Le Chatelier’s Principle

1. Le Chatelier’s Principle • 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.

2. The effect of a change in concentration • 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.

3. Example: Using Le Chatelier’s Principle • Arsenic can be extracted from its ores by first reacting the ore with oxygen (called roasting) to form solid As4O6, which is then reduced using carbon: • As4O6(s) + 6C(s) ⇌ As4(g) + 6CO(g) • Predict the direction of the shift of the equilibrium position in response to each of the following changes: • Addition of carbon monoxide • Addition or removal of carbon or As4O6 • Removal of gaseous arsenic (As5)

4. As4O6(s) + 6C(s) ⇌ As4(g) + 6CO(g) • Addition of carbon monoxide • The shift will be away from the substance whose concentration is increased. The equilibrium position will shift to the left. • Addition or removal of carbon or As4O6 • Since a pure solid has no effect on the equilibrium position, there is no shift. • Removal of gaseous arsenic (As4) • The shift will be toward the substance whose concentration is decreased. The equilibrium position will shift to the right.

5. The Effect of a Change in Pressure • There are three ways to change the pressure of a reaction system involving gaseous components: • Add or remove a gaseous reactant or product. • Add an inert gas (one not involved in the reaction). • 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.

6. Example: Using Le Chatelier’s Principle • Predict the shift in equilibrium position that will occur for each of the following processes when the volume is reduced: • P4(s) + 6Cl2(g) ⇌ 4PCl3(l) • Only consider Cl2 (only gas). The volume is decreased, so the position will shift to the right to decrease the number of gaseous molecules. • PCl3(g) + Cl2 (g) ⇌ PCl5(g) • Equilibrium position will shift to the right since the product side contains only one gaseous molecule while the reactant side has two. • PCl3(g) + 3NH3(g) ⇌ P(NH2)3(g) + 3HCl(g) • Both sides contain four gaseous molecules so no shift is seen in this case.

7. The Effect of a Change in Temperature • 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.

8. Example: Using Le Chatelier’s Principle • For each of the following reactions, predict the shift in equilibrium position as the temperature is increased. • N2(g) + O2(g) ⇌ 2NO(g) ΔHo = 181 kJ • The reaction is endothermic so write energy as a reactant. • N2(g) + O2(g) + energy ⇌ 2NO(g) • Increase in temperature will cause the equilibrium to shift to the right. • b. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔHo = -198 kJ • The reaction is exothermic so write energy as a product. • 2SO2(g) + O2(g) ⇌ 2SO3(g) + energy • Increase in temperature the equilibrium will shift to the left.

9. Summary of Le Chatelier’s Principle