Solubility Equilibria

1. SolubilityEquilibria Lead (II) iodide precipitates when potassium iodide is mixed with lead (II) nitrate. Graphic: Wikimedia Commons user PRHaney

2. Ksp Values for Some Salts at 25C

3. Solving Solubility Problems For the salt AgI at 25C, Ksp = 1.5 x 10-16 AgI(s)  Ag+(aq) + I-(aq) O O +x +x x x 1.5 x 10-16 = x2 x = solubility of AgI in mol/L = 1.2 x 10-8 M

4. Solving Solubility Problems For the salt PbCl2 at 25C, Ksp = 1.6 x 10-5 PbCl2(s)  Pb2+(aq) + 2Cl-(aq) O O +2x +x 2x x 1.6 x 10-5 = (x)(2x)2 = 4x3 x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M

5. Use Solubility to find Ksp

6. Solving Solubility with a Common Ion For the salt AgI at 25C, Ksp = 1.5 x 10-16 What is its solubility in 0.05 M NaI? AgI(s)  Ag+(aq) + I-(aq) 0.05 O 0.05+x +x 0.05+x x 1.5 x 10-16 = (x)(0.05+x)  (x)(0.05) x = solubility of AgI in mol/L = 3.0 x 10-15 M

7. Precipitation and Qualitative Analysis

8. Rates of Chemical Reactions [Kinetics]

9. Kinetics • Studies the rate at which a chemical process occurs. The change in concentration or reactants per unit time or The change in concentration of products per unit time • Besides information about the speed at which reactions occur, kinetics also explores the reaction mechanism (how the reaction occurs).

10. Reaction Rates: 2NO2(g)  2NO(g) + O2(g) 1. Can measure disappearance of reactants 2. Can measure appearance of products 3. Are proportional stoichiometrically

11. Factors Affecting Rate • Temperature Increasing temperature always increases the rate of a reaction. • Surface Area Increasing surface area increases the rate of a reaction • Concentration Increasing concentration USUALLY increases the rate of a reaction • Presence of Catalysts

12. Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • A plot of concentration vs. time for this reaction yields a curve like this. • The slope of a line tangent to the curve at any point is the instantaneous rate at that time.

13. Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • All reactions slow down over time. • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning.

14. Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between reactant molecules.

15. Collision Model • Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy). • Reactants must have proper orientation to allow the formation of new bonds.

16. The Collision Model Furthermore, molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.

17. Activation Energy The minimum energy required to transform reactants into the activated complex (The minimum energy required to produce an effective collision) Flame, spark, high temperature, radiation are all sources of activation energy

18. Activation Energy • In other words, there is a minimum amount of energy required for reaction: the activation energy, Ea. • Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.

19. Processes in which energy is released as it proceeds, and surroundings become warmer Exothermic Processes Reactants  Products + energy

20. Exothermic Reaction w/Catalyst

21. Processes in which energy is absorbed as it proceeds, and surroundings become colder Endothermic Processes Reactants + energy  Products

22. Endothermic Reaction w/Catalyst

23. Enzymes • Enzymes are catalysts in biological systems. • The substrate fits into the active site of the enzyme much like a key fits into a lock.

24. Catalysis • Catalyst: A substance that speeds up a reaction by lowering activation energy • Enzyme: A large molecule (usually a protein) that catalyzes biological reactions. • Homogeneous catalyst: Present in the same phase as the reacting molecules. • Heterogeneous catalyst: Present in a different phase than the reacting molecules.

25. Rate Laws • A rate law shows the relationship between the reaction rate and the concentrations of reactants. • The exponents tell the order of the reaction with respect to each reactant. • This reaction is First-order in [NH4+] First-order in [NO2−]

26. The Reaction Mechanism • Thereaction mechanism is the series of steps by which a chemical reaction occurs. • A chemical equation does not tell us how reactants become products; it is a summary of the overall process. Reactants  Products The  sign has represents the reaction mechanism, but gives no indication of the steps in the mechanism

27. The Rate-Determining Step In a multi-step reaction, the slowest step is the rate-determining step. It therefore determines the rate of reaction.

28. Catalysts • Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction. • Catalysts change the mechanism by which the process occurs.