In the Brønsted-Lowry system, a Brønsted-Lowry acid is a species that donates H + and a Brønsted-Lowry base is a species that accepts H +.

Transcription

1 16.1 Acids and Bases: A Brief Review Arrhenius concept of acids and bases: an acid increases [H + ] and a base increases [OH ] BrønstedLowry Acids and Bases In the BrønstedLowry system, a BrønstedLowry acid is a species that donates H + and a BrønstedLowry base is a species that accepts H +. Therefore a BrønstedLowry base does not need to contain OH. NH 3 is a BrønstedLowry base but not an Arrhenius base. Consider NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq): H 2 O donates a proton to ammonia. water is acting as an acid. NH 3 accepts a proton from water. ammonia is acting as a base. Amphoteric substances can behave as acids and bases. Thus water is an example of an amphoteric species. Conjugate AcidBase Pairs Whatever is left of the acid after the proton is donated is called its conjugate base. Similarly, a conjugate acid is formed by adding a proton to the base. 1

2 Sample Exercise 16.1 (p. 675) a) What is the conjugate base of each of the following acids: HClO 4 H 2 S PH 4 + HCO 3? b) What is the conjugate acid of each of the following bases? CN SO 4 2 H 2 O HCO 3 Practice Exercise 1 (16.1) Consider the following equilibrium reaction: HSO 4 (aq) + OH 2 (aq) SO 4 (aq) + H 2 O (l) Which substances are acting as acids in the reaction? a) HSO 4 and OH b) HSO 4 and H 2 O c) OH 2 and SO 4 2 d) SO 4 and H 2 O Practice Exercise 2 (16.1) Write the formula for the conjugate acid of each of the following: HSO 3 F 3 PO 4 CO 2

3 The hydrogen sulfite ion (HSO 3 ) is amphoteric. Sample Exercise 16.2 (p. 676) a) Write an equation for the reaction of HSO 3 with water, in which the ion acts as an acid. b) Write an equation for the reaction of HSO 3 with water, in which the ion acts as a base. In both cases identify the conjugate acidbase pairs. Practice Exercise 1 (16.2) The dihydrogen phosphate ion, H 2 PO 4, is amphiprotic. In which of the following reactions is this ion serving as a base? (i) H 3 O + (aq) + H 2 PO 4 H 3 PO 4(aq) + H 2 O (l) (ii) H 3 O + 2 (aq) + HPO 4 H 2 PO 4 + H 2 O (l) 2 (iii) H 3 PO 4(aq) + HPO 4 2 H 2 PO 4 + H 2 O (l) a) (i) only b) (i) and (ii) c) (i) and (iii) d) (ii) and (iii) e) (i), (ii) and (iii) Practice Exercise 2 (16.2) When lithium oxide (Li 2 O) is dissolved in water, the solution turns basic from the reaction of the oxide ion (O 2 ) with water. Write the reaction that occurs, and identify the conjugate acidbase pairs. 3

4 Relative Strengths of Acids and Bases The stronger an acid is, the weaker its conjugate base will be. 1. Strong acids completely transfer their protons to water. 2. Weak acids only partially dissociate in aqueous solution. 3. Substances with negligible acidity do not transfer a proton to water. In every acidbase reaction, the position of the equilibrium favors the transfer of a proton from the stronger acid to the stronger base. H + is the strongest acid that can exist in equilibrium in aqueous solution. OH is the strongest base that can exist in equilibrium in aqueous solution. Sample Exercise 16.3 (p. 677) For the following protontransfer reaction, use the above figure (Figure 16.4) to predict whether the equilibrium lies to the left (K c < 1) or to the right (K c > 1): HSO 4 (aq) + CO 3 2 (aq) SO 4 2 (aq) + HCO 3 (aq) 4

5 Practice Exercise 1 (16.3) Based on the information in Figure 16.4, place the following equilibria in order from smallest to largest value of K c : (i) CH 3 COOH (aq) + HS (aq) CH 3 COO (aq) + H 2 S (aq) (ii) F (aq) + NH 4 + (aq) (iii) H 2 CO 3 (aq) + Cl (aq) HF (aq) + NH 3(aq) HCO 3 (aq) + HCl (aq) a) (i) < (ii) < (iii) b) (ii) < (i) < (iii) c) (iii) < (i) < (ii) d) (ii) < (iii) < (i) e) (iii) < (ii) < (i) Practice Exercise 2 (16.3) For each of the following reactions, use Figure 16.4 to predict whether the equilibrium lies predominantly to the left or to the right: a) HPO 4 2 (aq) + H 2 O (l) H 2 PO 4 (aq) + OH (aq) b) NH 4 + (aq) + OH (aq) NH 3(aq) + H 2 O (l) 16.3 The Autoionization of Water autoionization of water: 2H 2 O(l) H 3 O + (aq) + OH (aq) The Ion Product of Water equilibrium constant expression for the autoionization of water: 1.0 [ ] = K w = H O OH 3 Sample Exercise 16.4 (p. 679) Calculate the values of [H + ] and [OH ] in a neutral solution at 25 o C. 5

6 Practice Exercise 1 (16.4) In a certain acidic solution at 25 o C, [H + ] is 100 times greater than [OH ]. What is the value for [OH ] for the solution? a) 1.0 x 10 8 M b) 1.0 x 10 7 M c) 1.0 x 10 6 M d) 1.0 x 10 2 M e) 1.0 x 10 9 M Practice Exercise 2 (16.4) Indicate whether solutions with each of the following ion concentrations is neutral, acidic, or basic: a) [H + ] = 4 x 10 9 M b) [OH ] = 1 x 10 7 M c) [OH ] = 7 x M Calculate the concentration of H + (aq) in Sample Exercise 16.5 (p. 680) a) a solution in which [OH ] is M (1.0 x M) b) a solution in which [OH ] is 1.8 x 10 9 M (5.0 x 10 6 M) Assume T = 25 o C. 6

7 Practice Exercise 1 (16.5) A solution has [OH ] = 4.0 x 10 8 M. What is the value of [H + ] for the solution? a) 2.5 x 10 8 M b) 4.0 x 10 8 M c) 2.5 x 10 7 M d) 2.5 x 10 6 M e) 4.0 x 10 6 M Calculate the concentration of OH (aq) in a solution in which a) [H + ] = 2 x 10 6 M Practice Exercise 2 (16.5) b) [H + ] = [OH ] c) [H + ] = 100 x [OH ] 7

8 16.4 The ph Scale In most solutions [H + ] is quite small. We express the [H + ] in terms of ph: ph = log[h + ] = log[h 3 O + ] Sample Exercise 16.6 (p. 681) Calculate the ph values for the two solutions described in Sample Exercise (a) (12.00) b) (5.25) Practice Exercise 1 (16.6) A solution at 25 o C has [OH ] = 6.7 x 10 3 M. What is the ph of the solution? a) 0.83 b) 2.2 c) 2.17 d) e) 12 Practice Exercise 2 (16.6) a) In a sample of lemon juice [H + ] is 3.8 x 10 4 M. What is the ph? (3.42) b) A commonly available windowcleaning solution has a [OH ] of 1.9 x 10 6 M. What is the ph? (8.28) 8

9 Sample Exercise 16.7 (p. 683) A sample of freshly pressed apple juice has a poh of Calculate [H + ]. (1.7 x 10 4 M) Practice Exercise 1 (16.7) A solution at 25 o C has poh = Which of the following statements is or are true? (i) The solution is acidic. (ii) The ph of the solution is (iii) For this solution, [OH ] = M. a) Only one of the statements is true. b) Statements (i) and (ii) are true. c) Statements (i) and (iii) are true. d) Statements (ii) and (iii) are true. e) All three statements are true. Practice Exercise 2 (16.7) A solution formed by dissolving an antacid tablet has a poh of Calculate [H + ]. (6.6 x M) 9

10 Other p Scales poh = log[oh ] log[h + ]+ ( log[oh ]) = ph + poh = logk w = Measuring ph ph meter. acidbase indicators. 10

11 16.5 Strong Acids and Bases Strong Acids The most common strong acids are HCl, HBr, HI, HNO 3, HClO 3, HClO 4, and H 2 SO 4. MEMORIZE THESE! Strong acids are strong electrolytes and ionize completely in solution: HNO 3 (aq) + H 2 O(l) H 3 O + (aq) + NO 3 (aq) or HNO 3 (aq) H + (aq) + NO 3 (aq) In solution the strong acid is usually the only source of H +. Therefore, the ph of a solution of a monoprotic acid may usually be calculated directly from the initial molarity of the acid. What is the ph of a M solution of HClO 4? (1.40) Sample Exercise 16.8 (p. 685) Practice Exercise 1 (16.8) Order the following three solutions from smallest to largest ph. (i) 0.20 M HClO 3 (ii) M HNO 3 (iii) 1.50 M HCl a) (i) < (ii) < (iii) b) (ii) < (i) < (iii) c) (iii) < (i) < (ii) d) (ii) < (iii) < (i) e) (iii) < (ii) < (i) Practice Exercise 2 (16.8) An aqueous solution of HNO 3 has a ph of What is the concentration of the acid? ( M) 11

12 Strong Bases The most common strong bases are ionic hydroxides of the alkali metals or the heavier alkaline earth metals (e.g., NaOH, KOH, and Ca(OH) 2 are all strong bases). MEMORIZE THESE Strong bases are strong electrolytes and dissociate completely in solution. For example: NaOH(aq) Na + (aq) + OH (aq) The poh (and thus the ph) of a strong base may be calculated using the initial molarity of the base. Not all bases contain the OH ion. Ionic metal oxides, hydrides, and nitrides are basic. The oxide, hydride and nitride ions are stronger bases than hydroxide. They are thus able to abstract a proton from water and generate OH. What is the ph of a) a M solution of NaOH? (12.45) Sample Exercise 16.9 (p. 685) b) a M solution of Ca(OH) 2? (11.34) Practice Exercise 1 (16.9) Order the following three solutions from smallest to largest ph: (i) M Ba(OH) 2 (ii) M KOH (iii) Pure water a) (i) < (ii) < (iii) b) (ii) < (i) < (iii) c) (iii) < (i) < (ii) d) (ii) < (iii) < (i) e) (iii) < (ii) < (i) What is the concentration of a solution of a) KOH for which the ph is 11.89? (7.8 x 10 3 M) Practice Exercise 2 (16.9) b) Ca(OH) 2 for which the ph is 11.68? (2.4 x 10 3 M) 12