CHAPTER 14. Acids and Bases

Transcription

1 CHAPTER 14 Acids and Bases

2 14.1 The Nature of Acids and Bases Arrhenius model Acid supplies H + to an aqueous solution Base supplies OH - to an aqueous solution Bronsted-Lowry model Acid is a proton (H + ) donor; base is a proton (H + ) acceptor More inclusive model (many bases don t have OH - ; ex. NH 3 )

3 General Rxn - Acid in Water HA(aq) + H 2 O(l) <--> A - (aq) + H 3 O + (aq) 2 CONJUGATE acid-base PAIRS: HA (acid), A - (base) H 2 O (base), H 3 O + (acid) Short form: HA(aq) <--> A - (aq) + H + (aq) Eq expression: K a = [H + ][A - ] [HA]

4 [H + ] = [H 3 O + ]

5 Example 14.1 A Conjugate Pairs Write the dissociation reaction for each of the following acids and identify the conjugate acid-base pairs. a) Formic acid (HCOOH) b) Perchloric acid (HClO 4 )

6 Example 14.1 B Equilibrium Expressions for Acid Dissociations Write an equilibrium expression for each of the equations in Example 14.1 A.

7 14.2 Acid Strength Acid strength is indicated by K a. Small K a = eqbm lies to the left = weak acid (mostly molecules at eqbm) Large K a = eqbm lies to the right = strong acid (mostly ions at eqbm) Be sure you have memorized the strong acids

8 ACID STRENGTH HAS NOTHING TO DO WITH MOLARITY!!!!!! THERE IS NO K a VALUE FOR THE STRONG ACIDS!

9 The stronger the acid, the weaker its conjugate base. The stronger the base, the weaker its conjugate acid.

10 Water Is Weird! Amphoteric - can act as acid OR as base. Even when it s pure, water autoionizes: H 2 O(l) + H 2 O(l) <--> OH - (aq) + H 3 O + (aq) If you know hydroxide or hydrogen ion concentration, you can always calculate the other!

11 Kw K w =[H + ][OH - ] At 25 C, K w =1.0 X 10-14

12 Example 14.2 A Relative Acid and Base Strengths Using Table 14.2 (page 628) in your book, and knowing that K w for H 2 O = 1 x at 25 O C, arrange the following acids in order of their strength. Then arrange their conjugate bases in order. HOC 6 H 5, H 2 O, HSO 4-, NH 4+, HNO 3

13 Example 14.2 B Conversion Between [H + ] and [OH - ] At 10 O C, K w for the autoionization of water equals 2.9 x Calculate [H + ] or [OH - ] as necessary under each of the following conditions. Calculate [H + ] if [OH - ] = 9.3 x 10-4 M. Is the solution acidic or basic? Calculate [H + ] and [OH - ] for a neutral solution. Calculate [OH - ] if [H + ] = 6.7 x M. Is the solution acidic or basic?

14 14.3 The ph Scale p means take the logarithm of the concentration, and change the sign. ph = -log[h + ] or -log[h 3 O + ] poh = -log[oh - ] New sig fig rule: # of sig figs in concentration equals the # of decimal places in the logarithm.

15 Example 14.3 A Converting between p and Concentration Calculate the p or [ ] as necessary for each of the following. (Careful with sig figs!) a) Calculate [Cl - ] if pcl = 7.32 b) Calculate pag if [Ag + ] = M c) Calculate pno 3 if [NO 3- ] = 15 M d) Calculate [NH 4+ ] if pnh 4 = 11.87

16 Useful Relationships ph = -log[h + ] or -log[h 3 O + ] poh = -log[oh - ] K w = [H + ][OH - ] = 1.0 x at 25 O C ph + poh = 14 at 25 O C

17 Example 14.3 B Converting among ph, poh, [H + ], and [OH - ] Fill in the blanks. Solution ph poh [H + ] [OH - ] Acid/ base/ neutral? a 6.88 b 8.4 x c 3.11 d 1.0 x 10-7

18 14.4 Calculating the ph of Strong Acid Solutions Recognize that there are always at least two equilibria to consider in an acid-base problem (sometimes more). Assess which equilibrium has the dominant effect on the acid-base character of the solution. FOCUS on that one.

19 Strong acids completely ionize in solution, so they dominate the [H + ] production: [H + ] [HA] O for strong acids EXCEPT in very dilute solutions!

20 Example 14.4 A The ph of a Strong Acid Calculate the ph and [OH - ] of a 5 x 10-3 M HClO 4 solution.

21 Example 14.4 B Practice with Strong Acids A solution is prepared by adding 15.8 g of HCl to enough water to make a total volume of 400. ml. What is the ph of the solution? How much hydrogen ion is contributed by the autoionization of water?

22 Mixture of SA If you have a mixture of strong acids get the total [H + ] by adding the MOLES H + s from EACH strong acid. Calculate molarity from this information with the NEW VOLUME

23 Mixture of SA A solution is prepared by mixing 90.0 ml of 5.00 M HCl with 30.0 ml of 8.00 M HNO 3. Water is added until the final volume is 1.00 L. Calculate [H + ], [OH - ], and the ph of this mixture. (Always assume 25 C if not mentioned in the problem.)

24 14.5 Calculating the ph of Weak Acid Solutions Always think carefully about which eqbm will dominate. (Compare K a values) Have to solve an eqbm expression to find [H + ] (unlike strong-acid problems).

25 Example 14.5 A ph of a Weak Acid Calculate the ph of a M aqueous solution of formic acid, HCOOH (K a = 1.77 x 10-4 ).

26 Example 14.5 B Practice with Weak Acids The value for K a = 7.45 x 10-4 for citric acid (C 6 H 10 O 8 ). Calculate the ph of a M citric acid solution.

27 An example of a mixture of a strong acid and weak acid: What is the ph of a solution containing 0.10 M HCl and 0.10 M HOCl? K a for HOCl is 3.5 X 10-8

28 Example 14.5 C The ph of a Mixture of Weak Acids Calculate the ph of a mixture of 2.00 M formic acid (HCOOH, K a = 1.77 x 10-4 ) 1.50 M hypobromous acid (HOBr, K a = 2.06 x 10-9 ). What are the concentrations of both the hypobromite ion (OBr - ) and hydroxide (OH - ) ion at equilibrium?

29 Percent Dissociation of Acid % = amount dissociated (M) x 100 initial concentration (M) or % = [H + ] X 100 [HA]

30 Example 14.5 D Percent Dissociation Determine the percent dissociation of the formic acid solution in the previous problem.

31 Example 14.5 E K a from Percent Dissociation In a M solution, uric acid (HC 5 H 3 N 4 O 4 ) is 1.6% dissociated. Calculate the value of K a for uric acid.

32 14.6 Bases Strong bases completely dissociate in water: BOH(s) --> B + (aq) + OH - (aq) [OH - ] [BOH] O for strong bases

33 Example 14.6 A ph of a Strong Base Calculate the ph of a solution made by putting 4.63 g of LiOH into water and diluting to a total volume of 400. ml.

34 Weak Bases Weak bases react with water: B(aq) + H 2 O(l) <--> BH + (aq) + OH - (aq) Eqbm position is far left for weak bases Use same method to solve weak base problems as weak acid problems.

35 Many substances are bases but do not have OH (Bronsted-Lowery). These bases are proton acceptors; the OH - comes from the water. NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH - (aq)

36 Example 14.6 B ph of a Weak Base Calculate the ph of a M solution of methylamine, CH 3 NH 2 (K b = 4.38 x 10-4 ).

37 Page 647: Table of K b values A larger K b means a stronger base. A smaller K b means a weaker base. You could use the table of K b values to rank the strength of bases. K b for the conjugate base of a strong acid is almost 0.

38 Don t forget about WATER!! If [OH - ] is close to (or less than) the [OH - ] due to the ionization of water (1 X 10-7 M), you must take water into account. (This only happens when an acid is very, very dilute.)

39 Percent ionization is similar to that of acids (except that you use OH - instead of H + ). % = [OH - ] X 100 [BOH]

40 14.7 Polyprotic Acids Can give off more than one H + Protons get successively harder to remove Generally, first dissociation is dominant source of H + Use the other eqbm expressions to get conc. of all ions in solution

41 H 2 SO 4 is a polyprotic acid (it has 2 H s) The first H dissociating: H 2 SO 4 H + + HSO 4 - K a1 = [H+ ] [HSO 4- ] [H 2 SO 4 ] The second H dissociating: HSO 4 - H + + SO 4-2 K a2 = [H + ] [SO 4-2 ] [HSO 4- ]

42 Example 14.7 A ph of Oxalic Acid Calculate the ph of a 1.40 M H 2 C 2 O 4 (oxalic acid) solution and the equilibrium concentrations of H 2 C 2 O 4, HC 2 O 4-, C 2 O 4 2-, and OH -. K a1 = 6.5 X 10-2 K a2 = 6.1 X 10-5

43 Example 14.7 B Phosphoric Acid Using the information below, calculate the ph, [PO 4 3- ], and [OH - ] in a 6.0 M phosphoric acid (H 3 PO 4 ) solution. K a1 = 7.5 X 10-3 K a2 = 6.2 X 10-8 K a3 = 4.8 X 10-13

44 Be careful with H 2 SO 4 : H 2 SO 4 K a1 = strong (none) HSO 4 - K a2 = 1.2 x 10-2 Below 0.1 M, 2nd dissociation contributes to [H + ]

45 Example 14.7 C Dilute sulfuric acid Calculate the ph of a 1.00 X 10-2 M sulfuric acid solution. (Note the concentration is below 0.1 M as mentioned in the last slide.)

46 14.8 Acid-Base Properties of Salts Key consideration: What are the acid-base properties, and strengths, of each component of a salt? Strong acids and bases have weak conjugates which exhibit no acid-base properties They are NEUTRAL

47 HCl(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) Cl-(aq) is a conjugate base of HCl but it is very, very, very weak. (K b almost 0) so we say it is NEUTRAL

48 14.8 Strong Acids and conjugates HCl Cl - HBr Br - HI I - HNO 3 NO - 3 H 2 SO 4 *HSO - 4 HClO 4 ClO - 4 HIO 4 IO - 4 HClO 3 ClO - 3

49 The conjugates of weak acids and bases will have some effect on the ph Useful equation: K w = K a x K b Given K a, you can calc. K b and figure out ion concentrations, ph, etc.

50 Example 14.8 A Calculating K a or K b Using data given below, calculate K a or K b as required, and write the reaction with water for each of the following aqueous ions: a) NO 2 - (K a for HNO 2 = 4.0 x 10-4) b) F - (K a for HF = 7.2 x 10-4 ) c) C 6 H 5 NH 3 + (K b for aniline, C 6 H 5 NH 2 = 3.8 x ) d) HC 3 H 4 NH 2 + (K b for imidazole, C 3 H 4 NH 2 = 9.0 x 10-8 )

51 What if salt contains both a conjugate of a weak acid and a weak base? Compare the K a and K b to see which one will dominate.

52 Example 14.8 B Predicting Acid- Base Behavior Using data from Tables in your book (pages 663, 685, 689), predict whether each of the following will create an acid, base, or neutral aqueous solution. a) Na 3 PO 4 b) KI c) HC 5 H 5 NCl (pyridinium chloride) d) NH 4 F

53 Example 14.8 C ph of a Salt Calculate the ph of a M NaNO 2 solution. (K a for HNO 2 = 4.0 x 10-4 )

54 14.9 The Effect of Structure on Acid-Base Properties 1. What are the two structural properties that determine if a molecule will act as an acid? Bond strength & Polarity 2. Why is HF weak compared to HCl 3. Explain why increasing the number of oxygens in an H-O-X grouping increases acid strength.

55 Example 14.9 Structure and Properties Given the two acids HOI and HIO 3, and two values for K a, 0.17 and 2 x 10-11, which value goes with which acid?

56 14.10 Acid-Base Properties of Oxides 1. Define acidic oxide. 2. Define basic oxide.

57 Example Acidic and Basic Oxides Determine whether each of the following oxides will give an acidic or basic solution when added to water. a) K 2 O b) SO 3 c) MgO

58 14.11 The Lewis Acid-Base Model Most general acid-base model Lewis acids are electron acceptors. Lewis bases are electron donors. Many Lewis acids don t even contain hydrogen! Ligand: another name for a Lewis base Ex: Cu +2 and NH+ 3

59 Example Lewis Acids and Bases Identify the Lewis acid and base in each of the following reactions. a) Cu 2+ (aq) + 4NH 3 (aq) <--> Cu(NH 3 ) 4 2+ (aq) b) I - (aq) + I 2 (aq) <--> I 3- (aq) c) Fe 3+ (aq) + 6H 2 O(l) <--> Fe(H 2 O) 6 3+ (aq)