Chapter 15. Aqueous Acid-Base Equilibria 15.1 Proton Transfers in Water 15.2 The pH Scale 15.3 Weak Acids and Bases 15.4 Recognizing Acids and Bases 15.5 Acidic and Basic Salts and Oxides 15.6 Factors Affecting Acid Strength 15.7 Multiple Equilibria Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Learning objective: Using the Brønsted-Lowry theory of acids and bases. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd Proton Transfers in Water

Proton Transfers in Water 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. HNO 3, HCl, NH 4 +, H 2 PO 4 - Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Proton Transfers in Water Brønsted-Lowry definition of acid-base reactions Base – any substance that can accept a proton (H + ) from another substance.  Can be neutral or anions  e.g. NH 3, CO 3 2-, PO 4 3- Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Conjugate Acid-Base Pairs Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. conjugate acid-base pair The acid has an extra proton in each case!

Example 15 – 1 Conjugate Acid-Base Pairs Write the chemical formula and the Lewis structure and draw a molecular picture of each of the following: (a) the conjugate acid of ammonia, NH 3 (b) the conjugate base of formic acid, HCO 2 H (c) the conjugate acid of the hydrogen sulphate ion, HSO 4 -. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Types of Acids and Bases Monoprotic acids: only capable of donating one proton  HCl, HI, HS -, HPO 4 -, HSO 4 - Polyprotic acids: capable of donating two or more protons  H 2 CO 3, H 3 PO 4, H 2 PO 4 -, H 2 SO 4, H 2 S Monoprotic bases: only capable of accepting one proton  Cl -, HPO 4 -, HSO 4 - Polyprotic bases: capable of accepting two or more protons  SO 4 2-, CO 3 2-, PO 4 3- 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.

Autohydrolysis of Water Water is the solvent for most acid/base reactions Hydronium ion: hydrated hydrogen ion 2 H 2 O (l) Ý H 3 O + (aq) + OH - (aq) Referred to as autohydrolysis – self-hydrolysis. Occurs in any aqueous solution. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Water Autohydrolysis Constant Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. 2 H 2 O (l) Ý H 3 O + (aq) + OH - (aq)

Strong Acids (Strong Electrolytes) Strong Bases (Strong Electrolytes) HClHydrochloric acidLiOHLithium hydroxide HBrHydrobromic acidNaOHSodium hydroxide HIHydroiodic acidKOHPotassium hydroxide HNO 3 Nitric acid HClO 4 Perchloric acid H 2 SO 4 Sulfur acid 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.

Example 15 – 2 Ion Concentrations in a Solution of Strong Acid Determine the ion concentration in a 5.0 x M aqueous solution of HClO 4, a strong acid. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 3 Ion Concentrations in a Strong Base Solution What are the ion concentrations in 0.500L of an aqueous solution that contains 5.0 g of NaOH? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.2 The pH Scale Learning objective: Relating pH to concentrations of ions in solution. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.2 The pH Scale Used to express the hydronium ion concentration Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Features of the pH Scale 1. pH = 7.00 defines a neutral solution. Acid solutions have pH The more acidic the solution, the lower its pH. 3. A change in pH of one unit reflects a tenfold change in the hydronium ion concentration. 4. The immense range of concentrations from > 1M to < M is compressed into a more convenient range, from ~ -1 to ~ 15 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 4 pH and Ion Concentrations What are the concentrations of hydronium and hydroxide ions in a beverage with a pH of 3.05? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

pH and pOH Recall that K w = [H 3 O + ][OH - ] log 10 K w = pK w = log([H 3 O + ][OH - ]) = log[H 3 O + ] + log[OH - ] = pH + pOH = 14 …true in any aqueous solution Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 5 pH and pOH What is the pH of a 0.25M solution of NaOH? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Overview of pH and pOH Given any one of these following: [H 3 O + ],[OH - ], pH or pOH, the other three can be determined. pH = - log [H 3 O + ][H 3 O + ] = 10 -pH pOH = - log [OH - ][OH - ] = 10 -pOH [H 3 O + ][OH - ] = K w pH + pOH = pK w At 25.0 °C: [H 3 O + ][OH - ] = 1.0 x At 25.0 °C: pH + pOH = Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.3 Weak Acids and Bases Learning objective: Calculating concentrations and pH in weak acid and base solutions. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.3 Weak Acids and Bases Think back to the table of strong acids and bases. There aren’t too many listed. What about the other acids and bases? They are referred to as weak acids and bases because they do not totally dissociate in solution.  Weak acids contain the acid and water as major species.  Weak bases contain the base and water as major species Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Weak Acids Proton transfer to water is not quantitative, so there exits an equilibrium where only a small fraction of the acid molecules have transferred their protons to water. In a solution, the major species are water molecules and the acid, HA. HA + H 2 O Ý H 3 O + + A - Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 6 Calculating K a The pH of a 0.25 M aqueous HF solution is Calculate the K a for this weak acid. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Percent Hydrolysis 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.

Example 15 – 7 Percent Hydrolysis Determine the percent hydrolysis for an aqueous solution of HF that is 25 mM. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Weak Bases Proton transfer from water is not quantitative. In solution, the major species are thus water molecules and the base, A -. A - + H 2 O Ý HA + OH - Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 8 pH of a Weak Base Ammonia has K b = 1.8 x What is the pH of 0.25 M aqueous ammonia? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.4 Recognizing Acids and Bases Learning objective: Recognizing and naming some common acids and bases. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.4 Recognizing Acids and Bases Oxoacids – an acid that contains an inner atom bonded to a variable number of oxygen atoms and acidic OH groups. General formula H x EO y Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Some Weak Oxoacids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Carboxylic Acids RCO 2 H All carboxylic acids are weak acids. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Some Weak Acids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 9 Identifying Acids Examine the following formulas. Decide if each represents a strong acid, a weak acid or neither. Justify your conclusions. (a) Cl 3 CCO 2 H (b) CH 3 CH 2 CH 2 OH (c) HCN (d) HClO 4 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Weak Bases Water is a weak base Another common weak base is NH 3. Many other weak bases are derivatives of ammonia called amines, where some of the N – H bonds have been replaced with C – H bonds. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Representative Organic Bases Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 10 Acidic and Basic Properties of Amino Acids Lysine and aspartic acids are two of the amino acids found in proteins. Describe the acid-base characteristics of these compounds. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.5 Acidic and Basic Salts Learning objective: Calculating the pH of solutions of salts of weak acids or bases. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.5 Acidic and Basic Salts An aqueous solution of a soluble salt contains cations and anions that can often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. Cations that are conjugate acids of weak bases make a solution acidic. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

For instance, dissolve some NaF in water: NaF (s) → Na + (aq) + F - (aq) The solution is basic because: H 2 O (l) + F - (aq) Ý HF (aq) + OH - (aq) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. A Basic Salt

Relating K a, K b and K w Where K a is the ionization constant for a weak acid (e.g. HF) and K b is the ionization constant for its conjugate base (F - ). Also, pK a + pK b = Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Relative Magnitudes of K a and K b Acid Strength KaKa Conjugate Base Strength KbKb Strong>1Very Weak< Weak to 1Weak to 1 Very Weak< Strong>1 Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Strong acids have weak conjugate bases and vice-versa

Example 15 – 11 Salt of a Weak Acid 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.

Example 15 – 12 Salt of a Weak Base What are the important acid-base equilibria in an aqueous solution of pyridinium chloride (C 5 H 5 NHCl)? What are the values of their equilibrium constants? Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.6 Factors Affecting Acid Strength Learning objective: Explaining the factors that contribute to the strength of an acid. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Factors Affecting Acid Strength Effect of charge – affects the ability to donate and accept protons (remember opposites attract) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Factors Affecting Acid Strength Effect of charge – affects the ability to donate and accept protons (remember opposites attract) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Factors Affecting Acid Strength Effect of charge – affects the ability to donate and accept protons (remember opposites attract) Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Structural Factors In order to donate a proton, a molecule must break a H – X bond. This becomes easier as bond strengths decrease, and therefore the acids become stronger. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 13 Acidities of Simple Carboxylic Acids Oxalic acid, HO 2 C – CO 2 H, has K a1 = 1.3 x and K a2 = 1.4 x Formic acid HCO 2 H, has K a = 1.8 x Explain why the first proton of oxalic acid is substantially more acidic than the proton of formic acid, but the second proton is less acidic. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.7 Multiple Equilibria Learning objective: Calculating concentrations in solutions involving multiple equilibria. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

15.7 Multiple Equilibria There are always at least two equilibria involving acids and bases.  The expression for the reaction of the acid (or base) with water  The expression for the autoionization of water Usually, you only need to deal with the equilibrium that contains the major species. Polyprotic acids provide the most common example of multiple equilibria that play a role in determining concentrations of minor species. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Polyprotic Acids e.g. Sulphurous Acid, H 2 SO 3 In water, three equilibria exist: H 2 O + H 2 O Ý H 3 O + + OH - K w = 1.0 x (usually negligible) H 2 SO 3 + H 2 O Ý H 3 O + + HSO 3 - K a1 = 1.4 x HSO H 2 O Ý H 3 O + + SO 3 2- K a2 = 6.3 x K a2 < K a1 due to charge effects Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Representative Polyprotic Acids Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 14 Ion Concentrations in a Polyprotic Acid Solution Carbonated water contains carbonic acid, a diprotic acid that forms when carbon dioxide dissolves in water. CO 2 (g) + H 2 O (l) Ý H 2 CO 3 (aq) A typical carbonate beverage contains M H 2 CO 3. Determine the concentrations of the ions present in this solution. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 15 Molecular View of a Polyprotic Acid The drawing shows a molecular view of a very small region of an aqueous solution of oxalic acid. For clarity, water molecules are not shown. Redraw this molecular picture to show the solution (a) after two hydroxide ions react with these molecules, and (b) after four hydroxide ions react with these molecules. Include in your drawings the water molecules that form as products. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Example 15 – 16 Ion Concentrations in a Polyprotic Anion Solution Potassium sulphite is commonly used as a food preservative, because the sulphite anion undergoes reactions that release sulphur dioxide, an effective preservative. Determine the concentrations of the ionic species present in a solution of potassium sulphite that is M. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Chapter 15 Visual Summary