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17 Acids and Bases

17 Acids and Bases. A group of chemical properties is related to acidity. According to these properties, a substance can be called an acid or a base . The word acid comes from Latin acere meaning sour. A base is an alkaline , which is derived from Arabic alqali .

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17 Acids and Bases

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  1. 17 Acids and Bases A group of chemical properties is related to acidity. According to these properties, a substance can be called an acid or a base. The word acid comes from Latin acere meaning sour. A base is an alkaline, which is derived from Arabic alqali. Presence of acids and bases have been recognized by ancient people. Lavoisier named oxygen as the element from which acids are derived. Liebig (German) proposed that an acid contains hydrogen. Many chemical reactions are called acid-basereactions; they are not necessarily neutralizations. The acid-base concept is interesting, especially the way it evolved. Understand acid or base, important properties of substances. 17 Acid and Base

  2. Answer these questions What are acids? What are bases? What are acid-basereactions?What are neutralization reactions?What are the relationships between acids and bases?What is the role of the solvent in acid or base solutions? How does the acid-baseconcept evolve and why?What are Arrhenius acid, Bronsted-Lowry acid and Lewis acids?What is the relationship between conjugateacids and bases? What does pH mean and how is the pH scale related to acidity? What are salts? Understand important properties of substances 17 Acid and Base

  3. Arrhenius acids and bases Sevante Arrhenius proposed that substances exists as ions in solution in his dissertation, which was awarded a fourth class (D) in 1884. He was unable to find a job in his native Sweden. He was awarded the Nobel Prize in 1903 for his electrolytic dissociation theory. The fundamental concept: (Text PHH: 17-1) Acid - any substance which delivers hydrogen ion (H+) to the solution. HA  H++ A¯ Base - any substance which delivers hydroxide ion (OH¯) to the solution. BOH  X+ + OH¯ 17 Acid and Base

  4. Problems with Arrhenius theory Like Dalton’s atomic theory, Arrhenius theory have problems today: Acidity did not show in other solvent. What is the solvent role? Some salts produce acidic or basic solutions, not neutral. Why? Which is the base, NH3 or NH4OH? Is OH¯really the only base? How can H+ be stable? Are proton donated? Some chemists want students to learn Brønsted-Lowry theory of acids and bases before learning Arrhenius theory, because the former is more general. Question the established theory. 17 Acid and Base

  5. Brønsted-Lowry theory of acids and bases An acid is a substance from which a proton can be removed.Acids are proton donors. A base is a substance that can remove a proton from an acid.Bases are proton acceptors.Because of strong desire for protons, bases rip protons off acids. Acid-basereactions are competitions for protons.HCl + H2OH3O+ + Cl¯ HNO3 + H2OH3O+ + NO3¯ HC2H3O2 + H2OH3O+ + C2H3O2¯ H2O + NH3NH4+ + OH¯ conjugate acids and bases 17–2 17 Acid and Base

  6. Problems with B-L theory The theory works very nicely in all protic solvent, but fails to explain acid-base behavior in aprotic solvents and non-solvent situations. A more general concept on acid and base was proposed by G.N. Lewis at about the same time Bronsted-Lowry theory was proposed. Question the established theory. 17 Acid and Base

  7. Evolution of the acid-base concept • year thinker Acid Base acid-base reaction • 1884 Arrhenius ionize ionize H+ + OH¯ = HOHH+OH¯ • Bronsted- ProtonprotonHA + B = HB + A Lowry Donoracceptorconjugation • 1923 Lewis electrophilnucleophilE + Nu = E:Nu 17 Acid and Base

  8. Lewis acids and bases Gilbert Newton Lewis (1875-1946) influential American chemist. His theories include the Lewis dot structure taught in Chem120 and covalent bond theories. Lewis acids are electrophils: H+, Na+, BF3, Lewis bases are nucleophils: NH3, H2O, PH3 Acid base reactions:BF3 + :NH3 F3B:NH3 Lewis at his desk.He generalized an idea. 17 Acid and Base 17–9

  9. HClO4ClO4 ¯H2SO4 HSO4¯HI I¯HBr Br¯HCl Cl¯HNO3 NO3¯H3O+ H2O HSO4¯SO42¯ H2SO3 HSO3 ¯ H3PO4 H2PO4 ¯ HNO2 NO2¯ HF F ¯ HC2H3O2 C2H3O2 ¯ Al(H2O)63+ Al(OH)(H2O)52+ H2CO3 HCO3 ¯ H2S HS ¯ Relativestrengthsofacidsandbases The stronger the acid, the weaker its conjugate base. HClO ClO¯ HBrO BrO¯ NH4+ NH3 HCN CN ¯ HCO3¯ CO32 ¯ H2O2 HO2 HS ¯ S2¯ H2O OH¯ ROH RO¯ 17–4 & 17–5 Table 17–3 17 Acid and Base

  10. Skills regarding strength of acids What are strong acids?What are strong bases? Which is the strongest acid, HF, HCl, HBr, or HI?How about HNO3 and HNO2? H2SO4, H2SO3? How about HClO4, HClO3, HClO2, HClO, and HCl? What is the strongest acid in an aqueous solution?What is leveling effect? Order a given list according to the strength. Interpret reactions as due to strength of acids and bases. Explain strength with related acids and bases. 17 Acid and Base

  11. Molecular structure and acid strength A strong acid loses its proton easily. A strong base holds onto a proton tightly. The more polarized is the H—X bond in an acid, the easier the molecule releases H+ thus a stronger acid. The weaker the H —X bond, the easier the molecule releases H+ thus a stronger acid. The bond strength and the polarity are related to electronegativity and electrostatic interactions – size of the ions. Able to predict acidity from molecular structure. 17–8 17 Acid and Base

  12. Self-ionization of water Water molecules autoionize 2 H2O (l) = H3O+ (aq) + OH¯ (aq) [H3O+] [OH¯]Kc = ———————— [H2O] (= 1000/18 = 55.6) Kw = [H2O] Kc = 1e–14 only at 25oC, it’s T dependent. The ion product, Kw increases as T increases, and its value remains the same in the presence of acid or base. The molecule of life, H2O, and its acidity. 17 Acid and Base 17 – 3

  13. Solutions of strong acids and bases Strong acids and strong basescompletely ionize in their solutions. HCl (aq) + H2O (l)  H3O+ (aq)+ Cl– (aq) KOH (aq)  K+ (aq)+ OH– (aq) In a 0.100 M HCl or HNO3 solution, [H+] = 0.100 Mand [OH–] = 1e–14 / 0.100 = 1e–13 M at 25oC In a 0.100 M NaOH or KOH solution, [OH–] = 0.100 Mand [H+] = 1e–14 / 0.100 = 1e–13 M at 25oC In a 0.010 M Ca(OH)2 solution,[OH–] = 2*0.010 = 0.020 M [H+] = 1e–14 / 0.020 = 5e–13 M at 25oC 17 Acid and Base

  14. Some strong acids and bases Strong acids Hydrogen halides HCl HBr HI Oxyacids of halogens HClO3 HBrO3 HIO3 HClO4 HBrO4 HIO4 Other oxyacids H2SO4 HNO3 Strong Bases Hydroxides of alkali metals NaOH KOH CsOH Hydroxides of alkali earth Ca(OH)2 Sr(OH)2 17 Acid and Base 17 – 4

  15. Neutralization Reactions The neutralization reaction between strong acids and strong bases has the net ionic reaction H3O+ (aq)+ OH– (aq) = 2 H2O The anions are bystander ions. They do not participate the reaction. These ions are I–, Br–, Cl–, NO3 –, HSO4 –, CClO4 –, Na+, K+, Cs+, Ca2+ etc. When dried, the ions left behind in neutralization reactions form salts. 17 Acid and Base

  16. The pH scale Sorensen introduced the pH scale in 1909 using the symbol pH. The p is from the German word potenz, power of (10). pH = – log [H+]; [H+] = 10 –pHpOH = – log [OH–]; [OH–] = 10 –pOHpK = – log [K]; K = 10 –pK pH = 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 [H+] = 0.79 0.63 0.50 0.40 0.32 0.25 0.20 0.16 0.13 (not linear; need not copy, figure out yourself) For aqueous solution; Kw = [H+] [OH–] - log Kw = pH + pOH = 14 only at 25oC 17 Acid and Base 17–3, p–672

  17. pH meter and pH electrodes The pH meter is based on the principle to be discussed in electrochemistry. This topic is also related to the equilibrium constant K and Gibbs free energy, G. 17 Acid and Base

  18. Neutrality In a neutral solution, [H+] = [OH–]. (Different from Fig. 17-5) In an acidic solution, [H+] > [OH–], and a basic solution, [H+] < [OH–]. In a saturated CaO (same as Ca(OH)2) solution, [OH–] = 0.025.Calculate pH, [H+], [Ca2+], and pOH at 25oC. Solution: pOH = – log 0.025 = 1.60 pH = 14.00 – 1.60 = 12.40 only applicable at 25oC [H+] = 10–12.40 = 4.0e–13 M Note that 4.0e-13*0.025 = 1e-14 [Ca2+] = [OH–] / 2= 0.013 M 17 Acid and Base

  19. Answer these questions – review What are acid-basereactions?What are neutralization reactions?What is the role of the solvent in acidity? What does pH mean and how is the pH scale related to acidity? What are salts? 17 Acid and Base

  20. Acid-Base Equilibria Strong acids and bases completely ionize in their aqueous solutions. HCl  H+ + Cl– KOH  K+ + OH– Weak acids and bases ionize but not completely in their solutions. HCH3COO  H+ + CH3COO–acetic acid, vinegar spiritHNic  H+ + Nic–HNic = Niacine, C5NH5COOH, a water-soluble vitamin required by the body for health, growth and reproduction; part of the vitamin B complex. It was first prepared in pure form by oxidizing nicotine using conc. HNO3. Nicotine is a major chemical in tobacco . N N CH3 <= Nicotine Niacine =>Vitamine B3 N COOH 17 Acid and Base

  21. Caffeine CaffeineC8H10N4O2 is a weak base, (pH of 1% soln 6.9) taste bitter smell like tea, a cardiac stimulant, (boost of energy), mild diuretic, addictive, operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. Crystals are hexagonal prisms by sublimation, mp 238°C. Sublimes 178°. Fast sublimation is obtained at 160-165° under 1 mm Hg pressure. Many organic bases are interesting drugs, and their chemistry is fascinating. 17 Acid and Base

  22. Ionization of weak acids Ionization of acetic acid, HCH3COO, HA HA = H+ + A–same as HA + H2O = H3O+ + A– [H+] [A–] A– = CH3COO– (acetate)Ka = ————— Kaacid ionization constant, important for an acid [HA] Ka equilibrium constant with solvent effect The pKa is defined similar to the pH, pKa = – log KaKa = 10 –pKa See Table 17-3 on page 678 17 Acid and Base

  23. Some weak acids and bases Common Weak Acids Formic acid HCOOH Acetic acid CH3COOH Trichloroacetic acid CCl3COOHHydrofluoric HF Hydrocyanic HCN Hydrogen sulfide H2S Water H2O Conjugate acids of weak bases NH4+ Common Weak Bases ammonia NH3 trimethyl ammonia N(CH3)3 pyridine C5H5N ammonium hydroxide NH4OH water H2O HS- ion HS- conjugate bases of weak acids e.g.: HCOO- 17 Acid and Base

  24. Determine Ka and percent ionization Nicotinic acid, HNic, is a monoprotic acid. A solution containing 0.012 M HNic, has a pH of 3.39. What is its Ka? What is the percent of ionization? Solution: HNic = H+ + Nic– 0.012-x x xx = [H+] = 10–3.39 = 4.1e-4 [HNic] = 0.012 – 0.00041 = 0.012 (4.1e-4)2Ka = ————— = 1.4e-5 0.012 Degree of ionization = 0.00041 / 0.012 = 0.034 = 3.4% 17 Acid and Base

  25. Application of Ka The Ka of nicotinic acid, HNic, is 1.4e-5. A solution containing 0.22 M HNic. What is its pH? What is the degree of ionization? Solution: HNic = H+ + Nic– 0.22-x x x x 2Ka = ———— = 1.4e-5 0.22 – x (use approximation, small indeed) x =  (0.22*1.4e-5) = 0.0018pH = – log (0.0018) = 2.76 Degree of ionization = 0.0018 / 0.22 = 0.0079 = 0.79% compare with 3.4% when the solution was 0.012 M 17 Acid and Base

  26. pH of a weak acid The pH of C M acid HA (Ka) solution. Method: HA = H+ + A– C-x x x x 2Ka = ————C – xx 2 + Kax – C Ka = 0 – Ka + Ka2 + 4 C Ka x = ——————————— 2 pH = – log x The pOH of C M base BOH (Kb) solution. Method: BOH = B+ + OH– C-y y y y 2Kb = ————C – yy 2 + Kbx – C Kb = 0 – Kb + Kb2 + 4 C Kb y = ——————————— 2 pOH = -log y 17 Acid and Base

  27. Using the quadratic formula The Ka of nicotinic acid, HNic, is 1.4e-5. A solution containing 0.00100 M HNic. What is its pH? What is the degree of ionization? Solution: HNic = H+ + Nic– 0.001-x x x x2Ka = —————— = 1.4e-5 x2 + 1.4e-5 x – 1.4e-8 = 0 0.00100 – x –1.4e–5 +  (1.4e–5)2 + 4*1.4e-8x = —————————————————— = 0.000111 M 2pH = – log (0.000111) = 3.95 Degree of ionization = 0.000111/ 0.001 = 0.111 = 11.1% x 2 + Kax – C Ka = 0 – Ka + Ka2 + 4 C Ka x = ——————————— 2 Deg.’f ioniz’n 0.22 0.79%0.012 3.4 %0.001 11.1 % 17 Acid and Base

  28. Degree of or percent ionization % ionization The degree or percent of ionization of a weak acid always decreases as its concentration increases, as shown from the table given earlier. Deg.’f ioniz’n 0.22 0.79%0.012 3.4 %0.001 11.1 % Concentration of acid 17 Acid and Base

  29. Polyprotic acids Polyprotic acids such as sulfuric and carbonic acids have more than one hydrogen to donate. H2SO4 = H+ + HSO4–Ka1 very large completely ionized HSO4– = H+ + SO42–Ka2 = 0.012 H2CO3 = H+ + HCO3–Ka1 = 4.3e-7 HCO3– = H+ + CO32–Ka2 = 4.8e-11 Ascorbic acid (vitamin C) is a diprotic acid, abundant in citrus fruit. Others:H2S, H2SO3, H3PO4, H2C2O4 (oxalic acid) … C 17 Acid and Base

  30. Aspartame - nutrasweet A dipeptide methyl ester : L-aspartyl-L-phenylalanine methyl ester C14H18N2O5(molar mass = 294.31) Aspartame has two ionizable protons 1965. Jim Schlatter synthesized it and discovered it sweet leaving no bitter after tast. 17 Acid and Base

  31. Species concentrations of diprotic acids Evaluate concentrations of species in a 0.10 M H2SO4 solution. Solution: H2SO4 = H+ + HSO4– completely ionized (0.1–0.1) 0.10+y 0.10-y HSO4– = H+ + SO42–Ka2 = 0.012 0.10–y 0.10+y y Assume y = [SO42–] (0.10+y) y ————— = 0.012 (0.10-y) [SO42–] = y = 0.01M [H+] = 0.10 + 0.01 = 0.11 M; [HSO4–] = 0.10-0.01 = 0.09 M Y2 + 0.112 y – 0.0012 = 0 - 0.112+0.1122 + + 4*0.0012y = —————————————— = 0.0098 2 If concentration’f H2SO4 = 1.0 M, what doUdo? 17 Acid and Base

  32. Species concentrations of weak diprotic acids Evaluate concentrations of species in a 0.10 M H2S solution. Solution: H2S = H+ + HS–Ka1 = 1.02e-7 (0.10–x) x+y x-y Assume x = [HS–] HS– = H+ + S2–Ka2 = 1.0e-13 x–y x+y y Assume y = [S2–] (x+y) (x-y) (x+y) y ————— = 1.02e-7 ———— = 1.0e-13 (0.10-x) (x-y) [H2S] = 0.10 – x = 0.10 M[HS–] = [H+] = x  y = 1.0e–4 M; [S2–] = y = 1.0e-13 M 0.1>> x >> y: x+ y = x-y = xx = 0.1*1.02e-7 = 1.00e-4y = 1e-13 See Example 16.4 17 Acid and Base

  33. Beware of Misconceptions These equations show dynamic equilibria at the molecular level in a system. These equations are not separate reactions, but they indicate possible combination and dissociation in both directions. H2S = H+ + HS–Ka1 = 1.02e-7 HS–= H+ + S2–Ka2 = 1.0e-13 H2O = H+ + OH-Kw = 1e-14 H2S(g) HS- H2S OH- H2O H+ H+ H+ OH- H2O H2O H2S H2S H+ H2O S2- OH- OH- S2- S2- HS- 17 Acid and Base

  34. Base-ionization constant For a base BOH, BOH = B+ + OH– [B+] [OH–] Kb = ——————— [BOH] The pKb is defined similar to pKa pKb = – log Kb, Kb = 10–pKb Whatever you have learned for weak acids apply to weak bases For acid, HA HA = H+ + A– [H+] [A–] Ka = ————— [HA] The pKa is defined similar to the pH, pKa = – log KaKa = 10 –pKa From slide 3 June 18 17 Acid and Base

  35. Common weak bases Substance Formula KbAmmonia NH3 1.8e-5 aniline C6H5NH2 4.2e-10 dimethylamine (CH3)2NH 5.1e-4 ethylamine C2H5NH2 4.7e-4 Hydrozine N2H4 1.7e-6 Hydroxylamine H2NOH 1.1e-8 methylamine CH3NH2 4.4e-4 Pyridine C5H5N 1.4e-9 Urea NH2CONH2 1.5e-14no need to copy Table 16.2 Many drugs are salts of weak bases, such as AdvilPseudoephedrine HCl Buckley's mixtureDextromethorphan hydrobromide DristanTraxodone HCl 17 Acid and Base

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