1. Equilibrium

2. Equilibrium is a state in which there are no observable changes as time goes by. Although there are still changes occurring, they are not easily observed. Chemical equilibrium is achieved when: 1. the rate of the forward reaction is equal to the rate of the reverse reaction, and 2. the concentrations of the reactants and products remain constant (but not necessarily equal)

3.  There are two types of equilibrium: Physical and Chemical.  Physical Equilibrium  H 2 0 (l) ⇄ H 2 0 (g)  The compound is not changing, only the phase  Chemical Equilibrium  N 2 O 4 (g) ⇄ 2NO 2 (g)  The atoms are arranged into different compounds

6. N 2 O 4 (g) ↔ 2NO 2 (g)

7. Law of Mass Action - For a reversible reaction at equilibrium and constant temperature, the ratio of reactant and product concentrations has a constant value (K eq ). The Equilibrium Constant (K eq ) - A number equal to the ratio of the equilibrium concentrations of products to the equilibrium concentrations of reactants each raised to the power of its stoichiometric coefficient. (Not the initial concentrations)

8. For the general reaction: aA (g) + bB (g) cC (g) + dD (g)

9. N 2 O 4 (g) ⇄ 2 NO 2 (g)

10. Chemical equilibrium is defined by K eq. The magnitude of K eq will tell us if the equilibrium reaction favors the reactants or the products. If K eq » 1 – Products are favored If K eq « 1 – Reactants are favored

11.  Homogeneous Equilibrium – applies to reactions in which all reacting species are in the same phase. N 2 O 4 (g) ⇄ 2 NO 2 (g)

12.  Write the equilibrium constant expression for the reaction below. CO (g) + Cl 2 (g) ⇄ COCl 2 (g)

13.  Write the equilibrium constant expression for the reaction below. NO 2 (g) ⇄ NO (g) + O 2 (g)

14. Heterogeneous Equilibrium – applies to reactions in which more than one phase is present during the reaction. Can include liquids, gases and/or solids as reactants or products. The equilibrium expression does not include pure liquids and solids, because their concentrations don’t change. Liquids are included only if there are no gases. Liquid water is NEVER included.

15. CaCO 3 (s) CaO (s) + CO 2 (g) The concentration of solids and pure liquids are not included in the expression for the equilibrium constant.

17. Shifting Equilibrium

18. Chemical Equilibrium represents a balance between forward and reverse reactions. Changes in the following will alter the direction of a reaction: Concentration of Reactants or Products Pressure Volume Temperature

19. Le Châtlier’s Principle – if an external stress is applied to a system at equilibrium, the system adjusts in such a way to reduce the stress as the system reaches a new equilibrium position.

20. Increase in concentration of one or more reactants To balance the stress, the concentration of the reactants need to be reduced. The equilibrium will shift to the ________. Increase in concentration of one or more products To balance the stress, the concentration of the products need to be reduced. The equilibrium will shift to the ________.

21. Decrease in concentration of one or more reactants To balance the stress, the concentration of the reactants need to be increased. The equilibrium will shift to the ________. Decrease in concentration of one or more products To balance the stress, the concentration of the products need to be increased. The equilibrium will shift to the ________.

22. Change Increase in [Products] Decrease in [Products] Increase in [Reactants] Decrease in [Reactants] Shift in Equilibrium Left Right Left

23. FeSCN 2+ (aq) ⇄ Fe 3+ (aq) + SCN - (aq) Red Yellow a.) At equilibriumb.) Increase in SCN - (aq) c.) Increase in Fe 3+ (aq) d.) Increase in FeSCN 2+ (aq)

24. Changes in pressure and volume primarily only concern gases which can be compressed. As pressure is increased (and volume decreased), particles will be pushed closer and closer together. To relieve the stress, atoms will combine to form the fewest particles possible. As pressure is decreased (and volume is increased), particles will spread apart. To relieve the stress, atoms will form the most number of particles possible.

26. Change Increase in Pressure Decrease in Pressure Increase in Volume Decrease in Volume Shift in Equilibrium Side with fewest moles Side with most moles Side with fewest moles

27.  Example: N 2 O 4 (g) ⇄ 2NO 2 (g)  If pressure is increased, particles will be pushed closer together.  The reaction will shift to decrease the total number of particles  Shift: Left  If pressure is decreased, particles will spread out.  The reaction will shift to increase the total number of particles.  Shift: Right

28. A temperature increase favors an endothermic reaction and a temperature decrease favors and exothermic reaction. ChangeEndothermicExothermic Increase TRightLeft Decrease TLeftRight

29. Example: Heat + N 2 O 4 (g) ↔ 2NO 2 (g)  The forward reaction absorbs heat – endothermic  So the reverse reaction releases heat – exothermic  If temperature is raised, the reaction will shift to lower it.  Reaction will shift to the right.  If temperature is lowered, the reaction will shift to raise it.  Reaction will shift to the left.