1. (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

2. 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.

3. 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

4. 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

5. 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)

6. 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 10 -14 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 10 -7 M in pure water.

7. 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 10 -14 /0.10 = 1.0 x 10 -13 M

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

9. 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 10 -14 /0.10 = 1.0 x 10 -13 M

10. 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

11. The Definition of pH pH = -log 10 [H 3 O + ] Problem 20-2 : (a) For pure water, [H + ] = 1.0 x 10 -7 M, so pH = (b) The pH of 0.1 M HCl is pH = (c) The pH of 0.1 M NaOH is pH =

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

13. 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.

14. 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 10 -5 M pH =

15. Problem 20-4: Calculating pH The concentration of H 3 O + in a sample of wine is 2.0 x 10 -4 M. Calculate the pH of the wine. Ans:

16. Answers to Problems in Lecture 20 1. (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 = 13.00 3.pH = 4.56 4. pH = 3.70