Chemistry 130 Acid and Base equilibria Dr. John F. C. Turner 409 Buehler Hall

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.

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:

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

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.

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.

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.

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

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.

Chemistry 130 Conjugate acid and base strengths There are a variety of strong acids – any material that has a larger pK 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.

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.

Chemistry 130 pK W, pH and pOH The autoprotolysis of water relates the hydronium ion concentration and the hydroxide ion concentration to the autoprotolysis constant of water: This relationship dictates the concentrations of hydroxide and hydronium ion in all aqueous solutions

Chemistry 130 Simple pH and pOH calculations Q:What is the pH of a solution of M solution of HI?

Chemistry 130 Simple pH and pOH calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of M solution of HI?

Chemistry 130 Simple pH and pOH calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of M solution of KOH?

Chemistry 130 Weak acids and bases These calculations are straightforward as HI is a strong acid and KOH is a strong base. For weak acids and bases, which do not fully ionize in solution, we have to use the rules for equilibria with which we are already familiar. We also use which we have defined for the generic acid reaction and base reaction in water.

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of 0.5 M solution of acetic acid? At equilibrium, the new concentrations are:

Chemistry 130 Weak acid/base calculations The acid constant for acetic acid is whereas the autoprotolysis constant for water is The contribution of the self-ionization of water is of the order of to the concentration of hydronium ion. In general, if the acid constant for the weak acid is sufficiently large in comparison to the autoprotolysis constant for water, we can ignore the autoprotolysis of water with respect to the hydrogen ion concentration

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of 0.5 M solution of acetic acid?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of 0.5 M solution of acetic acid? There are two ways of solving this equation 1. use the quadratic formula2. make the assumption that

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, the pH and the concentration of hydroxide ion in a solution of 0.5 M solution of acetic acid?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine? At equilibrium, the new concentrations are:

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine?

Chemistry 130 Weak acid/base calculations Q:Calculate pOH, pH and the concentrations of both hydroxide ion and hydronium in a solution of 1.5 M solution of hydroxylamine?

Chemistry 130 Weak acid/base calculations Given that pH and pOH are related via the relationship then in aqueous solution, only one of these quantities is required for all to be calculable. Similarly, if K A is known for an acid or K B is known for a base, then using the change on the establishment of equilibrium will give the hydronium ion concentration or the hydroxide concentration and so all is known. If we know the pH or pOH, we can also calculate K A or K B for the system, using similar methods.

Chemistry 130 Weak acid/base calculations Q:Calculate pK B for dimethylamine given that a M solution has a pH of

Chemistry 130 Weak acid/base calculations Q:Calculate pK B for dimethylamine given that a M solution has a pH of

Chemistry 130 Weak acid/base calculations Q:Calculate pK B for dimethylamine given that a M solution has a pH of

Chemistry 130 Polyprotic acids A polyprotic acid is one that can ionize more than once. Common examples include Sulfuric acid Phosphoric acid Carbonic acid Note that acids such as acetic acid are not polyprotic

Chemistry 130 Polyprotic acids Each ionization of a polyprotic acid has an associated acid constant. For phosphoric acid Note that each acid constant differs from the one before and, although the first ionization, in this case, is strong, the others are not.

Chemistry 130 Polyprotic acids If the first ionization of a polyprotic acid is described by a large acid constant, then equal concentrations of the hydronium ion and dihydrogenphosphate ons are produced. The equilibrium for the second ionization is and the equilibrium constant is and so the acidity of the second ionization is independent of the initial concentration of phosphoric acid

Chemistry 130 Acid and Base equilibria Dr. John F. C. Turner 409 Buehler Hall

Chemistry 130 Salts of strong and weak acids When a salt is dissolved, the equilibria for the conjugate acid and base are established. Dissolving the salt introduces the conjugate acid or the conjugate base into the solution and the normal equilibria occur Salts of strong acids and strong bases form neutral solutions Salts of strong acids and weak bases form acidic solutions Salts of weak acids and strong bases form basic solutions The pH of solutions of salts of weak acids and weak bases depend on the acid constant of the acid and the base constant of the base

Chemistry 130 Salts of strong and weak acids The reaction that occurs when an anion associated with a weak acid is dissolved in water changes the pH of the solution. This happens because the anion is the conjugate base of the associated acid and the acid-base equilibrium for that acid is established. For nitrous acid, the associated anion is nitrite,

Chemistry 130 Salts of strong and weak acids For nitrous acid, the associated anion is nitrite,

Chemistry 130 Salts of strong and weak acids For nitrous acid, the associated anion is nitrite,

Chemistry 130 Strength of conjugate acids and bases The acid and base strength of a conjugate acid-base pair, such as nitrite-nitrous acid acetic acid-acetate ammonia-ammonium are related by the relationship

Chemistry 130 Strength of conjugate acids and bases This situation occurs because the conjugate base of a weak acid and the conjugate acid of a weak base are both appreciably strong. The acid or base strength of a conjugate acid or base of a strong base or acid is extremely weak and is negligible in most applications. The appreciable strength of a conjugate acid or base and the presence of an equilibrium, because the base or acid is weak means that additions of acid or base to a solution that contains the acid-base conjugate pair will not effect the pH of the solution greatly. These solutions are termed 'buffers'.

Chemistry 130 Buffer solutions A buffer solution is one that contains a conjugate acid-base pair and is used to provide a relatively constant pH in chemical reactions, biological and medical systems and in industrial settings. Because of the presence of the conjugate acid-base, we can write from which we can calculate the pH if we know the concentrations of the acid and conjugate base and the acid constant for the acid.

Chemistry 130 Buffer solutions Given that

Chemistry 130 Buffer solutions For a buffer, the pH is given by which is the Henderson-Hasselbalch equation.

Chemistry 130