Chemistry 1020—Lecture 18—Notes

We began class by showing the interactive drill web page for practicing interconversions of pH, [H⁺], [OH^-], and pOH. (Remember, pOH is defined as -log[OH^-].

We then went over Test 2 in class.

Acid-Base Reactions

So far we have talked about acids in relation to the formation of H⁺ in aqueous solution, and the relationship between [H⁺] and pH.

Now that we have introduced the pH scale, and related it and acid strength to the molar concentration of hydrogen ions, lets examine some different acids and bases and look at their properties.

First of all, we can distinguish between strong acids and weak acids in terms of the concentrations of hydrogen ions they produce.

Strong acids are substances that completely dissociate into hydrogen ions in aqueous solution. When an acid dissociates completely into H⁺ and its conjugate base, such as HCl:

HCl  ---> H⁺ + Cl^-

then the [H⁺] is simply related to the concentration of HCl in the solution. For example, 0.10 M HCl produces 0.10 M H⁺, and a pH of 1.0

Weak acids, on the other hand, only partly dissociate in aqueous solution. Recall in our demonstration of electrolytes in aqueous solution that acetic acid was a weak electrolyte, indicating that while it formed ions in aqueous solution, the concentration of ions produced was relatively small.

Acetic acid dissociates according to the following reversible reaction:

If you make a solution that is 0.1 M in acetic acid, and measure the pH, you will find the pH is 2.9, which corresponds to a [H⁺] of 0.0013 M. This means that only about one percent of the acetic acid molecules are dissociated.

Nevertheless, the protons on the acetic acid are available for reaction with a base that is stronger than the water molecule, such as in the following reaction, which is an acid-base neutralization:

CH₃COOH + NaOH ® H₂O + CH₃COO^-Na⁺

There are only a few common strong acids:

The rest of the acids we deal with are considered weak acids. Of course acids vary in their degree of dissociation, something that can be treated quantitatively, but which is beyond the scope of this course.