(a) The Nature of Acids and Bases; (b) Acid Strength; (c) The pH Scale; (d) Calculating the pH of Strong Acids Chemistry 142 B Autumn Quarter 2004 J. B. Callis, Instructor Lecture #20

Definition Brønsted-Lowery Acid - A substance that can donate a hydrogen ion, e.g. when chloric acid is dissolved in water: HClO 3 (aq) + H 2 O(l) = H 3 O + (aq) + ClO 3 - (aq) acid base acid base note: the hydrogen ion is symbolized as the hydronium ion to emphasize that it is hydrated. note: acids and bases occur as conjugate acid-base pairs, e.g. HClO 3 and ClO 3 - form such a pair.

Definition Brønsted-Lowry Base - A substance that can accept a hydrogen ion, e.g. when ammonia is dissolved in water: H 2 O(l) + NH 3 (aq) = NH 4 + (aq) + OH - (aq) acid base conjugate conjugate acid base note: The Brønsted-Lowry scheme is not limited to aqueous solutions: HCl(in NH 3 ) + NH 3 (l) = NH 4 + (in NH 3 ) + Cl - (in NH 3 ) acid base conjugate conjugate acid base

Problem 20-1: Identifying Acids and Conjugate Bases Which of the following can act as Bronsted-Lowry acids? Give the formula of the conjugate Bronsted-Lowry base for each of them. (a) Cl - (b) HSO 4 - (c) NH 4 + (d) NH 3 (e) H 2 O

Amphoteric Molecules - Can Function as either acids or bases, depending on reaction conditions. Water - (a) acts as an acid in donating a hydrogen ion to NH 3, (b) acts as a base in accepting a hydrogen ion from HClO 3 Hydrogen phosphate ion can act as an acid: HPO 4 2- (aq) + H 2 O(l) = H 3 O + (aq) + PO 4 3- (aq) or as a base: HPO 4 2- (aq) + H 2 O(l) = H 2 PO 4 - (aq) + OH - (aq)

Water Dissociates Slightly: H 2 O(l) + H 2 O(l) = H 3 O + (aq) + OH - (aq) acid base acid base We can express this process of autoionization in terms of an equilibrium: K w = [H 3 O + ][OH - ] Where K w = 1.0 x at 25 o C. Note that H 2 O(l) does not enter into the equilibrium. It is easy to show that the concentration of H + and OH - are 1 x M in pure water.

Aqueous Acid Solution: contains an excess of H 3 O + ions over OH - ions. Strong Acid: Ionizes completely in aqueous solution. Example: HCl(aq) + H 2 O(l) -> H 3 O + (aq) + Cl - (aq) The single arrow indicates that the reaction goes to completion. Other strong acids include HBr, HI, H 2 SO 4, HNO 3 and HClO 4 Calculation: What is the concentration of OH - ions when a 0.1 M solution of HCl is made up? [OH - ] = K w /[H 3 O + ] = 1.0 x /0.10 = 1.0 x M

Graphical representation of the behavior of acids of different strengths in aqueous solution. (a)Strong acid. (b)Weak Acid

Aqueous Basic Solution: contains an excess of OH - ions over H 3 O + ions. Strong Base: Ionizes completely in aqueous solution. Example: H 2 O(l) + NH 2 - (aq) -> NH 3 (aq) + OH - (aq) The single arrow indicates that the reaction goes to completion. Calculation: What is the concentration of H 3 O + ions when a 0.1 M solution of NaOH is made up? [H 3 O + ] = Kw/[OH - ] = 1.0 x /0.10 = 1.0 x M

The relationship of acid strength and conjugate base strength for the dissociation reaction HA(aq) + H 2 O(l) = H 3 O + (aq) + A - (aq) Acid Conjugate base

The Definition of pH pH = -log 10 [H 3 O + ] Problem 20-2 : (a) For pure water, [H + ] = 1.0 x M, so pH = (b) The pH of 0.1 M HCl is pH = (c) The pH of 0.1 M NaOH is pH =

Classification of pH Values pH < 7 Acidic Solution pH = 7 Neutral Solution pH > 7 Basic Solution

The pH scale and pH values of some common substances. The pH scale is a compact way to represent solution acidity. It involves base 10 logs (log), not natural logs (ln) The definition of pH in terms of [H + ] neglects any correction for non-ideality of the solutions.

Significant Figures for Logarithms The number of decimal places in the log is equal to the number of significant figures in the original number. Problem 20-3: [H + ] = 3.5 x M pH =

Problem 20-4: Calculating pH The concentration of H 3 O + in a sample of wine is 2.0 x M. Calculate the pH of the wine. Ans:

Answers to Problems in Lecture (a) This ion cannot act as a Bronsted-Lowry acid because it has no hydrogen. (b) An acid. Conjugate base: NH 3 (c) An acid. Conjugate base: NH 2 - (d) An acid. Conjugate base: OH - 2.(a) pH = 7.00 (b) pH 1.00 (c) pH = pH = pH = 3.70