GROUP AND DISCUSSION ACTIVITIES
Counterintuitive Examples, Discrepant Events
1. In a way, equilibrium systems are themselves discrepant events. The presence of all reactants and products in a reaction vessel is not expected by students who have generally been exposed earlier only to reactions that “go to completion.”
2. Students sometimes get the idea that higher temperatures drive reactions towards completion. If dissolution is treated as an equilibrium (Solute + Solvent Solution), for example, increasing the temperature is thought to increase the concentration of the solution. Although this is most often true with solid/ liquid combinations, there are exceptions (e.g., Li 2 SO 4 ). The solubility of NaCl is hardly affected by temperature at all. The solubility of a gas decreases with increased temperature. Opening a warm can of soda pop amply demonstrates this fact (see also Demonstration 2 in Solubility and Precipitation module).
3. Solubility equilibria are an excellent chance to challenge student ideas about physical vs. chemical changes. Ask students if a reaction resulting in a precipitate is a physical or chemical change [M + (aq) + A (aq) Æ MA (s)]. Then ask if the dissolution of an ionic salt is a physical or chemical change [MA(s) Æ M + (aq) + A (aq)]. Then show them that both reactions can be represented as one equilibrium reaction.
Pictures in the Mind
These diagrams represent molecular activity during a reversible reaction. Figure 1 is just one example from the Teacher’s Resource Manual, Concept Mastery section; Chemistry: the Study of Matter , Prentice-Hall, Dorin, Demmin, and Gabel, pp. CM-31 and 32. Have students label each drawing. Which drawing(s) represent the system at equilibrium (B and C) and which represent the system not at equilibrium (A and D)? X(g)
Analogies and Metaphors
1. In a football game, the number of players on the field is constant although exchange of players (substitution) changes actual persons.
2. Connected fish bowl analogy . Two fish tanks are connected by a tube large enough to allow passage of fish. A number of fish are placed in one of the tanks. At equilibrium, the number of fish in each tank will eventually become unchanged.
3. Two jugglers analogy.
Figure 2. Two jugglers analogy.
4. Drinking fountain line:
(a) Ten students waiting in line to get a drink of water on a hot day. As each gets a drink, the same student reenters the line (equilibrium in a closed system).
(b) Same situation as “a,” except as each student gets a drink and leaves, a new student enters the line (steady-state in an open system).
5. Picture a number of horses and wranglers in a corral. As each wrangler mounts a horse, the wrangler is bucked off. The equilibrium is: Horse + Wrangler Mounted wrangler Consider the effect (a la LeChatelier) of adding horses or wranglers.
Language of Chemistry
a. Physical Science Unit : Applies principles of chemistry, physics, and geology (e.g., identification of glass, soil samples, or tire marks) to the identification and comparison of crime-scene evidence.
b. Biology Unit: Examines blood, body fluids, hair, fibers, and botanical specimens.
c. Firearms Unit: Examines firearms, bullets, cartridge cases, shotgun shells, tool marks, latent prints, and ammunition of all types.
d. Document Examination Unit : Examines handwriting and typewriting on questioned documents; analyzes paper and ink, indented writings, obliterations, erasures, and burned or charred documents.
e. Toxicology Unit : Examines body fluids and organs for drugs and poisons.
f. Other units include the Identification Unit (photography, fingerprint, polygraph, and voice-print analysis), and Evidence Collection Unit.
Common Student Misconceptions
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