We can qualitatively predict the effects of changes in concentration, pressure, and temperature on a system at equilibrium by using Le Chatelier’s principle. Le Chatelier’s principle states that if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce the change.

If a component (reactant or product) is added to a reaction system at equilibrium (at constant T and P or constant T and V), the equilibrium position will shift in the direction that lowers the concentration of that component. If a component is removed, the opposite effect occurs. In other words, the system shifts in the direction that compensates for the imposed change.

Arsenic can be extracted from its ores by first reacting the ore with oxygen (called roasting) to form solid As 4 O 6, which is then reduced using carbon: As 4 O 6 (s) + 6C(s) ⇌ As 4 (g) + 6CO(g) Predict the direction of the shift of the equilibrium position in response to each of the following changes: a.Addition of carbon monoxide b.Addition or removal of carbon or As 4 O 6 c.Removal of gaseous arsenic (As 5 )

As 4 O 6 (s) + 6C(s) ⇌ As 4 (g) + 6CO(g) a.Addition of carbon monoxide The shift will be away from the substance whose concentration is increased. The equilibrium position will shift to the left. b.Addition or removal of carbon or As 4 O 6 Since a pure solid has no effect on the equilibrium position, there is no shift. c.Removal of gaseous arsenic (As 4 ) The shift will be toward the substance whose concentration is decreased. The equilibrium position will shift to the right.

There are three ways to change the pressure of a reaction system involving gaseous components: 1.Add or remove a gaseous reactant or product. 2.Add an inert gas (one not involved in the reaction). 3.Change the volume of the container. When an inert gas is added, there is no effect on the equilibrium position. When the volume of the container holding a gaseous system is reduced, the system responds by reducing its own volume. This is done by decreasing the total number of gaseous molecules in the system.

Predict the shift in equilibrium position that will occur for each of the following processes when the volume is reduced: a.P 4 (s) + 6Cl 2 (g) ⇌ 4PCl 3 (l) Only consider Cl 2 (only gas). The volume is decreased, so the position will shift to the right to decrease the number of gaseous molecules. b.PCl 3 (g) + Cl 2 (g) ⇌ PCl 5 (g) Equilibrium position will shift to the right since the product side contains only one gaseous molecule while the reactant side has two. c.PCl 3 (g) + 3NH 3 (g) ⇌ P(NH 2 ) 3 (g) + 3HCl(g) Both sides contain four gaseous molecules so no shift is seen in this case.

To use Le Chatelier’s principle to describe the effect of temperature change, treat energy as a reactant (in an endothermic process) or as a product (in an exothermic process), and predict the direction of the shift in the same way as when an actual reactant or product is added or removed.

For each of the following reactions, predict the shift in equilibrium position as the temperature is increased. a.N 2 (g) + O 2 (g) ⇌ 2NO(g)ΔH o = 181 kJ The reaction is endothermic so write energy as a reactant. N 2 (g) + O 2 (g) + energy ⇌ 2NO(g) Increase in temperature will cause the equilibrium to shift to the right. b. 2SO 2 (g) + O 2 (g) ⇌ 2SO 3 (g) ΔH o = -198 kJ The reaction is exothermic so write energy as a product. 2SO 2 (g) + O 2 (g) ⇌ 2SO 3 (g) + energy Increase in temperature the equilibrium will shift to the left.

Summary of Le Chatelier’s Principle