1. Chemistry 130 Acid and Base equilibria Dr. John F. C. Turner 409 Buehler Hall jturner@ion.chem.utk.edu

2. Chemistry 130 Acids and bases The Brønsted-Lowry definition of an acid states that any material that produces the hydronium ion is an acid. A Brønsted-Lowry acid is a proton donor: The hydronium ion, has the same structure as ammonia. It is pyramidal and has one lone pair on O.

3. Chemistry 130 Acids and bases The Brønsted-Lowry definition of a base states that any material that accepts a proton is a base. A Brønsted-Lowry base is a proton acceptor: In aqueous solution, a base forms the hydroxide ion:

4. Chemistry 130 Conjugate acids and bases For any acid-base reaction, the original acid and base are complemented by the conjugate acid and conjugate base: On the LHS, Hydroxide ion is the proton acceptor Ammonium ion is the proton donor On the RHS, Water is the proton donor Ammonia is the proton acceptor Water and hydroxide ion are conjugate acid and base Ammonia and ammonium ion are also conjugate acid and base

5. Chemistry 130 Conjugate acids and bases The anion of every acid is the conjugate base of that acid The cation of every base is the conjugate acid of that base Acid-base conjugates exist due to the dynamic equilibrium that exists in solution.

6. Chemistry 130 Acid and base constants We use equilibrium constants to define the position of equilibrium and to reflect the dynamic nature of the system. For an acid, we define Water is not included as the change in concentration of water is negligible (water is ~55 M when pure) for a dilute solution.

7. Chemistry 130 Acid and base constants We use equilibrium constants to define the position of equilibrium and to reflect the dynamic nature of the system. For a base, we define Again, water is not included as the change in concentration of water is negligible (water is ~55 M when pure) for a dilute solution.

8. Chemistry 130 Acid and base constants We also use the logarithm of the acid or base constant as an indicator of acid or base strength: Note that the logarithm used here is to base 10, not the natural log

9. Chemistry 130 Conjugate acid and base strengths The dynamic nature of the equilibrium between the conjugate acid-base pair means that a strong acid will have a weak conjugate base A strong base will have a weak conjugate acid. In both these cases, 'strong' and 'weak' are defined qualitatively the position of the acid or base equilibrium. A strong acid forms almost exclusively the hydronium ion and the concentration of the undissociated acid is negligible; the conjugate base must be weak.

10. Chemistry 130 Conjugate acid and base strengths There are a variety of strong acids – any material that has a larger pH A than the hydronium ion will form the hydronium ion in solution. Any material with an acid constant smaller than the hydronium ion will establish a measurable equilibrium between the acid and the dissociated hydronium ion and associated conjugate base – the anion. Water therefore acts as a leveling solvent and restricts the degree of acidity possible in aqueous solution.

11. Chemistry 130 Autoprotolysis of water Water can act as both an acid and a base – it can form the hydronium ion as well as the hydroxide ion in solution. Pure water also undergoes a 'self-equilibrium': This is autoprotolysis or autionization.

12. Chemistry 130