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Ch15. Acids and Bases: A Second Look

Ch15. Acids and Bases: A Second Look. Brady & Senese, 5th Ed. 1. Arrhenius acid is a substance that produces H + (H 3 O + ) in water. Arrhenius base is a substance that produces OH - in water. 4.3. A Br ø nsted acid is a proton donor A Br ø nsted base is a proton acceptor. base.

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Ch15. Acids and Bases: A Second Look

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  1. Ch15. Acids and Bases: A Second Look Brady & Senese, 5th Ed 1

  2. Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water 4.3

  3. A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid acid base conjugatebase conjugateacid base acid 15.1

  4. (a) Formic acid transfers a proton to a water molecule. HCHO2 is the acid and H2O is the base. (b) When a hydronium ion transfers a proton to the CHO2- ion, H3O+ is the acid and formate ion is the base.

  5. Identify the Conjugate Partner for Each 15.1. Brønsted-Lowry acids and bases exchange protons 6

  6. Identify the conjugate pair for each acid/base: Iodic acid Nitrous acid Acetic acid Bisulfite ion Hydrogen sulfate

  7. Your Turn! How many of the following pairs are conjugate pairs: HCN/CN- ii.HCl/Cl- iii. H2S/S2- 1 2 3 None of them are conjugate 15.1. Brønsted-Lowry acids and bases exchange protons 8

  8. An amphoteric substances can act as either an acid or base, also called amphiprotic, and can be either molecules or ions

  9. Your Turn! How many of the following are amphoteric? HC2H3O2 , H2PO42-, HSO4- 1 2 3 None of these 15.1. Brønsted-Lowry acids and bases exchange protons 13

  10. Strong Acid Weak Acid 15.4

  11. ACID-BASE THEORIES Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It is a WEAK ACID HOAc + H2O H3O+ + OAc- WEAK Abase acidSTRONG B Because [H3O+] is small, this must mean 1. H3O+ is a stronger acid than HOAc 2. OAc- is a stronger base than H2O

  12. Identify The Preferred Direction Of The Following H3O+ (aq) + CO32-(aq)↔ HCO-3(aq) + H2O (l) NH4+(aq) + HCO-3(aq) ↔NH3(aq) + H2CO3(aq) Cl- + HCN(aq)↔ HCl(aq) + CN-(aq) H2O HCN H3O+ NH4+ H2CO3 Increasing Acid Strength 15.1. Brønsted-Lowry acids and bases exchange protons 17

  13. 15.4

  14. Your Turn! Which reaction is not written as in the preferred direction. HC2H3O2 + H2AsO4-↔H3AsO4 + C2H3O2- H3AsO4 +H2AsO3- ↔ H2AsO4- + H3AsO3 HC2H3O2 + H2AsO3- ↔C2H3O2- + H3AsO3 All are preferred as written H2O H3AsO3 HC2H3O2 H3AsO4 H3O+ Increasing Acid Strength 15.1. Brønsted-Lowry acids and bases exchange protons 19

  15. Periodic Trends Of Binary Acids (HnX ) As we read left to right in a period, increasing electronegativityof X makes the H-X bond more polar Acid strength increases with increasing polarity As we read top to bottom in a group, the acid strength increases due to increasing bond length of the HX bond due to increased radius of the anion, X 15.2. Strengths of Brønsted acids and bases follow periodic trends 20

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

  17. Learning Check Which is a stronger acid? H2S or H2O CH4 or NH3 HF or HI δ- δ- δ+ δ+ δ- δ+ δ+ δ+ δ+ δ- δ- 15.2. Strengths of Brønsted acids and bases follow periodic trends 22

  18. Oxoacids ( A(O)m(OH)n) Increase in strength as the electronegativity of the central atom, A, increases Increase in strength as the number of oxygen atoms on (hence the oxidation state of) the central atom, A, increases Electrical induction through the central atom weakens strength of the bond to H δ+ δ- δ+ δ- 15.2. Strengths of Brønsted acids and bases follow periodic trends 23

  19. Learning Check Which is a stronger acid? H2SO4 or H3PO4 HNO3 or H3PO4 H2SO4 or H2SO3 HNO3 or HNO2 15.2. Strengths of Brønsted acids and bases follow periodic trends 24

  20. Anions Of Oxoacids Are Basic Oxygen atoms are electron withdrawing, thus the charge on an anion is located on the lone oxygens The more oxygen atoms there are that share the same charge, the less basic is the anion The stronger the base behavior of the anion, the greater the strength of the conjugate acid - - 2 O share the (-) charge 3 O share the (-) charge 15.2. Strengths of Brønsted acids and bases follow periodic trends 25

  21. Your Turn! Which statement about acid strength is false: Binary acids get stronger as the anion gets larger Ternary oxoacids get stronger as the central atom increases in size Ternary acid strength increases with increasing oxidation state None of these are false 15.2. Strengths of Brønsted acids and bases follow periodic trends 26

  22. Rank by Acid Strength • HI HBr HCl HF • HClO4 HClO3 HClO2 HClO • HClO HBrO HIO

  23. Lewis Acid/Base Reactions Lewis acids accept an electron pair to form coordinate covalent bonds Lewis bases donate lone pairs of electron to form coordinate covalent bonds Neutralization is the formation of a coordinate covalent bond between the donor and acceptor 15.3. Lewis acids and bases involve coordinate covalent bonds 28

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

  25. In general, most metal oxides react with water to form bases, and nonmetal oxides react with water to form acids • In Section 5.5 metal oxides were called base anhydrides and nonmetal oxides were called acid anhydrides • When cations dissolve in water, they form species called hydrated ions • Hydrated metal ions tend to be Brønsted acids

  26. For the monohydrate of the metal ion Mn+ the equilibrium can be represented as The metal ion makes the hydrogen on the water more acidic.

  27. The charge density of a cation is its charge divided by its volume • The higher the charge density, the better a cation is at drawing electron density from a O-H bond and the more acidic it is • Within a given period, the cation size increases, and the charge density decreases, from top to bottom • As a result, the most acidic hydrated cations are found at the top of a group • As the cation charge increases, it becomes more acidic

  28. Water undergoes self-ionization or autoionization making it a weak electrolyte • This equilibrium is described by the ion product of water

  29. The Ion Product of Water [H+][OH-] [H2O] = constant Kc = H2O (l) H+(aq) + OH-(aq) [H2O] Kc[H2O] = Kw = [H+][OH-] The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular temperature. Solution Is [H+] = [OH-] neutral At 250C Kw = [H+][OH-] = 1.0 x 10-14 [H+] > [OH-] acidic [H+] < [OH-] basic 15.2

  30. Learning Check Complete the following with the missing data 8.50 15.5. pH is a measure of the acidity of a solution 38

  31. Learning Check Complete the following with the missing data 15.5. pH is a measure of the acidity of a solution 39

  32. What is the [H3O+] in a HCl solution that has a concentration of 5.9E-2M?a) 5.9E-2 M b) 3.0 E-2 Mc) 1.0 E-2 M d) 1.2 E-1 M e) 1.23 M At 250C Kw = [H+][OH-] = 1.0 x 10-14

  33. What is the pH of a solution that has a [OH-] = 1.2×10-3 M? 2.92 11.1 1.20 None of the above 15.5. pH is a measure of the acidity of a solution 41

  34. At 250C Kw = [H+][OH-] = 1.0 x 10-14 What is [OH-] in a Ba(OH)2 solution that has a concentration of 3.3E-2M? a) 1.7 E-2 M b) 3.3 E-2 M c) 6.6 E-2 M d) 1.2 E-1 Me) 0.23 M

  35. Indicators Help Us Estimate pH 15.5. pH is a measure of the acidity of a solution 45

  36. pFUNCTIONS • pX = - log X • pH = - log [H+] • pOH = - log [OH-] • pK = -log K

  37. The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater? pH = -log [H+] 15.3

  38. The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood? 15.3

  39. Strong Acids Ionize 100% in Water As the substances are placed into water, they form H3O+ . The H3O+ formed by the acid suppresses water’s ionization. (if [acid] > 10-7 M) The pH can be calculated from the concentration of H3O+ produced by the strong acid The reaction of strong acids occurs irreversibly, so we show the reaction with a → instead of using a double arrow 15.6. Strong acids and bases are fully dissociated in solution 49

  40. Learning Check What is the pH of 0.1M HCl 15.6. Strong acids and bases are fully dissociated in solution 50

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