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Chapter Fifteen 1 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Notes.

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Presentation on theme: "Chapter Fifteen 1 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Notes."— Presentation transcript:

1 Chapter Fifteen 1 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Notes – Acids, Bases, & pH –Worksheet #1 Homework (Write in Planner): –Worksheet #1 due next class

2 Chapter Fifteen 2 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Acids, Bases, and Acid–Base Equilibria Chapter Fifteen

3 3 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Crash Course Introduction 10O5Yhttp://www.youtube.com/watch?v=LS67vS 10O5Y

4 Chapter Fifteen 4 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Review - The Flow Chart What is the pH if the [OH - 1 ]is 3.8 x M? - log[H 3 O +1 ] [H 3 O +1 ] [OH -1 ] 10 -pOH pHpOH - log[OH -1 ] 10 -pH pH + pOH = 14

5 Chapter Fifteen 5 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Sig Figs with pH Significant figures with pH are unique! pH has as many places AFTER THE DECIMAL as significant figures in the concentration. –Ex: If you have 3.4 x M HCl, the pH is –Try it out: The calculator tells you the pH of x M HNO 3 is What would you write as the pH?

6 Chapter Fifteen 6 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Review See if you remember how to do the math. Complete each problem. 1.[H 3 O +1 ] = 3.28 x 10 -4, find pH 2.pH = 2.5, find [OH -1 ] 3.[OH -1 ] = 1.23 x 10 -8, find [H 3 O +1 ] 4.pOH = 2.4, find pH 5.[H 3 O +1 ] = x 10 -3, find pOH

7 Chapter Fifteen 7 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Definitions of Acid & Base

8 Chapter Fifteen 8 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The Arrhenius Theory Arrhenius theory: an acid forms H + (H 3 O +1 ) in water; and a base forms OH – in water. But not all acid–base reactions involve water, and many bases (NH 3, carbonates) do not contain any OH – …

9 Chapter Fifteen 9 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The Brønsted–Lowry Theory Brønsted–Lowry theory defines acids and bases in terms of proton (H + ) transfer. A Brønsted–Lowry acid is a proton donor. A Brønsted–Lowry base is a proton acceptor. –The conjugate base of an acid is the acid minus the proton it has donated. –The conjugate acid of a base is the base plus the accepted proton.

10 Chapter Fifteen 10 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Ionization of HCl HCl acts as an acid by donating H + to H 2 O H 2 O is a base in this reaction because it accepts the H + Conjugate acid of H 2 O Conjugate base of HCl

11 Chapter Fifteen 11 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Ionization of Ammonia

12 Chapter Fifteen 12 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Water Is Amphiprotic H 2 O acts as an acid when it donates H +, forming the conjugate base ___ H 2 O acts as a base when it accepts H +, forming the conjugate acid ___ Amphiprotic: Can act as either an acid or as a base

13 Chapter Fifteen 13 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.1 Identify the Brønsted–Lowry acids and bases and their conjugates in: (a)H 2 S + NH 3 NH HS – (b)OH – + H 2 PO 4 – H 2 O + HPO 4 2–

14 Chapter Fifteen 14 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Let’s See if You’ve Got it… For each acid, write its conjugate base: 1.HC 2 H 3 O 2 2.H 3 PO 4 3.HCO 3 -1

15 Chapter Fifteen 15 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Strength of Acids & Bases

16 Chapter Fifteen 16 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Strength It is important to know whether or not an acid or base is considered strong or weak because it determines how we do calculations. Today we will only do calculations for strong acids because strong acids COMPLETELY DISSOCIATE. That means that their concentration is the SAME AS the [H 3 O +1 ].

17 Chapter Fifteen 17 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Strong Acids The “strong” acids—HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4 —are considered “strong” because they ionize completely in water. The “strong” acids all appear above H 3 O + in Table 15.1 on p The strong acids are leveled to the same strength—to that of H 3 O + —when they are placed in water. MEMORIZE THE 6 MOST COMMON STRONG ACIDS!

18 Chapter Fifteen 18 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Periodic Trends in Acid Strength The greater the tendency for HX (general acid) to transfer a proton to H 2 O, the more the forward reaction is favored and the stronger the acid. A factor that makes it easier for the H + to leave will increase the strength of the acid. Acid strength is inversely proportional to H—X bond-dissociation energy. Weaker H—X bond => stronger acid. Acid strength is directly proportional to anion radius. Larger X radius => stronger acid.

19 Chapter Fifteen 19 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Periodic Trends in Acid Strength

20 Chapter Fifteen 20 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Strength of Carboxylic Acids Carboxylic acids all have the –COOH group in common. Differences in acid strength come from differences in the R group attached to the carboxyl group. In general, the more that electronegative atoms appear in the R group, the stronger is the acid.

21 Chapter Fifteen 21 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Helpful Note Page 1 of the reference sheet that I gave you does a nice job of summarizing how acid strength increases.

22 Chapter Fifteen 22 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Strength of Conjugate Acid–Base Pairs A stronger acid can donate H + more readily than a weaker acid. The stronger an acid, the weaker is its conjugate base. The stronger a base, the weaker is its conjugate acid. An acid–base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid–base pair.

23 Chapter Fifteen 23 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The stronger the acid … … the weaker the conjugate base. And the stronger the base … … the weaker the conjugate acid.

24 Chapter Fifteen 24 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.2 Select the stronger acid in each pair: (a) nitrous acid, HNO 2, and nitric acid, HNO 3 (b) Cl 3 CCOOH and BrCH 2 COOH

25 Chapter Fifteen 25 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry A Little pH Theory

26 Chapter Fifteen 26 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Self-Ionization of Water Even pure water conducts some electricity. This is due to the fact that water self-ionizes: The equilibrium constant for this process is called the ion product of water (K w ). At 25 °C, K w = 1.0 x 10 –14 = [H 3 O + ][OH – ] This equilibrium constant is very important because it applies to all aqueous solutions—acids, bases, salts, and nonelectrolytes—not just to pure water. Why did we leave out water?

27 Chapter Fifteen 27 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The pH Scale Since pH is a logarithmic scale, cola drinks (pH about 2.5) are about ____ times as acidic as tomatoes (pH about 4.5)

28 Chapter Fifteen 28 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.4 By the method suggested in Figure 15.5, a student determines the pH of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated aqueous solution, and obtains a value of What is the molarity of Mg(OH) 2 in its saturated aqueous solution? The suspended, undissolved Mg(OH) 2 (s) does not affect the measurement.

29 Chapter Fifteen 29 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry 15.5 Example Is the solution 1.0 x 10 –8 M HCl acidic, basic, or neutral?

30 Chapter Fifteen 30 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry That’s enough for one day… Worksheet #1 is due next class!

31 Chapter Fifteen 31 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Get out WS#1 to check Our Plan: –Questions on WS#1 –Notes – Weak Acids –Practice from Reference Sheet –Investigation 14 Pre-Lab Homework (Write in Planner): –Worksheet #2 due next class –Investigation 14 Pre-Lab

32 Chapter Fifteen 32 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Quick Review Given a 5.5 x M H 2 SO 4 solution: 1.Is H 2 SO 4 an acid or a base? 2.Is H 2 SO 4 strong or weak? 3.What is the pH of the solution? 4.What is the [OH -1 ]? 5.What is the conjugate base of H 2 SO 4 ?

33 Chapter Fifteen 33 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Quick Review - Answers 1.Acid 2.Strong x M 5.HSO 4 -1

34 Chapter Fifteen 34 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Equilibrium in Solutions of Weak Acids and Bases

35 Chapter Fifteen 35 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry pH of WEAK ACIDS Weak acids do not dissociate completely Some of the acid molecules are converted to hydronium ion, but some are not To solve for hydronium or hydroxide, we use an equilibrium calculation (RICE Table)To solve for hydronium or hydroxide, we use an equilibrium calculation (RICE Table)

36 Chapter Fifteen 36 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry pH of Weak Acids To find pH, you will often have to write the equation for the dissociation of an acid or base, so let’s practice. 1.HNO 2 is added to water 2.NH 3 is added to water 3.Acetic acid is added to water

37 Chapter Fifteen 37 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry K a and K b The equilibrium constant for a Brønsted acid is represented by K a, and that for a base is represented by K b. CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO – (aq) [H 3 O + ][CH 3 COO – ] K a = ––––––––––––––––– [CH 3 COOH] [NH 4 + ][OH – ] K b = ––––––––––––– [NH 3 ] NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH – (aq) Notice that H 2 O is not included in either equilibrium expression.

38 Chapter Fifteen 38 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Acid/Base Strength and Direction of Equilibrium In Table 15.1, HBr lies above CH 3 COOH in the acid column. Since HBr is a stronger acid than CH 3 COOH, the equilibrium for the reaction: lies to the left. Weaker acid Stronger acid We reach the same conclusion by comparing the strengths of the bases (right column of Table 15.1). CH 3 COO – lies below Br – ; CH 3 COO – is the stronger base: Weaker base Stronger base

39 Chapter Fifteen 39 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Equilibrium in Solutions of Weak Acids and Weak Bases These calculations are similar to the equilibrium calculations performed in Chapter An equation is written for the reversible reaction. 2.Data are organized, often in RICE format. 3.Changes that occur in establishing equilibrium are assessed. 4.Simplifying assumptions are examined (the “5% rule”). 5.Equilibrium concentrations, equilibrium constant, etc. are calculated.

40 Chapter Fifteen 40 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The 5% Rule EXPLAIN IT.

41 Chapter Fifteen 41 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry

42 Chapter Fifteen 42 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry pH of WEAK ACIDS What is the pH of a solution of 0.1 M acetic acid if the K a for acetic acid is 1.8 x ? CH 3 COOH + H 2 O ↔ CH 3 COO -1 + H 3 O +1

43 Chapter Fifteen 43 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.6 Ordinary vinegar is approximately 1 M CH 3 COOH and as shown in Figure 15.6, it has a pH of about 2.4. Calculate the expected pH of 1.00 M CH 3 COOH(aq), and show that the calculated and measured pH values are in good agreement.

44 Chapter Fifteen 44 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.7 (application of 5% rule) What is the pH of M CH 2 ClCOOH (aq)?

45 Chapter Fifteen 45 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.8 What is the pH of M NH 3(aq) ?

46 Chapter Fifteen 46 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example 15.9 The pH of a M aqueous solution of dimethylamine is What are the values of K b and pK b ? The ionization equation is (CH 3 ) 2 NH + H 2 O ↔ (CH 3 ) 2 NH OH – K b = ? Dimethylamine Dimethylammonium ion

47 Chapter Fifteen 47 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Polyprotic Acids A monoprotic acid has one ionizable H atom per molecule. A polyprotic acid has more than one ionizable H atom per molecule. –Sulfuric acid, H 2 SO 4 Diprotic –Carbonic acid, H 2 CO 3 Diprotic –Phosphoric acid, H 3 PO 4 Triprotic The protons of a polyprotic acid dissociate in steps, each step having a value of K a. Values of K a decrease successively for a given polyprotic acid. K a1 > K a2 > K a3, etc. Simplifying assumptions may be made in determining the concentration of various species from polyprotic acids.

48 Chapter Fifteen 48 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example Calculate the following concentrations in an aqueous solution that is 5.0 M H 3 PO 4 : (a) [H 3 O + ] (b) [H 2 PO 4 – ] (c) [HPO 4 2– ] (d) [PO 4 3– ]

49 Chapter Fifteen 49 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Ions as Acids and Bases HCl is a strong acid, therefore Cl – is so weakly basic in water that a solution of chloride ions (such as NaCl) is virtually neutral. Acetic acid, CH 3 COOH, is a weak acid, so acetate ion, CH 3 COO –, is significantly basic in water. A solution of sodium acetate (which dissociates completely into sodium and acetate ions in water) is therefore slightly basic: CH 3 COO – + H 2 O ↔ CH 3 COOH + OH –

50 Chapter Fifteen 50 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Carbonate Ion as a Base A carbonate ion accepts a proton from water, leaving behind an OH – and making the solution basic.

51 Chapter Fifteen 51 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Ions as Acids and Bases (cont’d) Salts of strong acids and strong bases form neutral solutions: NaCl, KNO 3 Salts of weak acids and strong bases form basic solutions: KNO 2, NaClO Salts of strong acids and weak bases form acidic solutions: NH 4 NO 3 Salts of weak acids and weak bases form solutions that may be acidic, neutral, or basic; it depends on the relative strengths of the cations and the anions: NH 4 NO 2, CH 3 COONH 4.

52 Chapter Fifteen 52 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Ions as Acids and Bases (cont’d) In order to make quantitative predictions of pH of a salt solution, we need an equilibrium constant for the hydrolysis reaction. The relationship between K a and K b of a conjugate acid–base pair is: K a x K b = K w If instead we have values of pK a or pK b : pK a + pK b = pK w = (at 25 °C)

53 Chapter Fifteen 53 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example Calculate the pH of a solution that is 0.25 M CH 3 COONa(aq). Example What is the molarity of an NH 4 NO 3 (aq) solution that has a pH = 4.80?

54 Chapter Fifteen 54 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The Common Ion Effect Consider a solution of acetic acid. If we add acetate ion as a second solute (i.e., sodium acetate), the pH of the solution increases: LeChâtelier’s principle: What happens to [H 3 O + ] when the equilibrium shifts to the left?

55 Chapter Fifteen 55 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Consider: HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 AND HCl + H 2 O ↔ H 3 O + + Cl M – x + X M 0.100M

56 Chapter Fifteen 56 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry The Common Ion Effect (cont’d) The common ion effect is the suppression of the ionization of a weak acid or a weak base by the presence of a common ion from a strong electrolyte. Acetic acid solution at equilibrium: a few H 3 O + ions and a few CH 3 COO – ions When acetate ion is added, and equilibrium reestablished: more acetate ions, fewer H 3 O + ions

57 Chapter Fifteen 57 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example Calculate the pH of an aqueous solution that is both 1.00 M CH 3 COOH and 1.00 M CH 3 COONa.

58 Chapter Fifteen 58 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example A Calculate the pH of an aqueous solution that is 0.15 M NH 3 and 0.35 M NH 4 NO 3. NH 3 + H 2 O ↔ NH OH -1 K b = 1.8 x 10 -5

59 Chapter Fifteen 59 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Stop! To review the different calculations we’ve done so far in class, let’s try out p. 2 of the reference sheet. Then, do Worksheet #2. It’s due next class.

60 Chapter Fifteen 60 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Worksheet #2 Our Plan: –Questions on WS#2 –Quiz –Investigation 14 Homework (Write in Planner): –Lab Report Due Monday

61 Chapter Fifteen 61 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Lab Report Our Plan: –Quick Review (Weak Acid HO) –Notes Homework (Write in Planner): –Lab Report Due Friday

62 Chapter Fifteen 62 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Buffer Solutions Many industrial and biochemical reactions—especially enzyme-catalyzed reactions—are sensitive to pH. To work with such reactions we often need a solution that maintains a nearly-constant pH. A buffer solution is a solution that changes pH only slightly when small amounts of a strong acid or a strong base are added. A buffer contains significant concentrations of both –a weak acid and its conjugate base, or –a weak base and its conjugate acid.

63 Chapter Fifteen 63 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Buffer Solutions (cont’d) The acid component of the buffer neutralizes small added amounts of OH –, forming the weaker conjugate base which does not affect pH much: HA + OH – ↔ H 2 O + A – The base component neutralizes small added amounts of H 3 O +, forming the weaker conjugate acid which does not affect pH much. A – + H 3 O + ↔ H 2 O + HA Pure water does not buffer at all …

64 Chapter Fifteen 64 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Pure water increases in pH by about 5 pH units when the OH – is added, and decreases by about 5 pH units when the H 3 O + is added. In contrast, the same amounts of OH – and H 3 O + added to a buffer solution barely change the pH.

65 Chapter Fifteen 65 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry An Equation for Buffer Solutions In certain applications, there is a need to repeat the calculations of the pH of buffer solutions many times. This can be done with a single, simple equation, but there are some limitations. The Henderson–Hasselbalch equation: To use this equation, the ratio [conjugate base]/[weak acid] must have a value between 0.10–10 and both concentrations must exceed K a by a factor of 100 or more. [conjugate base] pH = pK a + log –––––––––––––– [weak acid]

66 Chapter Fifteen 66 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Buffer Capacity and Buffer Range There is a limit to the ability of a buffer solution to neutralize added acid or base. This buffer capacity is reached before either buffer component has been consumed. In general, the more concentrated the buffer components in a solution, the more added acid or base the solution can neutralize. As a rule, a buffer is most effective if the concentrations of the buffer acid and its conjugate base are equal or nearly so. Therefore, a buffer is most effective when the desired pH of the buffer is very near pK a of the weak acid of the buffer.

67 Chapter Fifteen 67 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example A buffer solution is 0.24 M NH 3 and 0.20 M NH 4 Cl. (a) What is the pH of this buffer? (b) If mol NaOH is added to L of this solution, what will be the pH?

68 Chapter Fifteen 68 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry A What is the final pH if 0.03 mol HCl is added to L of a buffer solution that is 0.24 M NH 3 and 0.20 M NH 4 Cl?

69 Chapter Fifteen 69 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example What concentration of acetate ion in M CH 3 COOH(aq) produces a buffer solution with pH = 5.00?

70 Chapter Fifteen 70 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry A What concentration of acetic acid in M CH 3 COONa (aq) is needed to produce a buffer solution with pH = 4.50?

71 Chapter Fifteen 71 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Acid–Base Indicators An acid–base indicator is a weak acid or base. The acid form (HA) of the indicator has one color, the conjugate base (A – ) has a different color. One of the “colors” may be colorless. In an acidic solution, [H 3 O + ] is high. Because H 3 O + is a common ion, it suppresses the ionization of the indicator acid, and we see the color of HA. In a basic solution, [OH – ] is high, and it reacts with HA, forming the color of A –. Acid–base indicators are often used for applications in which a precise pH reading isn’t necessary.

72 Chapter Fifteen 72 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Different indicators have different values of K a, so they exhibit color changes at different values of pH …

73 Chapter Fifteen 73 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example A Conceptual Example Explain the series of color changes of thymol blue indicator produced by the actions pictured in Figure 15.14: (a) A few drops of thymol blue are added to HCl(aq). (b) Some sodium acetate is added to solution (a). (c) A small quantity of sodium hydroxide is added to solution (b). (d) An additional, larger quantity of sodium hydroxide is added to solution (c).

74 Chapter Fifteen 74 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Stop Work on Worksheet #3 up to problem 8.

75 Chapter Fifteen 75 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Titration Curve Activity –Notes –Investigation 15 Pre-Lab Homework (Write in Planner): –Finish WS#3 by next Wednesday –Investigation 15 Pre-Lab

76 Chapter Fifteen 76 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Titration Curve Activity With a partner, complete p. 6 & 7 of your worksheet packet.

77 Chapter Fifteen 77 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Neutralization Reactions At the equivalence point in an acid–base titration, the acid and base have been brought together in precise stoichiometric proportions. The endpoint is the point in the titration at which the indicator changes color. Ideally, the indicator is selected so that the endpoint and the equivalence point are very close together. The endpoint and the equivalence point for a neutralization titration can be best matched by plotting a titration curve, a graph of pH versus volume of titrant.

78 Chapter Fifteen 78 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Titration Curve, Strong Acid with Strong Base Bromphenol blue, bromthymol blue, and phenolphthalein all change color at very nearly 20.0 mL At about what volume would we see a color change if we used methyl violet as the indicator?

79 Chapter Fifteen 79 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example Calculate the pH at the following points in the titration of mL of M HCl with M NaOH: H 3 O + + Cl – + Na + + OH –  Na + + Cl – + 2 H 2 O (a) before the addition of any NaOH (b) after the addition of mL of M NaOH (c) after the addition of mL of M NaOH (d) after the addition of mL of M NaOH

80 Chapter Fifteen 80 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Titration Curve, Weak Acid with Strong Base The equivalence-point pH is NOT 7.00 here. Why not?? Bromphenol blue was ok for the strong acid/strong base titration, but it changes color far too early to be useful here.

81 Chapter Fifteen 81 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example Calculate the pH at the following points in the titration of mL of M CH 3 COOH with M NaOH: CH 3 COOH + Na + + OH –  Na + + CH 3 COO – + H 2 O (a) before the addition of any NaOH (b) after the addition of 8.00 mL of M NaOH (c) after the addition of mL of M NaOH (d) after the addition of mL of M NaOH (e) after the addition of mL of M NaOH

82 Chapter Fifteen 82 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry B In a titration similar that that of Example 15.21, mL of M NaOH is titrated with M CH3COOH (aq). Calculate the pH after the addition of the following volumes of the titrant a mL b mL c mL d mL e.Sketch the titration curve

83 Chapter Fifteen 83 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Example A Conceptual Example This titration curve shown in Figure involves 1.0 M solutions of an acid and a base. Identify the type of titration it represents.

84 Chapter Fifteen 84 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Lewis Acids and Bases In organic chemistry, Lewis acids are often called electrophiles (“electron-loving”) and Lewis bases are often called nucleophiles (“nucleus-loving”). There are reactions in nonaqueous solvents, in the gaseous state, and even in the solid state that can be considered acid–base reactions which Brønsted–Lowry theory is not adequate to explain. A Lewis acid is a species that is an electron-pair acceptor and a Lewis base is a species that is an electron-pair donor. Sulfur accepts an electron pair from the oxygen of CaO CaO(s) + SO 2 (g) → CaSO 3 (s)

85 Chapter Fifteen 85 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Investigation 15 Homework (Write in Planner): –Lab Report Due Monday

86 Chapter Fifteen 86 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Lab Report Our Plan: –Review/Practice Homework (Write in Planner): –Homework due next class

87 Chapter Fifteen 87 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Homework Our Plan: –Homework Questions –Quiz –Review Activities Homework (Write in Planner): –Test Next Class

88 Chapter Fifteen 88 Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Today… Turn in: –Nothing Our Plan: –Questions on Review –Test Homework (Write in Planner): –Enjoy your weekend


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