MSJ2e_Ch14_ISM1_June26_p615

MSJ2e_Ch14_ISM1_June26_p615 - Chapter 14: Chemical...

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Chapter 14: Chemical Equilibrium Chapter 14: Chemical Equilibrium Teaching for Conceptual Understanding The most common misconception about equilibrium is that equal amounts of reactant and product are present when equilibrium is achieved. Many students who understand that both reactant and products are present in an equilibrium mixture have the misconception that the system is static. They do not realize that reactants are constantly converting to products to the same extent that products are converting to reactants (and at the same rate). One good way to get this across to them is to remind them that the molecules are not smart enough to stop doing something. Students often confuse equilibrium constant expressions with kinetic rate laws from Chapter 13. Clearly point out that kinetic rate laws are determined experimentally and not by the coefficients of the balanced chemical equation. Equilibrium constant expressions on the other hand are related to the coefficients of the balanced chemical equations. As mentioned in the introduction for Chapter 13, they will sometimes mix up the definitions of k and K. When writing equilibrium constant expression, some students will want to include all reactants and products. They do not understand that pure solids and pure liquids are usually present with essentially constant concentrations (related to their constant density at constant temperature). Any change in quantity does not change their affect on the equilibrium. Concentrations of gases and aqueous solutions, however, can change significantly and must be included in the expression. The concept of Le Chatelier’s principle can be taught using the “scales of justice”. Consider the equilibrium A + B C + D. If more A is added the equilibrium will shift to the right. The initial equilibrium can be thought of as the “scales of justice” balanced. When more A is added the scales will dip on the reactant side. A  +  B C  +   A +  w ith  m o re A  a dded  C  + D To reach equilibrium once again, some of the material on the left scale must be transferred to the right. Similar logic can be used when a reactant or product is removed from the system. Suggestions for Effective Learning A good demonstration of equilibrium involves the transfer of water between two identical fish tanks, battery jars or similar large containers. Set the tanks side-by-side on the table. Start with the tank on the left (reactants) three-fourths full with water (add food coloring for better viewing) and the tank on the right empty. Give two students identical small beakers (cups or ladles), assign one to the reactant tank and the other the product tank, then have them start transferring water from their respective tanks at the same rate. Eventually the tanks will have unequal amounts of water and the volume in each will no longer change no
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MSJ2e_Ch14_ISM1_June26_p615 - Chapter 14: Chemical...

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