Le Châtelier’s Principle

1. Le Châtelier’s Principle Chapter 14.3

2. Chemical Equilibrium • The point in a chemical reaction when dynamic equilibrium has been achieved and the concentration of the reactants and products remains constant

3. What do you do when your stomach growls from hunger? • Feed it!

4. What do you do when your foot falls asleep? • Adjust your legs so that the circulation will be restored to your feet.

5. What do you do when your checking account is at a zero balance? • Have your mom deposit more money, of course! • All of these are examples of how a system at equilibrium responds to a stress in order to regain the state of equilibrium

6. Le Châtelier’s Principle • The principle that states that a system in equilibrium will oppose the change in a way that helps eliminate the change.

7. In other words • Whenever a system at equilibrium is disturbed, the system will shift in the direction which counteracts the disturbance

8. Huh? • Chemical reactions respond to similar stresses to the system • Note: when a system returns to a state of equilibrium, there is a new equilibrium point because the original conditions have been changed.

9. For Example • In your classroom, as Mrs. Price is teaching, a very loud and obnoxious student wanders in. • Your classroom is at dynamic equilibrium when the teacher is instructing and the students are engaged and learning (Insert laugh here!) • A student disturbs the peaceful instructive classroom • Mrs. Price sends the student to Mr. Francis thereby restoring order, hence equilibrium

10. Stress According to Le Châtelier • Stresses including changes in • concentration, • temperature and • pressure • are subject to Le Châtelier’s Principle

11. Chemical Shift • A chemical shift is when either the forward or reverse reaction is favored by the introduction of a stress. Equilibrium

12. So . . . • A forward shift is to the right of the reaction in response to a stress • A reverse shift is to the left of the reaction in response to a stress

13. Effect of Concentration N2 + 3H2 2NH3 • If the [N2] is increased, in other words, we add more reactant, • Then, the reaction will shift to the right, forward shift, in order to remove any additional nitrogen

14. Forward Shift • So, let’s 1add some nitrogen and the system will become 2reactant heavy • As a result, the system will consume the nitrogen and the forward reaction will be favored; hence, the [NH3] increases and the [H2] decreases to once again 3achieve equilibrium 1. N2 + 3H2 2NH3 2. N2 + 3H2 2NH3 3N2 + 3H2 2NH3

15. Reverse Shift • Suppose instead of nitrogen, the [NH3] is 1increased. The reaction becomes 2product heavy • As a result, the system will decompose the ammonia and the reverse reaction will be favored; hence, the [NH3] decreases and the [N2] and [H2] decreases to once again 3achieve equilibrium 1. N2 + 3H2 2NH3 2. N2 + 3H2 2NH3 3N2 + 3H2 2NH3

16. Common Ion Effect • In a saturated solution of an ionic compound, the ions are in equilibrium with it’s solid form AgCl(s) Ag+(aq) + Cl-(aq) • If you add additional Cl- from a different ionic parent, more AgCl will be produced

17. Reduction of Solubility • The common ion effect reduces the solubility of slightly soluble compounds

18. Effect of Volume Change • What happens when you reduce the volume of a system? • The pressure increases, and the particles are closer together • The stress can be relieved by producing a smaller number of particles

19. Example • Let’s look again at the Haber process • There are 4 moles of reactants and 2 moles of product • Reducing the volume would shift the reaction to the right where there are fewer particles N2(g) + 3H2(g) 2NH3(g)

20. Another Example • Let’s look at the reaction of Hydrogen and Chlorine to form Hydrochloric Acid H2(g) + Cl2(g) 2HCl(g) • There are 2 moles of reactant AND product so an increase or a decrease in volume would not cause the reaction to shift

21. Pressure Changes • Pressure changes have almost no effect on equilibrium reactions in solution • Pressure effects the equilibrium of gaseous species

22. Changing the Temperature • We can raise the temperature of a system by adding energy in the form of heat • Adding heat to a system is endothermic • Removing heat from a system is exothermic

23. Lower the Temperature • Because an exothermic reaction releases heat, it will favor a decrease in the temperature • Lower the temperature in a system and the reaction will shift to the exothermic side in order to replace some of the lost heat

24. Raise the Temperature • Because an endothermic reaction absorbs heat, it will favor an increase in the temperature • Raise the temperature in a system and the reaction will shift to the endothermic side in order to absorb the excess

25. Lower the Temperature in the Haber Process • When we lower the temperature, the reaction shifts toward the exothermic side • The [NH3] is increased and the [N2] and [H2] is decreased N2(g) + 3H2(g) 2NH3(g) + 91.8kJ

26. Effects of a Catalyst • At equilibrium, a catalyst increases the forward and reverse reactions equally • However, if a system is NOT at equilibrium, a catalyst will shorten the time needed to achieve equilibrium

27. In General, here are the rules

28. Now for the chemistry version . . 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy • What would happen if you . . . • Added more product of NO(g)? • Answer: The reaction would shift to the left in order to consume some of the added NO

29. Now for the chemistry version . . 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy • What would happen if you . . . • Removed an amount of the reactant O2(g)? • Answer: the reaction would shift to the left to replace the missing O2(g)

30. Now for the chemistry version . . 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy • What would happen if you . . . • Increase the pressure by decreasing the volume? • Answer: The reaction would shift toward the left which is the side with fewest gas molecules.

31. Now for the chemistry version . . 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy • What would happen if you . . . • Decreased the temperature of the system? • Answer: The reaction would shift to the right which is the side that produces energy in the form of heat.

32. Now for the chemistry version . . 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g) + energy • What would happen if you . . . • Added a catalyst? • Answer: There would be no change in the equilibrium. A catalyst simply changes the rate of a reaction without being consumed or changed significantly.

33. Summary of Effects