What we learn from Chap 17

Transcription

1 Acids and Bases hapter 17 What we learn from hap This chapter concerns the nature and uses of acids and bases. It is the second in our three-chapter h discussion i about equilibrium, and reinforces the idea of looking at acids and bases as an extension of the core, consistent understanding and application of equilibrium. The chapter opens with the phrase, ome is where the heart is. ome is also where the phosphoric acid...is. That is, our case study is on the practical science of acid- base chemistry. We make the case that acids and bases are used in many parts of the home, even in the construction of the home itself, a fact that we revisit several times in the chapter. Prof. Myeong ee Moon 1 2

2 APTER UTLINE 17.3 I. What Are Acids and Bases? II. Acid Strength A. Strong and Weak Acids B. Why Do Acids ave Different Strengths? III. The p Scale A. Water and The p Scale IV. Determining the p of Acidic Solutions A. Le hatelier s Principle and the Supply of ydroxide Ion in Acid Solutions B. p of Weak Acid Solutions. p of a Mixture of Monoprotic Acids V. Determining the p of Basic Solutions VI. Polyprotic Acids A. Production and Uses of Phosphoric and Sulfuric Acids B. The p of Polyprotic Acids VII. Assessing the Acid-Base Behavior of Salts in Aqueous Solutions A. The Relationship of Ka to Kb B. Acid-Base Properties of Amino Acids VIII.Anhydrides in Aqueous Solutions Prof. Myeong ee Moon ommon hemicals (Acid & Bases) Prof. Myeong ee Moon 1 4

3 The Use of Acids & Bases 17.5 Prof. Myeong ee Moon What are Acids & Bases 17.6 Arrhenius Acids and Bases Brønsted-Lowry Acid and Bases Lewis Acid and Bases Prof. Myeong ee Moon 1 6

4 Arrhenius Acids and Bases 17.7 An acid produces hydrogen ions in solution. l(aq) + (aq) + l (aq) A base produces hydroxide ions in solution. Na(aq) Na + (aq) + (aq) Prof. Myeong ee Moon 1 7 A shell of 21 water molecules 17.8 Prof. Myeong ee Moon 1 8

5 Brønsted-Lowry Acid and Bases 17.9 An acid is a proton donor. A base is an proton acceptor hydronium ion l l : + : 3 2 a conjugate acid-base pair. Prof. Myeong ee Moon 1 9 Brønsted-Lowry Reaction Prof. Myeong ee Moon 1 10

6 Lewis Acids and Bases An acid accepts a previously non-bonded pair of electrons (a lone pair) to form a coordinate covalent bond. A base donates a lone pair of electrons to form a coordinate covalent bond. B 3 (g) +:N 3 (g) B 3 N 3 a Lewis acid a Lewis Base Prof. Myeong ee Moon 1 11 Acid-Base Properties Prof. Myeong ee Moon 1 12

7 17.2 Acid Strength K a is the acid dissociation constant Strong acids Dissociate essentially completely Equilibrium is product favored ave a very large K a Weak acids Partially dissociate aveasmallk a a Equilibrium is reactant favored Prof. Myeong ee Moon 1 13 Acid Strength Prof. Myeong ee Moon 1 14

8 Acid Dissociation o onstant t (l) + (aq) (aq) K a Prof. Myeong ee Moon N 3 - Prof. Myeong ee Moon 1 16

9 Acid Strength Prof. Myeong ee Moon 1 17 Structure and Acid Strength Binary Acids : Polarity of the -X bond (size) The larger X (lower electron density) is, the stronger the acid F < l < Br < I igher electron density stronger bond between and atom Prof. Myeong ee Moon 1 18

10 Polarity (ont.) I Br l F Bond length Bond energy > > > 91.7 pm 297 < 368 < 431 < 569 kj/mol Acid strength 10 9 > 10 8 > 1.3x10 6 >> 6.6x F [F 3 ] F ion pair free ions -bonding Prof. Myeong ee Moon 1 19 Structure and Acid Strength Acid strength increases with electronegativity 2 < F Prof. Myeong ee Moon 1 20

11 Structure and Acid Strength Y- oxoacids Acid strength increases with electronegativity of Y I < Br < l Acid strength increases with number of oxygens (higher oxidation state) ) Br<Br 2 <Br 3 <Br 4 Prof. Myeong ee Moon 1 21 oxidation state (ont.) S S K a 10 3 K a =1.3x S - + S Prof. Myeong ee Moon 1 22

12 - Strengths of rganic Acids acetic acid ethanol K a = 1.8x10-5 K a =1.3x10-16 Prof. Myeong ee Moon 1 23 rganic Acids (ont.) Focus on the Anions Formed Prof. Myeong ee Moon 1 24

13 rganic Acids (ont.) K a = 1.8x K a = 1.3x Prof. Myeong ee Moon 1 25 rganic Acids (ont.) K a = 1.8x K a = 1.4x10-3 l - Prof. Myeong ee Moon 1 26

14 Ion Product of Water In any aqueous solution, there is an equilibrium between () ions and - ions. +(l) (l) 2 3 (aq) (aq) K W = [ 3 + ] [ ] = at 25 : (ion product const. of water) Prof. Myeong ee Moon p Scale Solution is p = -log [+] At 25 o neutral [ + ] = [ - ] [ + ] = p = 7 acidic basic [ + ] > [ - ] [ + ] > p < 7 [ + ] < [ - ] [ + ] < p > 7 [ + ] = 10 -p Prof. Myeong ee Moon 1 28

15 p Prof. Myeong ee Moon p and p K = + - w [ ][ ] pk w = p + p p+p=1400 p p = -log[ - ] [ - ] = 10 -p Prof. Myeong ee Moon 1 30

16 Sample Problem The p of orange juice is about 4.2. What is p, [ + ] and the [ - ]? p = 14.0 p = = [ + ] = 10 -p = = [ - ] = 10 -p = = Prof. Myeong ee Moon p of Strong Acid alculate the p of 0.15 M solution of l. Strong acid are completely dissociated, therefore, [ + ] = [l] p = -log [ + ]= -log(0.15) = 0.82 Prof. Myeong ee Moon 1 32

17 p of a Weak Acid Weak acids partially dissociate. i Let s abbreviate 3 as Ac Two important equilibria i are: Ac(aq) + (aq) + Ac - (aq) K a = (l) + (aq) + - (aq) K w = ( ) ( q) ( q) w K a >>K w hence the dissociation of Ac is the major contributor to the [ + ] in solution. Prof. Myeong ee Moon 1 33 p of a Weak Acid alculate the p of a 0.15M solution of Ac. Ac + + Ac x x x K a - Ac Ac -5 x x 5 x x x assume that x is small compared to concentration of the acid. x x = M -3 p=-log Prof. Myeong ee Moon 1 34

18 p of a Weak Acid Applying the 5% rule: x 100% 5% M M 100% = 1.1% 5% 015M 0.15 If the 5% rule is not satisfied, the quadratic equation must be used. Prof. Myeong ee Moon p of a Strong Base alculate the p of a 0.25 M K solution. Strong bases are completely dissociated, therefore, [ - ] = [K] p = -log [ - ] =-log(0.25) = 0.60 p = = Prof. Myeong ee Moon 1 36

19 p of a Weak Base alculate the p of a 0.25 M N 2 solution. The two important equilibria are: N 2 (aq) + 2 (l) N 3 + (aq) + - (aq) K b = (l) + (aq) + - (aq) K w = K b >>K w hence the hydrolysis of N 2 is the major contributor to the [ - ]. Prof. Myeong ee Moon 1 37 p of a Weak Base N 2 (aq) + 2 (l) N 3 + (aq) + - (aq) 0.25 M-x x x K = b + - [N3 ][ ] [N ] 2 x x x x x assume that x is small compared to the concentration of the base xx x p log p Prof. Myeong ee Moon 1 38

20 p of a Weak Base hecking with the 5% rule: M 100% = 0.021% 5% 025M 0.25 Prof. Myeong ee Moon 1 39 Strengths of Amines as Bases N 2 N 2 N 2 methylamine ethylamine propylamine pk b = pk b = pk b = Prof. Myeong ee Moon 1 40

21 17.6 Polyprotic Acids Polyprotic acids have more than one ionizable hydrogen. 3 P P K = a1 2 P 4 P K a2 = P P K a3 = Prof. Myeong ee Moon 1 41 Polyprotic Acids Prof. Myeong ee Moon 1 42

22 17.7 Acid-Base Behavior of Aqueous Salt Solutions Salt : cation ( + ) + anion ( - ) Aqueous salt solutions can be acidic, basic or neutral. The nature of the cation and the anion determine the p of the solution Prof. Myeong ee Moon ations Spectator(neutral) : derived from strong bases. Li +, Na +, K + ; a 2+, Sr 2+, Ba 2+ Acidic: all other cations, including those of the transition metals. Anions Spectator(neutral) t t : derived d from strong acids l -, Br -, I -, N - 3 Basic: anions derived from weak acids, such as F-, N - 2 Prof. Myeong ee Moon 1 44

23 Acid-Base Behavior of Aqueous Salt Solutions If cation is acidic, c, anion basic, compare Ka, Kb values. N 4 F: K a = , K b = ; acidic Prof. Myeong ee Moon 1 45 The Relationship of K a to K b A(aq) + (aq) + A - (aq) K a A - (aq) + 2 (l) A(aq) + - (aq) K b 2 (l) + (aq)+( - (aq) K w K w = K a K b K a K K w b K b K K w a Prof. Myeong ee Moon 1 46

24 Acid-Base Behavior of Aqueous Salt Solutions Neutral solutions Salts containing an alkali or alkaline earth metal and the conjugate base of a strong acid. Nal(s) + 2 (l) Na + (aq) + l - (aq) Basic solutions Salts made from strong base and a weak acid. 3 Na(s) Na + (aq) (aq) 3 - (aq) + 2 (l) 3 (aq) + - (aq) Prof. Myeong ee Moon 1 47 Acid-Base Behavior of Aqueous Salt Solutions Acidic Solutions Salts made from a strong acid and a weak base. N 4 l(s) N 4+ (aq) + l - (aq) N (aq) N 3 (aq) (aq) Prof. Myeong ee Moon 1 48

25 Anhydrides in Aqueous Solution Basic Anhydride A binary compound formed between metals with very low electronegativity and oxygen. Na 2 (s) + 2 (l) 2Na(aq) Acid Anhydride A binary compound formed between nonmetals and oxygen. 2 (g) + 2 (l) 2 3 (aq) Prof. Myeong ee Moon 1 49 Problems ,24,32, 50,64,84,97 Prof. Myeong ee Moon 1 50