Understanding Acid-Base Theories: Bronsted-Lowry & More

1. http:\\asadipour.kmu.ac.ir.......44 slides

2. Acids And Bases

3. The Arrhenius Theory • acid – a substance that produces H+ ion when dissolved in water • base - a substance that produces OH- ion when dissolved in water • Limitation of Arrhenius Theory http:\\asadipour.kmu.ac.ir.......44 slides

4. The Brønsted-Lowry Theory • Lowry was an English scientist,. • Bronsted was a Danish scientist • Acid – proton donor • Base – proton acceptor • Conjugate acid and base, HA/A-, differ by one proton. The conjugate acid of a base is the base plus the attached proton and the conjugate base of an acid is the acid minus the proton. (HCl/Cl-) • A substance that can act either as an acid or a base is amphiprotic. (H2PO4-) http:\\asadipour.kmu.ac.ir.......44 slides

5. The BrØnsted-Lowry Theory • Conjugate acid-base pairs are species that differ by a H+. • Some examples: http:\\asadipour.kmu.ac.ir.......44 slides

6. Amphoterism • Look at this reaction in more structural detail. reaction of zinc hydroxide with nitric acid. http:\\asadipour.kmu.ac.ir.......44 slides

7. Zn(OH)2 behaves as an acid in presence of strong bases. Amphoterism • In more structural detail. http:\\asadipour.kmu.ac.ir.......44 slides

8. Strong Acid Dissociation(makes 100 % ions) http:\\asadipour.kmu.ac.ir.......44 slides

9. Weak Acid Dissociation(only partially ionizes) http:\\asadipour.kmu.ac.ir.......44 slides

10. Ionization of Ammonia http:\\asadipour.kmu.ac.ir.......44 slides

11. http:\\asadipour.kmu.ac.ir.......44 slides

12. Strengths Of Conjugate Acid-Base Pairs • 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. • HCl+H2O→H3O++Cl- • CH3COOH + H2O H3O+ + CH3COO-????????? • Ka and Kb values are used to compare the strengths of weak acids and bases. http:\\asadipour.kmu.ac.ir.......44 slides

13. Leveling and differentiating effect • Water has a leveling effect; when the strong acids are dissolved in water, they all completely ionize to the hydronium ion. • HCl+H2O→H3O++Cl- • HClO4+H2O→H3O++ClO4- • Which one is stroger? • Water has a differentiating effect; when the weak acids are dissolved in water, they all partially ionize to the hydronium ion. • CH3COOH + H2O H3O+ + CH3COO- • HCOOH + H2O H3O+ + HCOO- Which one is stroger? Ka http:\\asadipour.kmu.ac.ir.......44 slides

14. Weak conj. bases Strong acid H2O → → Leveling effect → → → Strongest acid Weak bases Weak acids    Differentiating effect Differentiating effect     Strongest base Weak conj. acid ← Strong base http:\\asadipour.kmu.ac.ir.......44 slides

15. CH3COOH  Strongest acid  CH3COOH2+ Differentiating effect    NH3 → Leveling effect → CH3COOH ← Leveling effect ← Strongest base NH3 Differentiatingeffect  NH2- http:\\asadipour.kmu.ac.ir.......44 slides

16. Ionization constant of an acid For a monoprotic weak acid (HA) dissolved in water, HA(aq) + H2O(l)H3O+(aq) + A-(aq) conjugate acid conjugate base acid base http:\\asadipour.kmu.ac.ir.......44 slides

17. Ionization constant of an acid Ionization constant of the acid http:\\asadipour.kmu.ac.ir.......44 slides

18. Acid And Base Ionization Constants weak acid: CH3COOH + H2O  H3O+ + CH3COO- [H3O+][CH3COO-] Acid ionization constant: Ka = [CH3COOH] weak base: NH3 + H2O  NH4+ + OH- [NH4+][OH-] Base ionization constant: Kb = [NH3] Ka and Kb are the measure of the strengths of weak acids and bases. http:\\asadipour.kmu.ac.ir.......44 slides

19. Acids H –X Hydrid acids H – O – E Oxo acids http:\\asadipour.kmu.ac.ir.......44 slides

20. Representative Trends In Strengths of hydrid Acids S≈E & R http:\\asadipour.kmu.ac.ir.......44 slides

21. Relative Strengths Of hydrid Acids H –X in a period: (E↑↑↑↑ & R↓) • The larger the electronegativity difference between H and X, the more easily the proton is removed and the stronger is the acid. EN 0.4 < 0.9 < 1.4 < 1.9 Acid strength CH4 NH3 H2O HF • The strengths of binary acids increase from left to right across a period of the periodic table. http:\\asadipour.kmu.ac.ir.......44 slides

22. Relative Strengths Of Binary Acids H –X in a periodic group: (E↓ & R↑↑↑↑) Anion radius: the larger the anion’s radius, the stronger the acid. Anion radius 136 < 181 < 195 < 216 HF HClHBr HI Acid strength Ka 6.6x10-4 < ~106 < ~108 < ~109 The strength of binary acids increase from top to bottom in a group of the periodic table. http:\\asadipour.kmu.ac.ir.......44 slides

23. Strengths Of Oxoacids S≈E & R H – O - X • E≈ Two factors: - electronegativity of the central atom (E) - number of terminal oxygen atoms (n) • As the electronegativity of the central atom increases. As the number of terminal oxygen atoms increases, the acid strength also increases. E↑S↑ HOClOn n↑S↑ ↑ ↑ http:\\asadipour.kmu.ac.ir.......44 slides

24. Strengths Of Oxoacids As the electronegativity of the central atom (E) increases the acid strength increases. Electronegativity 2.5 < 2.8 < 3.0 HOIHOBrHOCl Acid strength Ka 2.3x10-11 < 2.5x10-9 < 2.9x10-8 As the number of terminal oxygen atoms increases, the acid strength also increases. O O װװ H-O-Cl<H-O-Cl=O<H-O-Cl=O<H-O-Cl=O װO Acid strength 2.9x10-8 < 1.1x10-2 < ~1000 < ~108 http:\\asadipour.kmu.ac.ir.......44 slides

25. Strengths Of Oxoacids Conventional chargeorOxidation number O O װװ H-O-Cl<H-O-Cl=O<H-O-Cl=O<H-O-Cl=O װO Oxidation number Cl +1 +3 +5 +7 Conventional charge Cl 0 +1 +2 +3 -------------------------------------------------------- O OO װװװ H-O-P-H= H-O-P-O-H=H-O-P-O-H ו וו H H OH Oxidation number P +1 +3 +5 Conventional chargeP +1 +1 +1 http:\\asadipour.kmu.ac.ir.......44 slides

26. Strengths Of Carboxylic Acids O R – C – O – H R – CH2 – O - H 1)Carboxylic acids all have the -COOH group 2)Differences in acid strength must come from differences in the R group attached to the carboxyl group. 3)In general, the more that electronegative atoms are attached in the R group, the stronger the acid. http:\\asadipour.kmu.ac.ir.......44 slides

27. Strengths Of Carboxylic Acids O װ R – CH2 – O – H < R – C – O – H In general, the more that electronegative atoms are attached in the R group, the stronger the acid. I-CH2CH2COOH Cl-CH2CH2COOH CH3-CHClCOOH CH3CCl2COOH Ka 8.3x10-5 < 1.0x10-4 < 1.4x10-3 < 8.7x10-3 http:\\asadipour.kmu.ac.ir.......44 slides

28. Strengths Of Amines As Bases BrNH2 < NH3> C6H5NH2 Kb 2.5x10-8 1.8x10-5 7.4x10-10 Electron-withdrawing groups on the ring further diminish the basicity of aromatic amines relative to aniline • Me2NH>MeNH2>Me3N>NH3 Why? http:\\asadipour.kmu.ac.ir.......44 slides

29. N H H H N N N H R H R R R R H R Amine bases: ammonia R= CH3, CH2CH3 aliphatic amine bases N H aromatic amine base H http:\\asadipour.kmu.ac.ir.......44 slides

30. Self-Ionization Of Water • Even the purest of water conducts electricity. This is due to the fact that water self-ionizes, that is, it creates a small amount of H3O+ and OH-. • 2 H2O (l) = H3O+ (aq) + OH¯ (aq) • [H3O+] [OH¯]Keq = ———————— [H2O] (1000/18 = 55.6) • Kw = Keq[H2O] = 1e–14only at 25oC, • it’s T dependent. http:\\asadipour.kmu.ac.ir.......44 slides

31. O H H H Self ionization reaction of water: + - O O + O + H H H H H T↑ Kw↑ http:\\asadipour.kmu.ac.ir.......44 slides

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33. pH and pOH • pH = - log[H3O+] [H3O+] = 10-pH pOH = - log[OH-] [OH-] = 10-pOH • pKw = pH + pOH = 14.00 • neutral solution: [H3O+] = [OH-] = 10 –7 M pH = 7.0 acidic solution: [H3O+] > 10-7 M pH < 7.0 basic solution: [H3O+] < 10-7 M pH > 7.0 http:\\asadipour.kmu.ac.ir.......44 slides

34. The pH Scale http:\\asadipour.kmu.ac.ir.......44 slides

35. An Example The pH of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated aqueous solution, is measured to be 10.52. What is the molarity of Mg(OH)2 in its saturated aqueous solution? http:\\asadipour.kmu.ac.ir.......44 slides

36. The Lewis Theory • Developed in 1923 by G.N. Lewis. • Acids are defined as electron pair acceptors. • Bases are defined as electron pair donors. • Neutralization reactions are accompanied by coordinate covalent bond formation. • In organic chemistry, • Lewis acids are often called electrophiles Lewis bases are often called nucleophiles http:\\asadipour.kmu.ac.ir.......44 slides

37. The Lewis Theory • Look at this reaction in more detail paying attention to the electrons. http:\\asadipour.kmu.ac.ir.......44 slides

38. The Lewis Theory • Again, a more detailed examination keeping our focus on the electrons. http:\\asadipour.kmu.ac.ir.......44 slides

39. The Lewis Theory • A third Lewis example is the autoionization of water. http:\\asadipour.kmu.ac.ir.......44 slides

40. The Lewis Theory • The reaction of sodium fluoride and boron trifluoride provides an example of a reaction that is • - only a Lewis acid-base reaction. • It does not involve H+ at all, thus it cannot be an Arrhenius nor a Brønsted-Lowry acid-base reaction. NaF + BF3  Na+ + BF4- • which is the acid and which is the base؟؟؟؟ • Electrophilic replacement http:\\asadipour.kmu.ac.ir.......44 slides

41. The Lewis Theory http:\\asadipour.kmu.ac.ir.......44 slides

42. Evolution of the acid-base concept year thinker Acid Base acid-base reaction ==================================================== 1884 Arrhenius ionize ionizeH+ + OH¯HOHH+OH¯ • Bronsted- ProtonprotonHA + BHB + ALowry Donoracceptorconjugation 1923 Lewis electrophilnucleophilE + NuE:Nu http:\\asadipour.kmu.ac.ir.......44 slides

43. Arrhenius Brønsted-Lowry Lewis Acid-Base Theories • Arrhenius, Brønsted-Lowry, and Lewis Acid-Base Theories expand on one another. http:\\asadipour.kmu.ac.ir.......44 slides

44. Arrhenius Brønsted-Lowry Lewis Acid-Base Theories • Arrhenius, Brønsted-Lowry, and Lewis Acid-Base Theories expand on one another. http:\\asadipour.kmu.ac.ir.......44 slides