1. Chemical Equilibrium A Review

2. Questions about reactions 1.What’s happening? The chemical equation ? 2.How fast is it happening? 3.To what extent does it occur? Kinetics Equilibrium Chapters 15-18 2 IO 3 - + 5 HSO 3 -  I 2 + 5 SO 4 2- + 3 H + + H 2 O H 2 O 2 (aq)  H 2 O(l) + O 2 (g) -- slow H 2 O 2 (aq) + catalyst  H 2 O(l) + O 2 (g) -- FAST HC 2 H 3 O 2 + H 2 O C 2 H 3 O 2 - + H 3 O +

3. Dynamic Equilibrium Equilibrium – opposite chemical processes taking place at equal rates.Equilibrium – opposite chemical processes taking place at equal rates. H 2 O(l) H 2 O(g) I 2 (H 2 O) I 2 (CCl 4 ) CO(g) + 2 H 2 (g) CH 3 OH(g) NaCl(s) NaCl(aq) H2OH2O

4. Dynamic Equilibrium At equilibrium: rate fwd = rate rev k fwd [N 2 O 4 ] = rate fwd For the dinitrogen tetroxide - nitrogen dioxide equilibrium: k fwd [N 2 O 4 ] = k rev [NO 2 ] 2 k fwd [NO 2 ] 2 k rev [N 2 O 4 ] = = K c N 2 O 4 (g, colorless) 2 NO 2 (g, brown) rate rev = k rev [NO 2 ] 2

5. Magnitude of K 1) Small K c CH 3 CO 2 H (aq) + H 2 O (l) H 3 O + (aq) + CH 3 CO 2 - 3) Intermediate K c 2 BrCl (g) Br 2 (g) + Cl 2 (g) K c = 5 2) Large K c 2 CO (g) + O 2 (g) 2 CO 2 (g) K c = 2.2 x 10 22 K c = 1.8 x 10 -5

6. Pure Liquids and Solids Equilibrium constant expressions do not contain concentration terms for pure solid or pure liquid componentsEquilibrium constant expressions do not contain concentration terms for pure solid or pure liquid components K c = [H 2 O] [CO][H 2 ] C (s) + H 2 O (g) CO (g) + H 2(g)

7. Altering Equilibrium Conditions: Le Châtelier’s Principle If an external stress (such as change in concentration, temperature, etc.) is applied to a system in a state of dynamic equilibrium, the equilibrium shifts in the direction that minimizes the effect of the stress.If an external stress (such as change in concentration, temperature, etc.) is applied to a system in a state of dynamic equilibrium, the equilibrium shifts in the direction that minimizes the effect of the stress. - Henri Louis LeChâtelier in 1888

8. The Effect of a Change in Concentration Given the system at equilibrium: CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g) Adding ammonia to the reaction mixture at equilibrium will force the reaction to produce more product. Likewise, removing ammonia from the equilibrium mixture will cause the reverse reaction to produce more reactants. CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g) Add NH 3 System restores equilibrium by producing products Equilibrium shifts to the left to replace some reactant Remove NH 3

9. Predicting Changes in Concentration Problem: Carbon will react with water to yield carbon monoxide and and hydrogen, in a reaction called the water gas reaction that can be used to convert coal into a useful fuel. C (s) + H 2 O (g) CO (g) + H 2 (g) What happens to: (a)[CO] if C is added? (b)[CO] if H 2 O is added? (b)[H 2 O] if H 2 is added? (c)[H 2 O] if CO is removed? (a) No change, since carbon is a solid, as long as some carbon is present to allow the reaction to proceed in the first place. (b) The reaction moves to the product side, and [CO] increases. (c) The reaction moves to the reactant side, and [H 2 O] increases. (d) The reaction moves to the product side, and [H 2 O] decreases.

10. Lab This Week Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) YellowColorlessRed

11. Lab Continued Establish a new equilibrium for each stressEstablish a new equilibrium for each stress Observe and record the effectObserve and record the effect Justify/explain changes before leaving labJustify/explain changes before leaving lab Note that balanced equations should be part of the ‘Calculations’ section in the reportNote that balanced equations should be part of the ‘Calculations’ section in the report Hints: 1.Only Fe 3+ reacts with SCN - to give color 2.Sn 2+ is a reducing agent, Fe 2+ doesn’t react with SCN - 3.SCN - ppts with Ag + 4.PO 4 3- forms a soluble colorless complex with Fe 3+ 5.Ammonia is a base. What do bases do with transition metal ions?

12. Procedure Proposal Predict the effect of each of the stresses and provide a 1 sentence justification for each prediction.