le Châtelier's Principle: Pressure

n2o4.jpg (6452 bytes)

This video shows the reaction of N₂O₄ and NO₂ as the pressure is changed.

Since the effect of pressure on the volume of a solid or liquid is almost negligible, we will only consider the effect of a change in pressure on gases.

For example, consider this reaction:

2NO₂(g)		N₂O₄ (g)
2 gas molecules		1 gas molecule

Now consider what will happen if we increase the pressure (which we could do by decreasing the volume).

Click for help on creating a stress table

Use le Châtelier's principle by applying the following three steps:

1. Identify the stress. In this case the applied stress is an increase in pressure.

2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to decrease the pressure. A reaction can do this be shifting towards the side with less gas molecules.

Applied Stress	Le Châtelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)
Increase P		Increase

3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the right of the equilibrium sign will go up, and everything on the left side will go down. The reaction will shift to the right.

Applied Stress	Le Châtelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)
Increase P	Decrease	Increase

Since N₂O₄ is colorless, the container will become lighter than it would, had there been no reaction (as shown in the video for an increase in pressure).

Exactly the opposite will happen if we decrease the pressure.

Use le Châtelier's principle by applying the following three steps:

1. Identify the stress. In this case the applied stress is a decrease in pressure.

2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to increase the pressure. A reaction can do this by shifting towards the side with more gas molecules.

Applied Stress	Le Châtelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)
Decrease P	Increase

3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the left of the equilibrium sign will go up, and everything on the right side will go down. The reaction will shift to the left.

Applied Stress	Le Châtelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)
Decrease P	Increase	Decrease

Since NO₂ is reddish-brown, the container will become darker than it would, had there been no reaction (as shown in the video for a decrease in pressure).

Check your understanding of le Châtelier's principle and pressure with these questions.

This experiment using carbonated water demonstrates the effect of pressure and le Châtelier's principle.

A butane lighter is a good example of pressure and le Châtelier's principle.

Acclimatization to high altitudes illustrates how le Châtelier's principle can be applied to an equilibrium in the body between oxygen and hemoglobin.