Home, Chemistry, Physics

ISSR CLASSES
Checkpoint Science
iGCSE Chemistry
iGCSE Physics
iGCSE Coordinated Science
A-Level Chemistry

PRACTICALS
Practicals Home
Practicals A-Z
Study Plan for Practical Work

Nitrogen dioxide equilibrium explanation

The effect of temperature change is the easiest to observe. On heating the equilibrium mixture, the brown colour darkens, despite the increase in volume, which would be expected to spread out and lighten the gas colour. There must be an increase in the amount of brown gas, nitrogen dioxide, present. That is, the equilibrium has shifted to the right. This is explained by Le Chatelier's principle that states that when a change is applied to a system in equilibrium it will shift so as to oppose that change. In this case, when we raise the temperature, the equilibrium shifts so that the endothermic reaction takes place, so reducing the increase in temperature. The endothermic reaction is the left to right one, and this is the direction of change that we observe.

On cooling the equilibrium mixture, the reverse occurs. The brown colour lightens despite the reduction in volume, because there is less nitrogen dioxide gas present. This is because the exothermic, right to left reaction took place producing more of the colourless dinitrogen tetroxide, as predicted by Le Chatelier's principle.

The colour changes that occur on change in pressure are much more difficult to observe. Well done, if you made the correct observations! On reduction of pressure, the following changes should have been seen from the video. First, the colour lightens, as the brown gas is now spread out over a much larger volume. Second, the colour darkens as the equilibrium shifts to the right producing more brown, nitrogen dioxide gas. The reaction is not particularly fast, and so it takes a few seconds for this second change to occur. It may be easier to see by looking at the following three, still photographs taken from the video. The first is just before the pressure is released, the second is immediately after the pressure is released and the third is about twenty seconds after the pressure is released:

The explanation for the second change is given by Le Chatelier's principle. As the pressure was reduced, the equilibrium shifts in a direction which will increase the pressure. This is the left to right reaction, as this results in the production of two gas molecules from one gas molecule. More gas means more pressure. The occurrence of the left to right reaction produces the observed darkening in the brown colour.