Aqueous Acid–Base Equilibria Instructor: Dr. Niloufar Choubdar Brønsted–Lowry definition of acid–base • In an acid–base reaction, a proton (H+ ) is transferred from one chemical species to another. A species that donates a proton is an acid, and a species that accepts a proton is a base. This identification of acids and bases is the Brønsted–Lowry definition of acid–base reactions. Acid – any substance that can donate a proton (H+) to another substance. Can be neutral, cations or anions e.g. HNO3, HCl, NH4+, H2PO4- Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 2 Base – any substance that can accept a proton (H+) from another substance. Can be neutral or anions e.g. NH3, CO32-, PO43- Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 3 From this perspective, every acid–base reaction has two reactants, an acid and a base. Every acid–base reaction also forms two products: Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 4 Dissociation of Water • These examples of acid–base reactions show that water can act as either an acid or a base: Water accepts a proton • from an HCl molecule, but it donates a proton to a anion PO43-. • As an acid, water donates a proton to a base and becomes a hydroxide anion. • As a base, water accepts a proton from an acid and becomes a hydronium cation. • A chemical species that can both donate and accept protons is said to be amphiprotic. • This ability to serve as both a proton donor and a proton acceptor suggests that a proton transfer equilibrium exists for pure water. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 5 • water equilibrium constant: Kw =1.0×10-14 • 2 H2O H3O+ (aq) + OH-(aq) Keq = Kw [H3O ][OH ] 1 10 14 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 6 Common Strong Acids & Strong Bases Strong acids will donate a proton to water to form hydronium ion. The hydronium ion concentration will be equal to the acid concentration. Strong bases will dissociate in solution to form hydroxide ion, the concentration of which can be calculated from the base’s molarity. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 7 Conjugate Acid-Base Pairs conjugate acid-base pair conjugate acid-base pair Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 8 Types of Acids and Bases Monoprotic acids: only capable of donating one proton HCl, HI, HS-, HPO4-, HSO4- Polyprotic acids: capable of donating two or more protons H2CO3, H3PO4, H2PO4-, H2SO4, H2S Monoprotic bases: only capable of accepting one proton Cl-, HPO4-, HSO4- Polyprotic bases: capable of accepting two or more protons SO42-, CO32-, PO43- Amphiprotic: molecules or ions which can behave either way (either an acid or a base) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 9 • Determine the ion concentrations in a 5×10-2 M aqueous solution of HCLO4 ? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 10 Initial concentration (M) Change in concentration (M) Equilibrium concentration 5×10-2 +X 5×10-2 + X 0 +X X equilibrium constant is much smaller than the initial concentration Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 11 Ion Concentrations in a Strong Base Solution • What are the ion concentrations in 0.500 L of an aqueous solution that contains 5.0 g of NaOH? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 12 • Initial concentration • Change in concentration • Equilibrium concentration 0 +X X Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 0.25 +X 0.25+ X 13 The pH Scale pH log10 [H3O (aq)] Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 14 Aqueous Acid–Base Equilibria Acidic solutions have pH <7, and basic solutions have pH>7 . Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 15 • What are the concentrations of hydronium and hydroxide ions in a beverage whose pH= 3.05? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 16 p scale Example: What is the pH of a 0.25 M solution of NaOH? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 17 Weak Acids and Bases • In a solution of a weak acid, the major species are water molecules and the acid, HA . The products of the proton transfer reaction, H3O+ and A- , are present in smaller concentrations as minor species • Example: • The pH of a 0.25 M aqueous HF solution is 1.92. Calculate Ka for this weak acid. Initial concentration (M) Change in concentration (M) 1.2×10-2 Equilibrium concentration 1.2×10-2 0.25 -1.2×10-2 0.24 0 1.2×10-2 0 1.2×10-2 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 18 Comparing strong and week acids % HA hydrolyzed 100% [H3O ]eq [HA]initial The hydronium ion concentration is at equilibrium. The weak acid concentration is its initial concentration . Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 19 Exercise: Determine the percent ionization for an aqueous solution of HF that is 25 mM. Ka = 6.0 × 10_4 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 20 We need to decide if we can make the approximation that 0,025-x ≈0.025 . The difference will be small if and initial concentrations are greater than 0.10 M. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd continue 21 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 22 Weak Bases: Proton Transfer from Water A- + H 2 O HA + OH- [HA][OH ] Kb [A ] Example: pH of a Weak Base example: Ammonia has (Kb = 1.8×10-5 ). What is the pH of 0.25 M aqueous ammonia? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 23 Comparing strong and week base Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 24 Oxoacids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 25 Carboxylic Acids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 26 Some Weak Acids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 27 Representative Polyprotic Acids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 28 Representative Organic Bases Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 29 Salts of Weak Acids Whenever two equilibria are added, the net equilibrium constant is the product of the individual ones. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 30 Exercise • Sodium hypochlorite (NaOCl) is the active ingredient in laundry bleach. Typically, bleach contains 5.0% of • this salt by mass, which is a 0.67 M solution. Determine the concentrations of all species and compute the • pH of laundry bleach. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 31 The sodium ion is neither an acid nor a base. So it is a spectator ion. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 32 Salts of Weak Bases Salts that contain cations of weak bases are acidic Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 33 Factors Affecting Acid Strength Effect of Charge Opposite electrical charges attract, and like charges repel. The hydroxide ion (OH-) is a strong base because its negative charge strongly attracts proton. The hydronium ion, H3O+, is a strong acid because its positive charge enhances the removal of one of its protons. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 34 Structural Factors Acid strength decrease as negative charge increases. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 35 The acidity of the binary hydrides increase from left to right across a row of the periodic table. As the electronegativity of X increases, the H-X bond polarity increases and so does the polarity. Moving down a column of a periodic table, the principal quantum number of the valence orbitals of the halogen Increases, orbital overlap in bonding decreases, and bond strength decreases. The H-F bond is substantially more polar, and hence stronger, than H-Cl bond. So HF is a weaker acid than HCl. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 36 The strength of an oxoacid increases as more and more electron density is withdrawn from the O—H bond. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 37 Example: Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 38 As the left-hand structure indicates, the second COOH group in oxalic acid is better at withdrawing electron density than is the —H in formic acid. This makes the O—H bonds in oxalic acid more polar than the O—H bond in formic acid, so the first proton of oxalic acid is more easily transferred, making oxalic acid stronger than formic acid. The resulting anion has a negative charge that hinders removal of the second proton and makes hydrogen oxalate a weaker acid than formic acid. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 39 Example: Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd Note that x is about 0.3% of 0.050, so the approximation is40valid Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 41 Salts of Polyprotic Acids Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 42 Example: • Potassium sulfite is commonly used as a food preservative, because the sulfite anion undergoes reactions that release sulfur dioxide, an effective preservative. Determine the concentrations of the ionic species present in a solution of potassium sulfite that is 0.075 M? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 43 Note that x is about 0.1% of 0.075, so the approximation is valid. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 44 Make that approximation that X<<1.1 ×10-4 [H2SO3] = 7.1× 10-13 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 45 Chapter 15 Visual Summary Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 46 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 47 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 48 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 49 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 50 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 51 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd 52
