Chapter 16
Chemical Equilibrium

We covered a few of the things in this chapter earlier in the semester: solubility, equilibrium constants, Kw, pH and pOH, and the reversibility of reactions, for example. The major new thing that I want you to learn about is Le Chatelier's principle:
If a stress is applied to a system that is in equilibrium, the system will respond in such a way as to relieve the stress and restore the equilibrium under the new set of conditions.
Concentration
When you add one of the reactants to a system in equilibrium, the reaction shifts to the products.
When you add one of the products, the reaction shifts toward the reactants.

Pressure (or Volume)
If you increase the pressure (or decrease the volume), the reaction shifts toward the side with the fewer total number of moles of gas.
If you decrease the pressure (or increase the volume), the reaction shifts toward the side with the greater number of moles of gas.

Temperature
When you increase the temperature, it's just like adding "heat" as a reactant, the reaction shifts away from the side where the heat is. That is, if it's an endothermic reaction (where heat is a reactant), then the reaction shifts toward the products. In an exothermic reaction, heat is a product, so the reaction would shift back toward the reactants.

Catalysts do not affect the equilibrium state of a reaction. Catalysts only allow a reaction to proceed more quickly toward a more stable equilibrium state. Check out Figure 16.4. The catalyst allows H2 and I2 to combine in a more efficient manner (note the lower, more stable, energy state of the activated complex, or the "hump").

We aren't covering quadratic equations in this class; you'll see plenty of them later.

Know how to write the equilibrium constant expression for a reaction.

Given the value for [H+], {OH-], pH, or pOH, you should be able to calculate any of the other three values.

Know how to calculate the pH of a solution of a strong acid or strong base, given the concentration (molarity) of the solution.

Return to the CHEM 151 Syllabus.
Return to the Schedule.
Go back to the study guide for Chapter 15 or Chapter 1.
Return to the Chem 151 main home page.

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