Academic journal article The Science Teacher

All Things Being Equal: A Guided Inquiry That Helps Students View an Equilibrium System from Particulate Level

Academic journal article The Science Teacher

All Things Being Equal: A Guided Inquiry That Helps Students View an Equilibrium System from Particulate Level

Article excerpt

Equilibrium systems and Le Chatelier's Principle are difficult concepts, leaving students with misconceptions (Orvis and Orvis 2005). Though students may recite that equilibrium is "when the rate of the forward and backward reaction are the same" or may indicate which direction a reaction "shifts" when a stress is placed on it, when asked what is occurring on a particulate level, they can't connect ideas about concentration and reaction rates or that all species in the equation are present in the system at the same time.

This guided inquiry activity was developed to help students "view" an equilibrium system from the particulate level and make connections to their macroscopic observations. Part I helps students observe a physical equilibrium system in which water is transferred between two larger containers. In Part II, students examine what happens to a chemical equilibrium system when various "stresses" are applied.

In Part III, the chemical system from Part II is examined using bingo-type chips to represent the reactants and products. This should help students visualize what happens to the reactant and product species in an equilibrium sys-tem when a stress is applied. Students use the equilibrium constant ([K.sub.eq]) to determine whether a reaction is at equilibrium. Then, by adding or removing "particles," students determine whether more reactants or products than before are made as a result of the changes. This article focuses on a portion of Part III of the activity (Figure 1, pp. 60-61). The full activity is available online (see "On the web").

FIGURE 1

Sample questions and answers from Part III: A particulate view of a chemical equilibrium system. (Answers are in italic.)

In this reaction, [Fe.sup.+3] is represented by an orange chip; [SCN.sup.-] is represented by a colorless chip. The product, [FeSCN.sup.+2], is represented by a red chip (the other side of the orange chip); and a colorless chip on top of it represents the combination of [Fe.sup.+3] and [SCN.sup.-] ions (i.e., the red-colored [FeSCN.sup.2+] ion).

Please note in the table for question #3 the [FeSCN.sup.+2] ion is listed as red. This is because the colors in the table represent what the students would see in a test tube, not what chips looks like.

Color: [Fe.sup.+3] Orange + [SCN.sup.-] Colorless [??] [FeSCN.sup.+2] Red

1. Write the equilibrium expression (K) for the reaction.

[K.sup.eq] = [FeSCN.sup.+2]/[Fe.sup.+3] x [SCB.sup.-]

2. This reaction is at equilibrium when there are two [Fe.sup.+3] (orange chips), six [SCN.sup.-] (colorless chips), and three [FeSCN.sup.+2] (colorless chip connected to a red chip). On the reaction paper, use the chips to represent this reaction at equilibrium.

3. Fill in the table below with the number of each type of ion in your equilibrium "test tube." When checking student work, make sure their "reaction" matches the arrangement shown here.

[[Fe.sup.+3]]     [[SCN.sup.-]]        [[FeSCN.sup.+2]]

Orange particles  Coioriess particles  Red particles

2                 6                    3

4. Using the equilibrium expression (K) and the number of each particle (question #3), calculate the equilibrium constant for this reaction (reduce fraction to lowest terms).

K = 3/(2x6) = 1/4 = 0.25

5. The color of a reaction is determined by which colored particles are most prevalent in the container. (In other words, if there are 10 green particles and 8 blue particles in a container, the reaction will look green-blue). If two colors have the same number of particles, then the color of the reaction is a mixture of both of them (10 red particles and 10 blue particles will look purple). Based on the ions on the paper, what color would we observe if we had moles of these ions in the same ratio as given by the chips? The reaction should look reddish-orange.

6. One stress that can be placed on an equilibrium system is to change the concentration of the reactants or products. …

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