Acid-Base Chemistry

This exercise investigates the acid-base behavior of carboxylic acids and amines, forcing students to relate observable changes in physical properties to ionization behavior in acid-base reactions. The operative concept at the physical level is that in comparing water solubility, a neutral carboxylic acid will be less soluble than its carboxylate ion, and a neutral amine will be less soluble than its ammonium ion.  The operative concept at the molecular level is an explicit definition of an acid-base reaction: the only thing that happens in an acid-base reaction is that a hydrogen ion moves from the acid to the base.

In the exercise, students examine benzoic acid and tributylamine in terms of solubility in water, and reaction with red and blue  litmus paper. They place a small scoop of benzoic acid into a tube and note physical properties, then add a dropperful of water to the tube and note solubility. A drop from the tube is placed on red and blue litmus. Then 3 M NaOH is added to the tube dropwise until the solution becomes clear. Finally, they add the same amount of 3 M HCl as they did NaOH and note the separation of solid benzoic acid from solution.

The same approach is repeated with tributylamine, using 5 drops of tributylamine, and 3M HCl followed by 3M NaOH.

In their write-up, students are asked to compare the acid/base behavior of carboxylic acids and amines. They are to write equations for the reaction between benzoic acid and NaOH; benzoate ion and HCl; tributylamine and water; tributylamine and HCl; and tributylammonium ion and NaOH. In each case, they are to name reactants and products, indicate whether each reactant and product is soluble in water, and label all reactants as acid or base..

This exercise is simple and elegant in the way it relates chemical changes and the equations that describe them to observable changes in the materials being examined.

 

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Revised 9/6/06