Acids, Bases, and Salts CHM 1010 PGCC Barbara A. Gage

Characteristics of Acids and Bases Litmus color Phenolphthalein color pH range Reaction with active metal (like Mg) Taste Formula component CHM 1010 PGCC Barbara A. Gage

Acids Strong Weak ionizes completely in water ionizes partially in water hydrochloric acid, HCl hydrofluoric acid, HF hydrobromic acid, HBr phosphoric acid, H3PO4 hydroiodic acid, HI acetic acid, CH3COOH (or HC2H3O2) nitric acid, HNO3 sulfuric acid, H2SO4 carbonic acid, H2CO3 perchloric acid, HClO4 CHM 1010 PGCC Barbara A. Gage

The extent of dissociation for strong acids. Strong acid: HA(g or l) + H2O(l) H3O+(aq) + A-(aq) H+ and H2O  H3O+ (hydronium ion) CHM 1010 PGCC Barbara A. Gage

The extent of dissociation for weak acids. Weak acid: HA(aq) + H2O(l) H3O+(aq) + A-(aq) CHM 1010 PGCC Barbara A. Gage

Bases (or alkalis) Strong Moderate Dissociates completely Dissociates completely but is not very soluble sodium hydroxide, NaOH magnesium hydroxide, Mg(OH)2 potassium hydroxide, KOH aluminum hydroxide, Al(OH)3 calcium hydroxide, Ca(OH)2 Weak strontium hydroxide, Sr(OH)2 barium hydroxide, Ba(OH)2 Dissociates partially ammonia, NH3 (NH4OH) carbonates, CO32- bicarbonates, HCO31- CHM 1010 PGCC Barbara A. Gage

An aqueous strong acid-strong base reaction on the atomic scale. MX is a “salt” – an electrolyte that is not an acid or base CHM 1010 PGCC Barbara A. Gage

Acid and Base Definitions Arrhenius Acid = compound that forms hydrogen (H+) ions in water Base = compound that forms hydroxide (OH-) ions in water CHM 1010 PGCC Barbara A. Gage

Acid and Base Definitions Bronsted-Lowry Acid = proton donor (H+ is a proton) Base = proton acceptor An acid-base reaction can now be viewed from the standpoint of the reactants AND the products. An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair. CHM 1010 PGCC Barbara A. Gage

Proton transfer as the essential feature of a Brønsted-Lowry acid-base reaction. Lone pair binds H+ HCl H2O + Cl- H3O+ + (acid, H+ donor) (base, H+ acceptor) NH3 H2O + Lone pair binds H+ NH4+ OH- + (base, H+ acceptor) (acid, H+ donor) CHM 1010 PGCC Barbara A. Gage

The Conjugate Pairs in Some Acid-Base Reactions + Base Acid + Conjugate Pair Reaction 1 HF H2O + F- H3O+ + Reaction 2 HCOOH CN- + HCOO- HCN + Reaction 3 NH4+ CO32- + NH3 HCO3- + Reaction 4 H2PO4- OH- + HPO42- H2O + Reaction 5 H2SO4 N2H5+ + HSO4- N2H62+ + Reaction 6 HPO42- SO32- + PO43- HSO3- + CHM 1010 PGCC Barbara A. Gage

Identifying Conjugate Acid-Base Pairs SAMPLE PROBLEM Identifying Conjugate Acid-Base Pairs PROBLEM: The following reactions are important environmental processes. Identify the conjugate acid-base pairs. (a) H2PO4-(aq) + CO32-(aq) HPO42-(aq) + HCO3-(aq) (b) H2O(l) + SO32-(aq) OH-(aq) + HSO3-(aq) Identify proton donors (acids) and proton acceptors (bases). conjugate pair2 conjugate pair1 SOLUTION: (a) H2PO4-(aq) + CO32-(aq) HPO42-(aq) + HCO3-(aq) proton donor proton acceptor proton acceptor proton donor conjugate pair2 conjugate pair1 (b) H2O(l) + SO32-(aq) OH-(aq) + HSO3-(aq) proton donor proton acceptor proton acceptor proton donor CHM 1010 PGCC Barbara A. Gage

Molecules as Lewis Acids An acid is an electron-pair acceptor. A base is an electron-pair donor. acid base adduct H2O(l) M(H2O)42+(aq) M2+ adduct CHM 1010 PGCC Barbara A. Gage

Identifying Lewis Acids and Bases SAMPLE PROBLEM Identifying Lewis Acids and Bases PROBLEM: Identify the Lewis acids and Lewis bases in the following reactions: (a) H+ + OH- H2O (b) Cl- + BCl3 BCl4- (c) K+ + 6H2O K(H2O)6+ PLAN: Look for electron pair acceptors (acids) and donors (bases). SOLUTION: acceptor (a) H+ + OH- H2O donor donor (b) Cl- + BCl3 BCl4- acceptor acceptor (c) K+ + 6H2O K(H2O)6+ donor CHM 1010 PGCC Barbara A. Gage

Acid Anhydrides Non-metal oxides react with water to form acidic solutions CO2 (g) + H2O (l)  H2CO3 (aq) N2O5 (s) + H2O (l)  2 HNO3 (aq) SO3 (g) + H2O (l)  H2SO4 (aq) Dissolved non-metal oxides cause acid rain. CHM 1010 PGCC Barbara A. Gage

Basic Anhydrides Metal oxides react with water to form alkaline solutions Na2O (s) + H2O (l)  2 NaOH (aq) CaO (s) + H2O (l)  Ca(OH)2 (aq) Al2O3 (s) + 3 H2O (l)  2 Al(OH)3 (aq) Lime (CaO) is used on lawns and is converted to Ca(OH)2 when it rains. CaO is less hazardous to handle. CHM 1010 PGCC Barbara A. Gage

Point of neutralization An acid-base titration. Start of titration Excess of acid Point of neutralization Slight excess of base CHM 1010 PGCC Barbara A. Gage

Finding the Concentration of Acid from an Acid-Base Titration Sample Problem Finding the Concentration of Acid from an Acid-Base Titration PROBLEM: You perform an acid-base titration to standardize an HCl solution by placing 50.00 mL of HCl in a flask with a few drops of indicator solution. You put 0.1524 M NaOH into the buret, and the initial reading is 0.55 mL. At the end point, the buret reading is 33.87 mL. What is the concentration of the HCl solution? SOLUTION: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) (33.87-0.55) mL x 1L 103 mL = 0.03332 L 0.03332 L X 0.1524 M = 5.078x10-3 mol NaOH Molar ratio is 1:1 5.078x10-3 mol HCl 0.05000 L = 0.1016 M HCl CHM 1010 PGCC Barbara A. Gage

H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Kc = [H3O+][OH-] [H2O]2 The Ion-Product Constant for Water Kc[H2O]2 = Kw = [H3O+][OH-] = 1.0 x 10-14 at 250C A change in [H3O+] causes an inverse change in [OH-]. In an acidic solution, [H3O+] > [OH-] In a basic solution, [H3O+] < [OH-] In a neutral solution, [H3O+] = [OH-] CHM 1010 PGCC Barbara A. Gage

The relationship between [H3O+] and [OH-] and the relative acidity of solutions. Divide into Kw [H3O+] [OH-] [H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-] ACIDIC SOLUTION NEUTRAL SOLUTION BASIC SOLUTION CHM 1010 PGCC Barbara A. Gage

Calculating [H3O+] and [OH-] in an Aqueous Solution SAMPLE PROBLEM Calculating [H3O+] and [OH-] in an Aqueous Solution PROBLEM: A research chemist adds a measured amount of HCl gas to pure water at 250C and obtains a solution with [H3O+] = 3.0x10-4M. Calculate [OH-]. Is the solution neutral, acidic, or basic? Use the Kw at 250C and the [H3O+] to find the corresponding [OH-]. SOLUTION: Kw = 1.0x10-14 = [H3O+] [OH-] so [OH-] = Kw/ [H3O+] = 1.0x10-14/3.0x10-4 = 3.3x10-11M [H3O+] is > [OH-] and the solution is acidic. CHM 1010 PGCC Barbara A. Gage

The pH values of some familiar aqueous solutions. pH = -log [H3O+] pOH = -log [OH-] pH + pOH = 14 CHM 1010 PGCC Barbara A. Gage

The relations among [H3O+], pH, [OH-], and pOH. CHM 1010 PGCC Barbara A. Gage

Calculating [H3O+], pH, [OH-], and pOH SAMPLE PROBLEM Calculating [H3O+], pH, [OH-], and pOH PROBLEM: In an art restoration project, a conservator prepares copper-plate etching solutions by diluting concentrated HNO3 to 2.0M, 0.30M, and 0.0063M HNO3. Calculate [H3O+], pH, [OH-], and pOH of the three solutions at 250C. PLAN: HNO3 is a strong acid so [H3O+] = [HNO3]. Use Kw to find the [OH-] and then convert to pH and pOH. SOLUTION: For 2.0M HNO3, [H3O+] = 2.0M and -log [H3O+] = -0.30 = pH [OH-] = Kw/ [H3O+] = 1.0x10-14/2.0 = 5.0x10-15M; pOH = 14.30 For 0.3M HNO3, [H3O+] = 0.30M and -log [H3O+] = 0.52 = pH [OH-] = Kw/ [H3O+] = 1.0x10-14/0.30 = 3.3x10-14M; pOH = 13.48 For 0.0063M HNO3, [H3O+] = 0.0063M and -log [H3O+] = 2.20 = pH [OH-] = Kw/ [H3O+] = 1.0x10-14/6.3x10-3 = 1.6x10-12M; pOH = 11.80 CHM 1010 PGCC Barbara A. Gage

Buffers Solutions that resist change in pH Can maintain any pH value between 0 and 14 (not just neutral pH 7) Composed of a weak acid and a salt made from the weak acid or weak base and salt made from the weak base Examples: HC2H3O2 and NaC2H3O2 NH4OH and NH4Cl CHM 1010 PGCC Barbara A. Gage

Buffers Reaction with acid: HC2H3O2 + C2H3O2- + H+  HC2H3O2 + HC2H3O2 Reaction with base: HC2H3O2 + C2H3O2- + OH-  C2H3O2- + C2H3O2- + HOH A buffer regenerates it’s own components. The pH it maintains depends on the ratio of salt to acid (or base) and the nature of the acid (or base). CHM 1010 PGCC Barbara A. Gage