Le Châtelier’s principle

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:

concentration

When the concentration of a reactant is increased, the system will shift right to consume some of it; when it is decreased, the system will shift left to replace some of it. The same thing happens with products, but in the opposite direction.

temperature

When temperature is increased and the reaction is endothermic, the system will shift right to consume some of the energy; when it is decreased, the system will shift left to replace some of the energy. The same thing happens with exothermic reactions, but in the opposite direction.

gas volume

When volume is increased (or pressure is decreased), the system will shift to the side that has a greater amount of gaseous entities to restore some pressure. When volume is decreased (or pressure is increased), the system will shift to the side that has a lesser amount of gaseous entities to relieve some pressure. The number of gaseous entities is the total number of moles of gas. If both sides have the same number, the system does not shift after changes in volume or pressure.

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:

• chemical nature of reactants
• presence of a catalyst
• presence of an inert gas

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.

Example

N₂O_4(g) + 57.2 kJ ⇌ 2 NO_2(g)