The Nature of Acids and Bases- Acid Strength and Ionization Constant Chapter 15.2-15.4

Acids and Bases in Nature Acids (sour in latin) are abundant, and diverse in their functions throughout the natural world. Citric acid (H3C6H5O7) gives citrus fruit their sour flavour Acetic Acid (HC2H3O2) is active in vinegar Hydrochloric acid (HCl) has a variety of uses including refining metal ore and digesting food in our stomachs Many organic acids contain a Carboxylic acid group Acids turn blue litmus paper red

Acids and Bases in Nature Characterized by a bitter taste and a slippery feel Sodium Hydroxide (NaOH) active in soap Alkaloids, usually present in poisonous plants, and dark chocolate Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH) are present in most labs Household cleaners such as ammonia (NH3) and drāno are basic and are characteristically slippery

Arrhenius Definition Acid: A substance that produces H+ ions in aqueous solution Base: A substance that produces OH- ions in a solution Example: HCl is considered an acid HCl(aq) → H+(aq) + Cl-(aq) HCl contains a covalent bond but it ionizes completely in water. These H+ ions are highly reactive and bond with water to form Hydronium H3O+

Arrhenius continued Example: NaOH is a base because it produces OH- ions NaOH(aq) → Na+(aq) +OH- (aq) NaOH dissociates in into its component ions when it is placed in water Under the Arrhenius definition, H+ released by acids and OH- released by bases combine to form water, neutralizing each other

Quick Review Based on the Arrhenius Definition of acids and bases, which of the following is a base? KOH NH4 H2CO3 In water, KOH readily dissociates into K+ and OH-, producing OH-

The Brømsted-Lowry Definition Acid: Proton (H+ion) donor Base: Proton (H+ion) acceptor Examples: HCl(aq) + H2O(l) → H3O+(aq) + Cl-(aq) NH3(aq) + H2O ⇋ NH4+(aq) + OH-(aq)

Surprise, Water is Special The Brønstead- Lowry definition allows for substances that can act as both acids and bases, Water is one of these amphoteric substances. NH3(aq) + H2O ⇋ NH4+(aq) + OH-(aq) Accepts proton (base) Donates proton (acid) Conjugate acid Conjugate Base NH4+(aq) + OH-(aq) ⇋ NH3(aq) + H2O Donates proton (acid) Accepts proton (base)

Practice! Identify the acid and base in the following reactions: H2SO4(aq) + H2O → HSO4-(aq)+ H3O+(aq) HCO3-(aq)+ H2O → H2CO3(aq)+OH-(aq)

More Practice! Write the conjugate base or acid. HCl ClO4- HCHO2 Cl- HClO4 CHO2-

15.4: Acid Strength and Ionization Constant (Ka)

Strong and Weak Acids The strength of an acid is determined by the degree to which it will ionize. A strong acid will ionize completely in solution whereas a weak acid will only ionize partially. This depends on equilibrium. HA(aq) + H2O ⇋ H3O+(aq)+ A-(aq) This is a generic formula, HA represents the acid and A represents its conjugate base. Ka<1→ weak acid Ka>1 → strong acid

Examples CH3COOH + H2O ⇋ H3O+ + CH3COO- Ka = [H3O+][CH3COO-]/ [CH3COOH] = 1.76 x 10-5 W E A K Carboxylic acids are notorious for being weak pKa= -logKa = 4.75 Important weak acids to know: HF, HC2H3O2,HCHO2,H2SO3(di),H2CO3, H3PO4(tri)

Example HCl is a strong acid, Ka= 1.3 x 106 Important strong acids to memorize: HCl, HBr,HI, HNO3, HClO4,H2SO4 Monoprotic acids: those that one contain one ionizable proton Polyprotic: an acid with 2 ionizable protons

Acid Ionization Constant (Ka) Using our general formula from earlier. HA(aq) + H2O ⇋ H3O+(aq)+ A-(aq) HA(aq) ⇋ H+(aq)+ A-(aq)

Ionization Constant Practice Calculate the concentration of ions in a solution of .15M acetic acid (HC2H3O2) and a Ka of 1.8 x 10-5. 3 x 10-6

AP expectation