Acid Equilibrium and p H Srensen AcidBase Definitions

Acid Equilibrium and p. H Sørensen

Acid/Base Definitions q Arrhenius Model q Acids produce hydrogen ions in aqueous solutions q Bases produce hydroxide ions in aqueous solutions q Bronsted-Lowry Model q Acids are proton donors q Bases are proton acceptors q Lewis Acid Model q Acids are electron pair acceptors q Bases are electron pair donors

Acid Dissociation HA Acid H+ + AProton Conjugate base Alternately, H+ may be written in its hydrated form, H 3 O+ (hydronium ion)

Dissociation of Strong Acids Strong acids are assumed to dissociate completely in solution. Large Ka or small Ka? Reactant favored or product favored?

Dissociation Constants: Strong Acids Perchloric Hydriodic HCl. O 4 HI Conjugate Base Cl. O 4 I- Hydrobromic Hydrochloric Nitric Sulfuric HBr HCl HNO 3 H 2 SO 4 Br. Cl. NO 3 HSO 4 - Very large Hydronium ion H 3 O + H 2 O 1. 0 Acid Formula Ka Very large

Dissociation of Weak Acids Weak acids are assumed to dissociate only slightly (less than 5%) in solution. Large Ka or small Ka? Reactant favored or product favored?

Dissociation Constants: Weak Acids Acid Iodic Oxalic Sulfurous Phosphoric Citric Nitrous Formula Conjugate Base Ka HIO 3 - 1. 7 x 10 -1 H 2 SO 3 HSO 3 - 1. 5 x 10 -2 H 2 C 6 H 5 O 7 - 7. 1 x 10 -4 H 2 C 2 O 4 H 3 PO 4 H 3 C 6 H 5 O 7 HNO 2 HC 2 O 4 - 5. 9 x 10 -2 H 2 PO 4 - 7. 5 x 10 -3 NO 2 - 4. 6 x 10 -4 F- 3. 5 x 10 -4 Hydrofluoric HF Formic HCOOH HCOO- 1. 8 x 10 -4 Benzoic C 6 H 5 COOH C 6 H 5 COO- 6. 5 x 10 -5 H 2 CO 3 HCO 3 - 4. 3 x 10 -7 Acetic Carbonic Hypochlorous Hydrocyanic CH 3 COOH HCl. O HCN CH 3 COO- 1. 8 x 10 -5 Cl. O- 3. 0 x 10 -8 CN- 4. 9 x 10 -10

Self-Ionization of Water H 2 O + H 2 O H 3 O+ + OH- At 25 , [H 3 O+] = [OH-] = 1 x 10 -7 Kw is a constant at 25 C: Kw = [H 3 O+][OH-] Kw = (1 x 10 -7) = 1 x 10 -14

Calculating p. H, p. OH p. H = -log 10(H 3 O+) p. OH = -log 10(OH-) Relationship between p. H and p. OH p. H + p. OH = 14 Finding [H 3 O+], [OH-] from p. H, p. OH [H 3 O+] = 10 -p. H [OH-] = 10 -p. OH

p. H and p. OH Calculations

The p. H Scale Graphic: Wikimedia Commons user Slower

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #1: Write the dissociation equation HC 2 H 3 O 2 C 2 H 3 O 2 - + H+

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #2: ICE it! I C E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x 0 0 +x +x x x

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #3: Set up the law of mass action E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #4: Solve for x, which is also [H+] E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x [H+] = 3. 0 x 10 -3 M

A Weak Acid Equilibrium Problem What is the p. H of a 0. 50 M solution of acetic acid, HC 2 H 3 O 2, Ka = 1. 8 x 10 -5 ? Step #5: Convert [H+] to p. H E HC 2 H 3 O 2 C 2 H 3 O 2 - + H+ 0. 50 - x x x