Lecture Log for 10/12 to 10/21 (EXAM 3)

• 10/21, Thur; Review of acidic an basic salts, section 8-11, and example problems. Polyprotic acids...see pages 765 and 767 of the textbook.
• 10/19, Tues; Weak Acid/Base Theory. Tried to get you to see the generic nature of all weak acid or weak base problems. Made constant references to my online help sheets. Discussed the way in which the equilibrium equation can be simplified with a simple assumption (TWO criteria must be met - reasonable concentration and a small enough value for K). Conjugate acids and bases have opposite strengths due to Kw = Ka Kb.
• 10/14, Thur; Le Chatlier's Principle! How a reaction at equilibrium responds to stress. What is the stress? Changes in conc, pressure, and temp. Can predict the direction of a reaction after a stress. You can quantify the change in K for temperature using the van't Hoff equation (looks like the C.C. eq and the Arrhenius Eq). Relationship between Kp and Kc was shown. Showed how you can use the table under the equation that you write (initial, change, final) for different types of questions. Might even have to work the table backwards to get an initial concentration on some problems. Introduced Chapter 18. REVIEW your electrolytes and concepts of acids (Chapter 10). Water is a weak electrolyte. Equilibrium constant is 1.0 x 10^-14 for water at 25°C. Defined pH and the limits of the pH scale.
• 10/12, Tues; Started by going over the 5 most missed questions from EXAM 2. Started Chapter 17 on equilibrium. Equil is DYNAMIC not static. Equilibrium can be acheived from all sorts of starting positions: from only reactants, or from only products, or from any mixture there of. Equilibrium constant is a state function yet can be shown as a ratio of rate constants (not state functions) or as a function of free energy (ΔGi, a state function). Showed the METHOD of writing the mass action expression. That expression (equation) will ALWAYS equal Q (reaction quotient) and IF at equilibrium, will equal K. Three possible Q outcomes: Q>K, Q<K, and Q=K - see page 708 in your book. Showed the different types of K, Kc, Kp, Ka, Kb, Kw, and Ksp. Plain K (no subscript) comes strait from thermodynamics (free energy) and the mass action expression should be written with activities instead of concentrations - there are NO UNITS on K!! Showed the method of HOW to SOLVE EQUILIBRIUM PROBLEMS. Make your table with the 3 lines: initial conditions, change in reaction (the x's), and the final equilibrium condition. Also mentioned how the SIZE of K tells you about the mixture of products and reactants. Large K means lots more products than reactants, Small K means lots more reactants than products. The rule of thumb for a 50/50 mix is when K = 1 (this is not exact, but a good reference point).