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 N 2 + 3H 2  2NH 3 If the [N 2 ] 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 1 add some nitrogen and the system will become 2 reactant heavy As a result, the system will consume the nitrogen and the forward reaction will be favored; hence, the [NH 3 ] increases and the [H 2 ] decreases to once again 3 achieve equilibrium 1. N 2 + 3H 2  2NH 3 2. N 2 + 3H 2  2NH 3 3 N2 + 3H 2  2NH 3

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

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 N 2 (g) + 3H 2 (g) 2NH 3 (g)

20. Another Example Let’s look at the reaction of Hydrogen and Chlorine to form Hydrochloric Acid H 2 (g) + Cl 2 (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 [NH 3 ] is increased and the [N 2 ] and [H 2 ] is decreased N 2 (g) + 3H 2 (g) 2NH 3 (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.. 4NH 3 (g) + 5O 2 (g) --> 4NO(g) + 6H 2 O(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.. 4NH 3 (g) + 5O 2 (g) --> 4NO(g) + 6H 2 O(g) + energy What would happen if you... Removed an amount of the reactant O 2 (g)? Answer: the reaction would shift to the left to replace the missing O 2 (g)

30. Now for the chemistry version.. 4NH 3 (g) + 5O 2 (g) --> 4NO(g) + 6H 2 O(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.. 4NH 3 (g) + 5O 2 (g) --> 4NO(g) + 6H 2 O(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.. 4NH 3 (g) + 5O 2 (g) --> 4NO(g) + 6H 2 O(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