le Châtelier's Principle: Changing Gas Pressurele Châtelier's Principle: Changing Gas PressureA syringe is filled with a mixture of nitrogen dioxide, NO2, and dinitrogen tetroxide, N2O4, gases. It is plugged so that no gas can escape. The system is in equilibrium. NO2 is a brown gas, and N2O4 is colorless, so the more NO2 is present, the darker brown the color of the syringe.
When the gas is rapidly compressed, the color first turns darker brown, and then gets slightly lighter. It ends up darker than it was before compression. When the gas is rapidly decompressed it becomes lighter, then gets slightly darker. It ends up lighter than it was before decompression. How do we explain these results?
The initial change in color results because when the gas is compressed, it is more concentrated, so that its color is darker. The gas will react to the pressure change as predicted by le Châtelier's principle by reducing the number of gas molecules. Two NO2 molecules will combine to make one N2O4. This takes a bit of time to happen, so the color fades gradually as the brown NO2 reacts, and the pressure decreases slightly.
When the gas is de-compressed, it is less concentrated, so that its color is lighter. As predicted by le Châtelier's principle, the system will respond by trying to increase the pressure. To do this it must make more gas molecules, so some colorless N2O4 will break up into brown NO2. This takes time to happen, so the color increases gradually, and the pressure increases slightly.
Notice that the system responds to any changes in pressure by decreasing or increasing the pressure slightly, but not back to its original value. Compression or de-compression cause changes in pressure, but as predicted by le Châtelier's principle, less than there would be if there was no reaction.