressure is driven by a change in volume because from the ideal gas law, we can see that P=(nRT)/V. Thus, a change in volume always results in a change in pressure. The change in pressure will never simply be a change in pressure in itself because that would mean calculating the increased rate that particles are pushing against the walls of a container, which we have yet to learn about.

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