Module 8
1. Module 8
1.23. Page 2
Module 8—Acid-Base Equilibrium
Explore
Your work in Lessons 3 and 4 has helped you add to your knowledge of acids and bases. Because of your work to understand the Brønsted-Lowry theory, you probably find it easier to define acids and bases by what they do in a reaction rather than by their empirical properties.
It is often easy to confuse the term strength with concentration when talking about acids and bases. Strength refers to the extent to which a substance ionizes. For instance, strong acids react 100% to produce hydronium ions. Concentration refers to the chemical amount of solute per unit volume of solution. It is possible to have a highly concentrated solution in which a weak acid is the solute.
In Module 7 your exploration of chemical equilibrium involved learning about percent reaction and the equilibrium constant for a system. As you have seen through your studies so far, both values are commonly used to describe the equilibrium position. You may wish to test your memory and write the formulas to calculate percent reaction and the equilibrium constant, and then adapt your calculations for an aqueous acid. How does the similar position of the term [H3O+(aq)] in both relationships account for the ability to use values for percent reaction and the equilibrium constant to describe equilibrium position?
Read
What would the equilibrium law expression and the equilibrium constant for acidic and basic systems look like? Read pages 737–738 in the textbook to find out. In your readings note that the equilibrium law expression for an acid is slightly different that what you may recall from Module 7. What assumption is being made in order to make this modification?
You may find the “Learning Tip” on page 738 useful in helping you distinguish between the function of a value for percent ionization and a value for Ka, the ionization constant for an acid.
Try This
Work through the “Sample problems” and “Communication examples” on pages 738–741 of the textbook to learn how to calculate Ka.
TR 1. Can you explain why an ICE table was used to solve these problems?
Read
In Module 7 you learned that ICE tables are an effective way to represent the changes that occur within a chemical system as the system moves from its initial set-up to equilibrium. You will recall that you only use equilibrium concentrations to calculate the value of the equilibrium constant, whether it be a Kc or a Ka. Completing an ICE table is a good way to sort out the changes that are occurring in a system moving to an equilibrium since, in the action of the forward reaction, reactants will be consumed to make products.
The rows in the ICE table represent the change in the concentration and the equilibrium concentration of each species. The information in these rows are reminders of the chemical processes that occur within the system when components mix and react as the system establishes an equilibrium.
You may notice that, in some cases (e.g., “Sample problem 16.2” and “Communication example 1” on pages 738–739), the concentration of hydronium ions produced as the acid ionizes is quite small. If you look at the first column of the ICE tables in these two textbook examples, you will notice that the “change in concentration” for the acid is so small that the value for the final and initial concentrations are the same. This inaccuracy affects the value for Ka and is the reason that values for Ka, like those that appear in the Chemistry Data Booklet, are often only shown with two significant digits.
As you see in “Communication example 2” on page 739, you cannot always assume that the change in initial concentration of the acid will be small. Therefore, it is a good practice to construct an ICE table when solving these types of problems. Doing so will ensure your calculations have the required accuracy and will demonstrate your complete understanding of the changes that occur as a system develops an equilibrium.