L2 - Predicting the Acid-Base Equilibrium

The Brønsted–Lowry concept unfortunately does not include any theoretical explanation about why any given entity attracts a proton more or less strongly. To predict the outcome of any acid–base combination, we must rely on empirical evidence, gained by measuring and recording the relative strengths of acid and base entities. Predictions must be restricted to only those acid–base combinations for which we already have data.  To help us, we can now look for a simple generalization that might allow us to predict the approximate position of equilibrium in an acid–base proton transfer.

Using the Relative Strength of Acid Table Empirical data is necessary to develop general trends about the tendency of acids and bases to transfer protons. Review the “Relative Strengths of Acids and Bases at 298.15 K” table located on pages 8 and 9 of the Chemistry Data Booklet. This table is also available on page 829 of the textbook. A few points about the structure of this table in the Chemistry Data Booklet are as follows: Columns 1 and 2 list the name and chemical formula for acids. Column 3 contains the chemical formula of the conjugate base for the acid shown in each row. Acids are listed by decreasing strength. For example, perchloric acid is a stronger acid than citric acid; as a result, citric acid is listed further down the table. The acids listed above the shaded hydronium ion row are strong acids; acids listed below the hydronium ion row are weak acids. Bases are listed by increasing strength, with the strongest base being hydroxide. The remaining substances listed in the conjugate base column are weak bases and are listed by increasing strength. For example, the perchlorate ion, ClO4–(aq), is the weakest base on the table. Substances listed below the perchlorate ion demonstrate increasing strength as bases.

Predicting Acid-Base Reactions

Earlier in this course you used the “Table of Selected Standard Electrode Potentials,” found on page 7 of the Chemistry Data Booklet, to predict and analyze reduction-oxidation reactions. Can the “Relative Strengths of Acids and Bases at 298.15 K” table (above) be used to predict acid-base reactions in a similar way?

Complete the following lab exercise.  Make sure to pay special attention as you examine the positions of the substances involved in the reactions.  Note the position of the equilibrium for each reaction and any general trends that you notice.

But knowing only the Brønsted–Lowry is not enough, we must go through five steps in order to properly predict the position of the equilibrium.