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Acids and Bases

Acids and Bases. Chapter 15 15.1-15.10 and 15.12. Br Ø nstead Acids and Br Ø nstead Bases. Recall from chapter 4: Br Ø nstead Acid- a substance capable of donating a proton. Br Ø nstead Base- a substance capable of accepting a proton.

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Acids and Bases

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  1. Acids and Bases Chapter 15 15.1-15.10 and 15.12

  2. BrØnstead Acids and BrØnstead Bases • Recall from chapter 4: • BrØnstead Acid- a substance capable of donating a proton. • BrØnstead Base- a substance capable of accepting a proton. • Conjugate Acid-Base Pair- an acid and its conjugate base or a base and its conjugate acid.

  3. BrØnstead Acids and BrØnstead Bases NH3 + H2O ↔ NH4+ + OH-

  4. BrØnstead Acids and BrØnstead Bases

  5. + H H O O [ ] + + - H H H O H O H H H2O (l) H+(aq) + OH-(aq) H2O + H2O H3O+ + OH- The Acid-Base Properties of Water autoionization of water conjugateacid base conjugatebase acid

  6. The Ion Product of Water • Hydrogen Ion concentration is key • Ion Product Constant (Kw)- the product of the molar concentrations of H+ and OH- ions at a particular temperature. At 250C Kw = [H+][OH-] = 1.0 x 10-14

  7. The Ion Product of Water Solution is [H+] = [OH-] neutral [H+] > [OH-] acidic [H+] < [OH-] basic

  8. = [OH-] = 1 x 10-14 Kw 1.3 [H+] The Ion Product of Water What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 1.3M? Kw = [H+][OH-] = 1.0 x 10-14 [H+]= 1.3M [OH-] [OH-] = 7.7 x 10-15M

  9. The Ion Product of Water

  10. pH a measure of Acidity • pH- the negative logarithm of the hydronium ion concentration (mol/L). pH = -log [H+] pOH = -log [OH-] pH + pOH = 14.00

  11. pH calculations 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+] [H+] = 10-pH [H+]= 10-4.82 [H+]= 1.5 x 10-5M

  12. pH Calculations The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood? pH + pOH = 14.00 pOH = -log [OH-] pOH = -log (2.5 x 10-7) pOH = 6.60 pH = 14.00 – pOH = 14.00 – 6.60 = 7.40 pH = 7.40

  13. Strength of Acids and Bases • Strong Acids- are strong electrolytes that are assumed to ionize completely in water. • Weak Acids- acids that ionize only to a limited extent in water. • Strong Bases- are strong electrolytes that ionize completely in water. • Weak Bases- weak electrolytes.

  14. Strength of Acids and Bases

  15. Conjugate Acid-Base Pairs

  16. Conjugate Acid-Base Pairs • Conjugate acid-base pairs: • The conjugate base of a strong acid has no measurable strength. • H3O+ is the strongest acid that can exist in aqueous solution. • The OH- ion is the strongest base that can exist in aqueous solution.

  17. HA (aq) + H2O (l) H3O+(aq) + A-(aq) HA (aq) H+(aq) + A-(aq) [H+][A-] Ka = [HA] Weak Acids and Acid Ionization Constants • Acid Ionization Constant (Ka)- the equilibrium constant for the ionization of an acid. • The larger the ka, the larger the stronger the acid.

  18. Weak Acids and Acid Ionization Constants HF (aq) H+(aq) + F-(aq) = 7.1 x 10-4 [H+][F-] Ka = [HF] What is the pH of a 0.5M HFsolution (at 250C)? ICE

  19. Weak Acids and Acid Ionization Constants HF (aq) H+(aq) + F-(aq) Initial (M) 0.50 0.00 0.00 Change (M) -x +x +x Equilibrium (M) 0.50 - x x x

  20. Weak Acids and Acid Ionization Constants = 7.1 x 10-4 = 7.1 x 10-4 x2 x2 Ka Ka = 0.50 - x 0.50 [HF] = 0.50 – x = 0.48 M Ka << 1 [H+] = [F-] = 0.019 M 0.50 – x 0.50 pH = -log [H+] = 1.72 x2 = 3.55 x 10-4 x = 0.019 M

  21. 0.019 M x 100% = 3.8% 0.50 M When can we use the Approximation Method? When can I use the approximation Method? When x is less than 5% of the value from which it is subtracted. x = 0.019 Less than 5% Approximation ok.

  22. -b ± b2 – 4ac x = 2a The Quadratic Equation • If approximation method does not work, use the quadratic equation. • Write the ionization expression in terms of x.

  23. Solving Weak Acid Ionization Problems • Solving weak acid ionization problems: • 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+]. • Use ICE to express the equilibrium concentrations in terms of single unknown x. • Write Kain terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly. • Calculate concentrations of all species and/or pH of the solution.

  24. Ionized acid concentration at equilibrium x 100% x 100% Percent ionization = Initial concentration of acid [H+] [HA]0 Percent Ionization • Percent Ionization- measure of the strength of an acid. percent ionization = For a monoprotic acid HA:

  25. Percent Ionization

  26. NH3(aq) + H2O (l) NH4+(aq) + OH-(aq) [NH4+][OH-] Kb = [NH3] Weak Bases and Base Ionization Constants • Base Ionization Constant- the equilibrium constant for the ionization of a base.

  27. Weak Bases and Base Ionization Constants

  28. Kw Kw Ka= Kb= Kb Ka Relationship between Ionization Constants and their Conjugate Bases KaKb = Kw Weak Acid and Its Conjugate Base

  29. Diprotic and Polyprotic Acids • An ionization constant expression can be written for each ionizable proton. • Two or more expressions may be necessary to calculate the concentrations of all species present in the acid solution.

  30. Diprotic and Polyprotic Acids

  31. Diprotic and Polyprotic Acids

  32. H+ + X- H X The stronger the bond Molecular Structure and Strength of Acids • Strength of an acid depends on: • Structure of the acid • Strength of the bond • Polarity of bond The weaker the acid

  33. Molecular Structure and Strength of Acids

  34. H+ + X- H X Hydrohalic Acids • Order of relative acid strength: HF << HCl < HBr < HI

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