Acids and Bases. Chapter 15. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 Acids and Bases Chapter 15 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Have a bitter taste. Bases Feel slippery. Many soaps contain bases. 4.3

3 Arrhenius acid is a substance that releases H + in water Arrhenius base is a substance that releases OH - in water + Na + Cl - 4.3

4 A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid acid base base acid conjugate acid conjugate base 15.1

5 Acid-Base Properties of Water H 2 O (l) H + (aq) + OH - (aq) autoionization of water H O + H O + [ ] + H O H H O - H H H conjugate base acid H 2 O + H 2 O H 3 O + + OH - acid conjugate base 15.2

6 The Ion Product of Water H 2 O (l) H + (aq) + OH - (aq) K c = [H+ ][OH - ] [H 2 O] [H 2 O] = constant K c [H 2 O] = K w = [H + ][OH - ] The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution Is neutral acidic basic 15.2

7 What is the concentration of OH - ions in a HCl solution whose hydrogen ion concentration is 1.3 M? K w = [H + ][OH - ] = 1.0 x [H + ] = 1.3 M [OH - ] = K w [H + ] 1 x = = 7.7 x M Acid or Base? 15.2

8 ph A Measure of Acidity ph = -log [H + ] Solution Is At 25 0 C neutral [H + ] = [OH - ] [H + ] = 1 x 10-7 ph = 7 acidic [H + ] > [OH - ] [H + ] > 1 x 10-7 ph < 7 basic [H + ] < [OH - ] [H + ] < 1 x 10-7 ph > 7 ph [H + ] 15.3

9 poh = -log [OH - ] [H + ][OH - ] = K w = 1.0 x log [H + ] log [OH - ] = ph + poh =

10 [H + ] [H + ] = K w [OH - ] [OH - ] = K w [H + ] [OH - ] ph = - log [H + ] [H + ] = 10 -ph [OH - ] = 10 -poh poh = - log [OH - ] ph ph = pk w -poh poh = pk w -ph poh

11 The ph of rainwater collected in a certain region of the northeastern United States on a particular day was What is the H + ion concentration of the rainwater? ph = -log [H + ] [H + ] = 10 -ph = = 1.5 x 10-5 M The OH - ion concentration of a blood sample is 2.5 x 10-7 M. What is the ph of the blood? ph + poh = poh = -log [OH - ] = -log (2.5 x 10-7 ) = 6.60 ph = poh = =

12 Strong Electrolyte 100% dissociation NaCl (s) H 2 O Na + (aq) + Cl - (aq) Weak Electrolyte not completely dissociated CH 3 COOH CH 3 COO - (aq) + H + (aq) Strong Acids are strong electrolytes HCl (aq) + H 2 O (l) HNO 3 (aq) + H 2 O (l) HClO 4 (aq) + H 2 O (l) H 2 SO 4 (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) H 3 O + (aq) + NO - 3 (aq) H 3 O + (aq) + ClO - 4 (aq) H 3 O + (aq) + HSO - 4 (aq) 15.4

13 Weak Acids are weak electrolytes HF (aq) + H 2 O (l) HNO 2 (aq) + H 2 O (l) HSO - 4 (aq) + H 2 O (l) H 2 O (l) + H 2 O (l) H 3 O + (aq) + F - (aq) H 3 O + (aq) + NO - 2 (aq) H 3 O + (aq) + SO 2-4 (aq) H 3 O + (aq) + OH - (aq) Strong Bases are strong electrolytes NaOH (s) H 2 O H KOH 2 O (s) Ba(OH) 2 (s) Na + (aq) + OH - (aq) K + (aq) + OH - (aq) H 2 O Ba 2+ (aq) + 2OH - (aq) 15.4

14 Weak Bases are weak electrolytes F - (aq) + H 2 O (l) NO 2 - (aq) + H 2 O (l) OH - (aq) + HF (aq) OH - (aq) + HNO 2 (aq) Conjugate acid-base pairs: The conjugate base of a strong acid has no measurable strength. H 3 O + is the strongest acid that can exist in aqueous solution. The OH - ion is the strongest base that can exist in aqeous solution. 15.4

15 15.4

16 Strong Acid Weak Acid 15.4

17 What is the ph of a 2 x 10-3 M HNO 3 solution? HNO 3 is a strong acid 100% dissociation. Start End M HNO 3 (aq) + H 2 O (l) 0.0 M 0.0 M 0.0 M H 3 O + (aq) + NO - 3 (aq) M M Start End ph = -log [H + ] = -log [H 3 O + ] = -log(0.002) = 2.7 What is the ph of a 1.8 x 10-2 M Ba(OH) 2 solution? Ba(OH) 2 is a strong base 100% dissociation M 0.0 M 0.0 M Ba(OH) 2 (s) Ba 2+ (aq) + 2OH - (aq) 0.0 M M M ph = pkw poh = log(0.036) =

18 Weak Acids (HA) and Acid Ionization Constants HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) HA (aq) H + (aq) + A - (aq) K a = [H+ ][A - ] [HA] K a is the acid ionization constant K a weak acid strength 15.5

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20 What is the ph of a 0.5 M HF solution (at 25 0 C)? HF (aq) H + (aq) + F - (aq) K a = [H+ ][F - ] [HF] = 7.1 x 10-4 HF (aq) H + (aq) + F - (aq) Initial (M) Change (M) x +x x Equilibrium (M) x x x K a = x x = 7.1 x 10-4 K a << x 0.50 K a x = 7.1 x 10-4 x 2 = 3.55 x 10-4 x = M [H + ] = [F - ] = M ph = -log [H + ] = 1.72 [HF] = 0.50 x = 0.48 M 15.5

21 When can I use the approximation? K a << x 0.50 When x is less than 5% of the value from which it is subtracted. x = M 0.50 M x 100% = 3.8% Less than 5% Approximation ok. What is the ph of a 0.05 M HF solution (at 25 0 C)? x 2 K a 0.05 = 7.1 x 10-4 x = M M 0.05 M x 100% = 12% More than 5% Approximation not ok. Must solve for x exactly using quadratic equation or method of successive approximation. 15.5

22 Solving weak acid ionization problems: 1. Identify the major species that can affect the ph. In most cases, you can ignore the autoionization of water. Ignore [OH - ] because it is determined by [H + ]. 2. Use ICE to express the equilibrium concentrations in terms of single unknown x. 3. Write K a in terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly. 4. Calculate concentrations of all species and/or ph of the solution. 15.5

23 What is the ph of a M monoprotic acid whose K a is 5.7 x 10-4? Initial (M) Change (M) Equilibrium (M) HA (aq) H + (aq) + A - (aq) x +x +x x x x K a = K a x 2 = 5.7 x x x = 5.7 x 10-4 K a << 1 x 2 = 6.95 x x x = M M M x 100% = 6.8% More than 5% Approximation not ok. 15.5

24 K a = x x = 5.7 x 10-4 x x 6.95 x 10-5 = 0 ax 2 + bx + c =0 x = -b ± b 2 4ac 2a x = x = Initial (M) Change (M) Equilibrium (M) HA (aq) H + (aq) + A - (aq) x +x +x x x x [H + ] = x = M ph = -log[h + ] =

25 percent ionization = Ionized acid concentration at equilibrium Initial concentration of acid x 100% For a monoprotic acid HA Percent ionization = [H + ] [HA] 0 x 100% [HA] 0 = initial concentration 15.5

26 Weak Bases and Base Ionization Constants NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) K b = [NH 4 + ][OH - ] [NH 3 ] K b is the base ionization constant K b weak base strength Solve weak base problems like weak acids except solve for [OH-] instead of [H + ]. 15.6

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28 Ionization Constants of Conjugate Acid-Base Pairs How do K a and K b relate to each other? HA (aq) A - (aq) + H 2 O (l) H + (aq) + A - (aq) OH - (aq) + HA (aq) K a K b H 2 O (l) H + (aq) + OH - (aq) K w K a K b = K w Weak Acid and Its Conjugate Base K a = K w K b K b = K w K a 15.7

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30 Molecular Structure and Acid Strength H X H + + X - The stronger the bond The weaker the acid HF << HCl < HBr < HI 15.9

31 Molecular Structure and Acid Strength - + Z O H Z O - + H + The O-H bond will be more polar and easier to break if: Z is very electronegative or Z is in a high oxidation state 15.9

32 Molecular Structure and Acid Strength 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z O O H O Cl O H O Br O Cl is more electronegative than Br HClO 3 > HBrO

33 Molecular Structure and Acid Strength 2. Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO 4 > HClO 3 > HClO 2 > HClO 15.9

34 Neutral Solutions: Acid-Base Properties of Salts Salts containing an alkali metal or alkaline earth metal ion (except Be 2+ ) and the conjugate base of a strong acid (e.g. Cl -, Br -, and NO 3- ). NaCl (s) H 2 O Na + (aq) + Cl - (aq) Basic Solutions: Salts derived from a strong base and a weak acid. NaCH 3 COOH (s) H 2 O Na + (aq) + CH 3 COO - (aq) CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) 15.10

35 Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) H 2 O NH 4+ (aq) + Cl - (aq) NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 15.10

36 Acid Hydrolysis of Al

37 Acid-Base Properties of Salts Calculate the ph of Salt: Determine the ph of a solution with a H NH 4 Cl (s) 2 O NH 4+ + Cl - Initial (M) Change (M) Equilibrium (M) 1mol NH Cl NH 4 + NH 3 + H x +x +x x x x volume of 275 ml containing 10.0 g of dissolved ammonium chloride mol =0.680 M NH Cl 10.0 g NH Cl mol g NH Cl L 4 Assumption: x K K = therefore NH K = w + w a 4 a Kb K b of NH a 5 NH K = K K a ph ph ph 2 X NH = 2 X X= = H log H log

38 Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion K a for the cation, solution will be neutral 15.10

39 Oxides of the Representative Elements In Their Highest Oxidation States AKA: Anhydrides (Without water) Because: Metal hydroxide heated metal oxide + water CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) N 2 O 5 (g) + H 2 O (l) 2HNO 3 (aq) 15.11

40 Definition of An Acid Arrhenius acid is a substance that releases H + in water A Brønsted acid is a proton donor A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons H + acid + OH - base H O H H + + H N H H H + N H acid H base H 15.12

41 Lewis Acids and Bases F F B F acid + N H F B H F H F base H N H H No protons donated or accepted! 15.12

42 Chemistry In Action: Antacids and the Stomach ph Balance NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g) Mg(OH) 2 (s) + 2HCl (aq) MgCl 2 (aq) + 2H 2 O (l)