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Acid-Base Equilibria

Acid-Base Equilibria. Chapter 16. HA ( aq ) + H 2 O ( l ) H 3 O + ( aq ) + A - ( aq ). HA ( aq ) H + ( aq ) + A - ( aq ). [H + ][A - ]. K a =. [HA]. weak acid strength. K a. Weak Acids (HA) and Acid Ionization Constants.

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Acid-Base Equilibria

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  1. Acid-BaseEquilibria Chapter 16

  2. HA (aq) + H2O (l) H3O+(aq) + A-(aq) HA (aq) H+(aq) + A-(aq) [H+][A-] Ka = [HA] weak acid strength Ka Weak Acids (HA) and Acid Ionization Constants Ka ≡ acid dissociation (ionization) constant

  3. Calculations with Weak Acids • Calculate Ka from pH (Use ICE table) • Calculate pH from Ka and given concentration (ICE table) • Calculate percent ionization = Concentration ionized x 100% Original concentration

  4. HF (aq) H+(aq) + F-(aq) = 6.8 x 10-4 = 6.8 x 10-4 = 6.8 x 10-4 [H+][F-] x2 x2 Ka Ka = Ka = 0.50 - x 0.50 [HF] HF (aq) H+(aq) + F-(aq) What is the pH of a 0.50M HFsolution (at 25°C)? Initial (M) 0.50 0.00 0.00 Change (M) -x +x +x Equilibrium (M) 0.50 - x x x If [HF] > 100 Ka 0.50 – x 0.50 x2 = 3.40 x 10-4 x = 0.018 M pH = -log [H+] = 1.72 [H+] = [F-] = 0.018 M [HF] = 0.50 – x = 0.48 M

  5. = 6.8 x 10-4 0.018 M 0.006 M x2 x 100% = 12% x 100% = 3.6% 0.05 M 0.50 M Ka 0.05 When can I use the approximation? If [HF] > 100 Ka Then 0.50 – x 0.50 Let’s determine error introduced: Less than 5% Approximation ok. x = 0.018 What is the pH of a 0.05M HFsolution (at 25°C)? x = 0.006 M More than 5% Approximation not ok. Must solve for x exactly using quadratic equation...

  6. = 5.7 x 10-3 x2 Ka = 0.122 - x HA (aq) H+(aq) + A-(aq) What is the pH of a 0.122M monoprotic acid whose Ka is 5.7 x 10-3? Initial (M) 0.122 0.00 0.00 Change (M) -x +x +x Equilibrium (M) 0.122 - x x x Is [HF] > 100 Ka? NO!! Approximation not ok.

  7. -b ± b2 – 4ac = 5.7 x 10-3 x = 2a x2 Ka = 0.122 - x HA (aq) H+(aq) + A-(aq) Initial (M) 0.122 0.00 0.00 Change (M) -x +x +x Equilibrium (M) 0.122 - x x x x2 + 5.7 X 10-3x – 6.95 X 10-4 = 0 ax2 + bx + c =0 x = 0.0237 x = - 0.0294 pH = -log[H+] = 1.625 [H+] = x = 0.0237 M

  8. Concentration ionized x 100% x 100% Percent ionization = Original concentration [H+] [HA]0 Percent ionization = For a monoprotic acid HA: [HA]0 = initial concentration Fig 16.9 The more dilute the acid, the greater the percent ionization:

  9. Polyprotic Acids • Have more than one ionizable proton • If difference between the Ka1 and subsequent Ka values > 103, the pH generally depends only on the first dissociation.

  10. NH3(aq) + H2O (l) NH4+(aq) + OH-(aq) [NH4+][OH-] Kb = [NH3] weak base strength Kb Weak Bases and Base Ionization Constants Kb ≡ base ionization constant Solve weak base problems like weak acid except solve for [OHˉ] instead of [H+].

  11. In class exercise What is the pH of a 0.15M solution of NH3 whose Kb is 1.8 x 10-5? Ans. = 11.20

  12. Table 16.4 Some Weak Bases and Aqueous Soln Equilibria

  13. HA (aq) H+(aq) + A-(aq) A-(aq) + H2O (l) OH-(aq) + HA (aq) H2O (l) H+(aq) + OH-(aq) Kw Kw Ka= Kb= Kb Ka Relationship Between Ka and Kb Ka Kb Kw KaKb = Kw Weak Acid and Its Conjugate Base

  14. Kw Kb= Ka Let’s say we need Kb for the acetate ion, CH3COOˉ. CH3COOˉ + H2O ⇌ CH3COOH + OHˉ From a Table of Ionization Constants of Some Weak Acids: Acetic acid CH3COOH Ka = 1.8 X 10ˉ5

  15. Table 16.5 Some Conjugate Acid-Base Pairs

  16. H2O NaCl (s)Na+ (aq) + Cl- (aq) H2O NaCH3COOH (s)Na+ (aq) + CH3COO- (aq) CH3COO-(aq) + H2O (l) CH3COOH (aq) + OH-(aq) Acid-Base Properties of Salt Solutions Neutral Solutions: Salts derived from a strong acidand a strong base Basic Solutions: Salts derived from a strong baseand a weak acid. Hydrolysis≡ the reaction of an anion or a cation of a salt with water

  17. H2O NH4Cl (s)NH4+ (aq) + Cl- (aq) NH4+(aq) NH3(aq) + H+(aq) 3+ 2+ Al(H2O)6(aq) Al(OH)(H2O)5(aq) + H+(aq) Acid-Base Properties of Salt Solutions Acid Solutions: 1) Salts derived from a strong acidand a weak base. 2) Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid:

  18. Acid Hydrolysis of Al3+

  19. Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: • Kb for the anion > Ka for the cation, solution will be basic • Kb for the anion < Ka for the cation, solution will be acidic • Kb for the anion Ka for the cation, solution will be neutral

  20. Factors Affecting Acid Strength Fig 16.12 Trends in acid-base properties of binary hydrides

  21. Factors Affecting Acid Strength

  22. Definition of An Acid + OH- H H + H O H + H+ H N H N H • • • • • • • • • • • • H H Arrhenius acid - a substance that produces H+ (H3O+) in water Brønsted acid - a proton donor Lewis acid - a substance that can accept a pair of electrons Lewis base - a substance that can donate a pair of electrons H+ acid base acid base

  23. H F F F B F B N H F F H H N H H • • Lewis Acids and Bases + acid base Coordinate covalent bond No protons donated or accepted!

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