When a chemical system at equilibrium is disturbed by a change in a property, the system adjusts in a way that opposes the change. This is called Le Châtelier’s principle (LCP).
Here are a few examples of applying LCP to predict how a system will respond when certain properties are changed:
The following properties affect the reaction rate, but since they affect forward and reverse rates equally, they do not disturb equilibrium and LCP does not apply:
The last one needs more explaining. If an inert gas is added to the system while keeping volume constant, then the total pressure increases, but the partial pressures of all the gases participating in the reaction stay the same, so there is no shift. If the inert gas is added and volume is allowed to increase, then the partial pressures increase and the reaction will shift.
N2O4(g) + 57.2 kJ ⇌ 2 NO2(g)
|add Ne(g) at constant pressure||Right|
|add Ne(g) at constant volume||—|