Chemistry II AP Objectives

Links:

College Board Topic III:
Reactions

Topics:

3. Equilibrium
   1. Concept of dynamic equilibrium, physical and chemical; Le Chatelier's principle; equilibrium constants
   2. Quantitative treatment
      1. Equilibrium constants for gaseous reactions: K_p, K_c
      2. Equilibrium constants for reactions in solution
         1. Constants for acids and bases; pK; pH
         2. Solubility product constants and their application to precipitation and the dissolution of slightly soluble compounds
         3. Common ion effect; buffers; hydrolysis

Time Range:
See course calendar

Suggested Teaching Strategies:
III. C. 1. and 2. a: Open with beaker demo - rate of forward reaction in an equilibrium system is eventually controlled by the rate of the reverse reaction. See Chem Source CD for specifics. Use demo to build idea that ration between reactants and products will remain the same regardless of how much material is in the central beaker. Introduce calculation of K_c and K_p. Use laboratory work to reinforce concepts. Introduce Q, the reaction quotient, a measure of how close a system is to equilibrium. Use the see saw analogy to explain that Le Chatelier correctly predicted that when systems at equilibrium are disturbed, they shift to counter the change. The Addison-Wesley lab on Le Chatelier's principle is helpful in building student understanding.

III. C. 2. b. i: Students have extensive practice in calculating pH, predicting acid and base strength and in the laboratory measuring acid-base molarity, K_a and molar mass.

III. C. 2. b. ii: A good demo of solubility product constant is the solubility of chalk in water, acid and base. Solubility can be predicted for any of these solutions if Ksp is known for the material. Students observe that there appears to be no reaction between water and chalk or base and chalk but chalk is rapidly dissolved in acid. Demo leads to a discussion of how the acid changes the equilibrium. Lab experience with K_sp for ionic solids is helpful.

III. C. 2. b. iii: Demo to introduce buffers and the common ion effect is the action of vinegar on a solution of sodium acetate. Demo should include the pH behavior of water when vinegar is added. Students then can compare the effect NaAc has on limiting the final pH. Conclude demo by adding a strong acid, like HCl, to the buffer to show that the solution pH is resistant to change. Introduce common ion calculation and have students predict how ionization is limited by common ion addition. Henderson-Hasselbalch will be taught at this point.

Aligned Resources:

• Core lab 6 - Standardization of an HCl Solution
• Core lab 15 - Finding an Equilibrium Constant for a Reaction
• Core lab 16 - Titrating a Diprotic Acid; Calculating K_a
• Former core lab - Common Ion Effect
• TV Ontario Video - Chemical Equilibrium
• Chem Demos - NO₂/N₂O₄ tubes
• Wings video on Catalysts fits with equilibrium discussion

Revision Date:
May 2001