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 ₂SO 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) MA (s)]. Then ask if the dissolution of an ionic salt is a physical or chemical change [MA(s) 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 TeacherÕs 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:

Consider the effect (a la LeChatelier) of adding horses or wranglers.

Key Questions

1. Predict the direction of shift in a chemical equilibrium system given changes in concentration, temperature, and pressure. [Answer this question by applying LeChatelierÕs Principle.]

2. State the conditions that result in some reactions reaching equilibrium while others do not. [All reactants and products of a chemical reaction must be present and in contact in order for an equilibrium state to be reached.]

3. Describe equilibrium and equilibrium shifts at the molecular level. [Use ideas of collision theory to answer this question.]

4. Determine a value for K for a chemical equilibrium system from concentration data. [See the discussion on Problem Solving in Tips for the Teacher.]

5. Determine the concentration of a reactant or product in a chemical equilibrium system given the value of K. [See a general chemistry text for the problem solving skills needed.]

6. Determine the equilibrium expression for a chemical equilibrium equation. [See the discussion on Problem Solving in this module.]

Other

1. Have students write essay answers to questions about Pictures in the Mind (see examples in Pictures in the Mind).

Examples of questions:

a. Tell whether each picture represents a system in equilibrium.

b. Explain your answer for each picture.

2. Simulation. Have students set up a factory to produce ammonia according to the Haber Process (see Industrial Inorganic Chemistry module). Include following steps.

Tips for the Teacher

Language of Chemistry

Key Words in LeChatelierÕs Principle

equilibrium the state of a chemical system in which the rate of product formation equals the rate of reactant formation.

stress refers to upsetting an established equilibrium system by adding more of a reactant or product to the system, or by changing reaction conditions such as temperature or pressure.

minimize means that a stressed equilibrium will respond to establish a new equilibrium state that reduces the added stress.

shift means that the adjustment of a stressed equilibrium system will result in the formation of more product (called a shift to the "right') or the formation of more reactant (called a shift to the "left").

Common Student Misconceptions

1. "Reactions always go to completion." The way we write chemical equations could imply that reactants completely become products. This is not necessarily true, especially in systems that establish equilibrium. The double reaction arrows serve to illustrate the fact that both reactants and products remain in a system at equilibrium.

2. "Reactions at equilibrium have equal concentrations of reactants and products.' Actually, this is seldom the case at equilibrium. Values of equilibrium constants indicate the extent to which reactants form products. Large values represent reactions that go farther to the right before equilibrium is established.

3. "Reactions at equilibrium have stoichiometrically related concentrations." Some students think that the amounts of reactants and products in a reaction at equilibrium are fixed by the coefficients of the equation. The coefficients fix the relative amount that will react, not the amounts that are present.

4. "Equilibria are static." It is common for students to see chemical equilibrium as a static balance rather than a dynamic balance between forward and reverse interactions of molecules.

5. "The right and left sides of an equation represent different regions of the reaction vessel." A chemical equation is simply a formalized way to depict the chemical system, and the terms "left" and "right" refer only to the written equation. There is no "sidedness" to the chemical system itself. Reactants and products are intermingled.

Problem Solving

1. If equilibrium constants are calculated, skills associated with the mathematical solution of the problem need to be stressed.



Addition of nitrogen gas, hydrogen gas, or ammonia gas to this system will cause shifts in the equilibrium. With heat on the right side of the equation, we see that the formation of ammonia is exothermic. Addition of heat to the system will cause a shift to the left. Increasing pressure on the system will cause a shift to the right because the smaller number of product particles would cause less pressure than the larger number of reactant particles, thus relieving the stress on the equilibrium caused by the increase in pressure.

3. Students are sometimes confused by shifts in equilibrium caused by temperature changes. If heat is represented as a product (in an exothermic reaction) or a reactant (in an endothermic reaction) in the equation for the reaction, then a change in temperature adds or takes away heat from the system. In this way, temperature is treated in the same way as increases or decreases in concentration. For example:



An increase in temperature increases the quantity of heat (a product), thus shifting the reaction to the left.

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