1. Chemical Equilibrium Chapter 18A
West Valley High School
General Chemistry
Mr. Mata

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved.
Standard 9A
Students will know how to use LeChatelier’s principle to predict the effect of changes in concentration, temperature, and pressure.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.

3. Essential Question:
How does Le Châtelier’s Principle describe the behavior of a chemical reaction?

4. Reversible Reactions & Chemical Equilibrium
Not all chemical reactions convert all reactant to products.
Sometimes , a reaction stops reacting EVEN THOUGH, some amount of reactant remains.

5. Reversible Reactions & Chemical Equilibrium
Reversible reaction = chemical reaction that can occur in either direction.
Forward reaction: A -> B
Reverse reaction: A <- B
Usually combined: A <-> B
Chemical equilibrium = point at which the rate of forward reaction equals the rate of reverse reaction.

6. Approaching Equilibrium
Insert figure 15.11

7. Reaching Equilibrium
Insert figure 15.12

8. Le Châtelier’s Principle
Proposed by Louis Le Châtelier (1888) to explain observations of change in concentration, pressure,& temperature.
“if a stress is applied to a system at equilibrium, the system changes in a direction to reduce that stress”.
Stress = concentration, pressure, or temperature changes.

9. Effects of Changes on a System
Concentration: system will shift away from the added component.
Temperature: system will change depending on the temperature (treat the energy change as a reactant or product).
energy + A B or A B + heat
endothermic exothermic

10. Effects of Changes on a System
Pressure: increase in pressure; shifts equilibrium toward side with fewer moles.
Decrease in pressure; shifts equilibrium toward side with more moles.
Equal moles of reactant & product; no shift.

11. Practical Example of Principle
Fritz Haber (1918) applied Le Châtelier’s principle to develop a method for the industrial synthesis of ammonia.
Developed Haber Process.
Won Nobel prize in chemistry (1918).
N2 (g) + 3H2 (g) -> 2NH3 (g) + energy

12. Practical Example of Principle
Haber process included:
Higher concentration of H2 or N2 decreased the other reactant; shifts equilibrium to the right; more NH3 made.
Removing NH3 as it forms shifts the equilibrium to the right.
Higher temperature (1020 C) shifts equilibrium to right; more NH3 made.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.

13. Practical Example of Principle
4. Catalyst used (U or Os); shift equilibrium to the right.
5. Higher pressure (200 atm) shifts equilibrium to right; more NH3 made.
Ammonia production very important for fertilizer & explosive applications.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.