Acid-Base chemistry Acidity of blood (pH range of Heartburn (acid-reflux) – Tums, Rolaids, Milk of Magnesia; The Purple Pill, Nexium Acidity regulation (tropical fish / goldfish tanks) Pepsi (most sodas); just how acidic? Loosens rusty bolts; cleans windshields Battery acid (H2SO4) Acid Rain (SO2, NO2, CO2) Drain cleaners (Drano, Liquid Plumber)

Nature of acids and bases Acids – sour, tart taste (vinegar, lemon juice) Bases – bitter taste, slippery feel between fingers (drano, detergent) Arrhenius definitions (1884): Acids: produce H+ ions (protons) in solution Bases: produce OH- (hydroxide) ions in solution

Lowry-Brønsted definitions Danish & British chemists More general definitions than Arrhenius definition: Acids: Proton donors Bases: proton acceptors H2O + HCl  H3O+ + Cl-

Lewis Acids / Lewis Bases LA: electron pair acceptor LB: electron pair donor NH3(aq) + H+(aq)  NH4+(aq) LB LA OH-(aq) + H+(aq)  H2O(l)

General reaction for HA + H2O HA(aq) + H2O(l)  H3O+(aq) + A-(aq) Acid Base Conjugate Conjugate Acid Base Position of equilibrium dictated between bases in equation (competition for H+) H2O stronger base: A- stronger base: Keq = [H3O+][A-] Ka = Acid dissociation constant [HA][H2O] [H3O+] = [H+] in H2O Ka = [H3O+][A-] Ka = Acid dissociation constant [HA]

Acid dissociation reactions Dissociation of acid (HA) most important here HA (aq)  H+(aq) + A-(aq) H2O still important (required for aqueous conditions) Ka for this equilibrium process? Can predict dissociation reaction for any acid (no matter how complex-looking) HCl (aq) HC2H3O2 (aq) NH4+ (aq) C6H5NH3+ (aq) [Al(H2O)6]3+ (aq)

Acid Strength Defined by equilibrium position of dissociation reaction: HA(aq) + H2O(l)  H3O+(aq) + A-(aq) Strong acid – lies far to right side (a) Weak acid – lies far to left side (b)

Describing acid strength Insert Figure 14.6 Insert table 14.1 strong acid weak base Property Strong Acid Weak Acid Ka value Equilibrium Posn. [H+]e vs [HA]o Strength of conj. base compared to water

Common strong acids HCl, HClO4, HNO3 (monoprotic acids) H2SO4 (diprotic acid) Ka values – very large

Relative base strengths Arrange H2O, F-, Cl-, NO2-, CN- in order of increasing base strength:

Water: Amphoteric substance Can exist as both an acid and a base Autoionization of water H2O(l) + H2O(l)  H3O+(aq) + OH-(aq) Kw = [H3O+][OH-]; Kw = Dissociation constant for H2O Kw = [H+][OH-] [H+]=[OH-] = 1.0 x 10-7 M (at 25 °C, in pure water) Kw = [H+][OH-] = 1.0 x 10-14 In any aqueous solution, the product of [H+] and [OH-] must always equal 1.0 x 10-14

Kw Kw = [H+][OH-] = 1.0 x 10-14 3 possible situations: Neutral solutions; [H+] = [OH-] Acidic solutions; [H+] > [OH-] Basic solutions; [H+] < [OH-] In all cases: Kw = [H+][OH-] = 1.0 x 10-14 Varies with T (as do all K values)

Calculating [H+],[OH-] [H+] with [OH-] = 1.0 x 10-5 M [OH-] with [H+] = 10.0 M [OH-] and [H+] in neutral solution at 60 °C (Kw = 1x10-13 at 60 °C)

pH scale (acidity measurement) pH = -log [H+] [H+] = 1.0 x 10-7 M; pH = pOH = -log[OH-]; pK = -log Ka Log scale; pH changes by 1 for every power of 10 change in [H+] pH decreases as [H+] increases

Calculating pH / pOH 1.0 x 10-3 M OH- 1.0 M H+ pH + pOH = 14.00 pH of sample of human blood = 7.41 @ 25 °C. Calculate pOH, [H+], [OH-]

pH of strong acid solutions What species are present? 1M HCl What are the major species that can furnish H+ ions? pH of 0.10 M HNO3 pH of 1.0 x 10-10 M HCl

pH of weak acid solutions pH of 1.00 M HF (weak acid); Ka = 7.2 x 10-4 Major species in solution? HF, H2O Major species which furnish H+? Which of these two is the stronger acid? Ka = [H+][F-] / [HF] = 7.2 x 10-4 In order to calculate pH, need equilibrium value of [H+] [HF]o = 1.00 M [H+]o = 0 M (approximation, as H+ from H20 not included here) [F-]o = 0 M Let x be the change required to reach equilibrium…..

pH of weak acid solutions Equilibrium concentration of [HF] = Equilibrium concentration of [H+] = Equilibrium concentration of [F-] = Substitute these into Ka = [H+][F-] / [HF] = 7.2 x 10-4 eqn, and solve for x….

Bases Cleaning solutions (ammonia, bleach) Antacids (Tums, Rolaids, Milk of Magnesia) Arrhenius: produces OH- ions Lowry – Brønsted: H+ acceptor Strong Bases - NaOH

pH of strongly basic solution 5.0 x 10-2 M NaOH solution (same procedure as for acidic pH calculations); Expected pH range? Major species:

Weak bases Many types of bases don’t contain OH-, but do increase [OH-] when dissolved in water (through reaction with water). Base Acid Lone pair of electrons on N picks up H+ from H2O

Weak bases General reaction with H2O: Kb = Kb always refers to reaction of a base with H2O to produce a conjugate acid and OH-

pH of weak bases - calculations Very similar to those of weak acids pH of 15.0 M solution of NH3 (Kb = 1.8 x 10-5)

Polyprotic acids More than 1 acidic H; H2SO4, H3PO4; H2CO3 Consider H3PO4 Only 1st dissociation step usually important for [H+] determination Ka1 >>> Ka2 >>> Ka3

Polyprotic acids pH of 5.0 M H3PO4 solution; eq. concs. of H3PO4, H2PO42-, HPO43- and PO43-.

Sulfuric acid (H2SO4) Unique acid Strong acid in 1st dissociation step Weak acid in 2nd dissociation step pH of 1.0 M H2SO4 solution Does HSO4- make significant contribution to [H+]? No.

Acid-base properties of salts Salts producing neutral solutions Salts producing basic solutions Salts producing acidic solutions

Acid-base properties of salts

Structure effects on acid/base properties Oxyacids Hydrogen halides