Position of equilibrium

In a nutshell

According to Le Chatelier's principle, an equilibrium will shift to oppose changes in pressure, temperature and concentration.

Dynamic equilibrium

In a reversible reaction, a dynamic equilibrium is established when the rate of the forward reaction is equal to the rate of the backwards reaction. Le Chatelier's principle states if a dynamic equilibrium is subjected to change, the equilibrium will shift its position to counteract the change.

Factors affecting the position of the equilibrium are:

Concentration
Temperature
Pressure

Catalysts speed up the rate of a reaction. Catalysts lower the activation energy (minimum energy required for particles to react) of a reaction by providing an alternative mechanism or pathway. Catalysts do not affect the equilibrium position because they speed up the rate of both the forward and backward reaction by an equal amount.

Concentration

In a dynamic equilibrium, based on Le Chatelier's principle, the following statements apply:

When the concentration of reactants increases, the equilibrium shifts to the right
When the concentration of reactants decreases, the equilibrium shifts to the left
When the concentration of products increases, the equilibrium shifts to the left
When the concentration of products decreases, the equilibrium shifts to the right

Pressure

For reactions in the gaseous state, a higher number of gaseous moles is associated with a higher pressure. Thus, in a dynamic equilibrium, based on Le Chatelier's principle, the following statements apply:

When pressure increases, the equilibrium shifts to the side with fewer moles of gas
When pressure decreases, the equilibrium shifts to the side with more moles of gas
If there are no gases or the moles of gas on each side is equal, change in pressure has no effect on the equilibrium

Example

$nitrogen ~dioxide\rightleftharpoons dinitrogen~tetroxide\newline \ \\2NO_2 (g) \rightleftharpoons N_2O_4 (g)$ 

For the reaction above, there are two moles of gas on the left-hand side and one mole of gas on the right-hand side. When pressure increases, the equilibrium shifts to the right as this is the side with fewer moles of gas. When pressure decreases, the equilibrium shifts to the left as this is the side with more moles of gas.

Temperature

For a reversible reaction, if the forward reaction is exothermic, the backward reaction is endothermic. Accordingly, if the forward reaction is endothermic, the backward reaction is exothermic. Based on Le Chatelier's principle, the following statements apply:

When temperature increases, the equilibrium shifts in the endothermic direction
When temperature decreases, the equilibrium shifts in the exothermic direction

Example

$iodine ~monochloride+chlorine\rightleftharpoons iodine~ trichloride\newline \ \\ ICl (l) + Cl_2 (g)\rightleftharpoons ICl_3 (s)$

For the dynamic equilibrium above, the forward reaction is exothermic and the backward reaction is endothermic. When temperature increases, the equilibrium shifts to the left, in the endothermic direction. When temperature decreases, the equilibrium shifts to the right, in the exothermic direction.