Acids and Bases Chapter and 15.12

Br Ø nstead Acids and Br Ø nstead Bases Recall from chapter 4: Recall from chapter 4: –Br Ø nstead Acid- a substance capable of donating a proton. –Br Ø nstead Base- a substance capable of accepting a proton. –Conjugate Acid-Base Pair- an acid and its conjugate base or a base and its conjugate acid.

Br Ø nstead Acids and Br Ø nstead Bases NH 3 + H 2 O ↔ NH OH -

Br Ø nstead Acids and Br Ø nstead Bases

The Acid-Base Properties of Water H 2 O (l) H + (aq) + OH - (aq) autoionization of water O H H+ O H H O H HH O H - + [] + H 2 O + H 2 O H 3 O + + OH - base acid conjugate acid conjugate base

The Ion Product of Water Hydrogen Ion concentration is key Hydrogen Ion concentration is key Ion Product Constant (K w )- the product of the molar concentrations of H + and OH - ions at a particular temperature. Ion Product Constant (K w )- the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x

The Ion Product of Water Solution is [H + ] = [OH - ] neutral [H + ] > [OH - ] acidic [H + ] < [OH - ] basic

The Ion Product of Water What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 1.3M? K w = [H + ][OH - ] = 1.0 x [H + ]= 1.3M [OH - ] = KwKw [H + ] 1 x = [OH - ] = 7.7 x M [OH - ]

The Ion Product of Water

pH a measure of Acidity pH- the negative logarithm of the hydronium ion concentration (mol/L). pH- the negative logarithm of the hydronium ion concentration (mol/L). pH = - log [H + ] pOH = -log [OH - ] pH + pOH = 14.00

pH calculations The pH of rainwater collected in a certain region of the northeastern United States on a particular day was What is the H + ion concentration of the rainwater? pH = - log [H + ] [H + ] = 10 -pH [H + ]= [H + ]= 1.5 x M

pH Calculations The OH - ion concentration of a blood sample is 2.5 x M. What is the pH of the blood? pH + pOH = pOH = -log [OH - ] pOH = -log (2.5 x ) pOH = 6.60 pH = – pOH = – 6.60 = 7.40 pH = 7.40

Strength of Acids and Bases Strong Acids- are strong electrolytes that are assumed to ionize completely in water. Strong Acids- are strong electrolytes that are assumed to ionize completely in water. Weak Acids- acids that ionize only to a limited extent in water. Weak Acids- acids that ionize only to a limited extent in water. Strong Bases- are strong electrolytes that ionize completely in water. Strong Bases- are strong electrolytes that ionize completely in water. Weak Bases- weak electrolytes. Weak Bases- weak electrolytes.

Strength of Acids and Bases

Conjugate Acid-Base Pairs

Conjugate acid-base pairs: The conjugate base of a strong acid has no measurable strength. H 3 O + is the strongest acid that can exist in aqueous solution. The OH - ion is the strongest base that can exist in aqueous solution.

Weak Acids and Acid Ionization Constants Acid Ionization Constant (K a )- the equilibrium constant for the ionization of an acid. Acid Ionization Constant (K a )- the equilibrium constant for the ionization of an acid. –The larger the k a, the larger the stronger the acid. HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) HA (aq) H + (aq) + A - (aq) K a = [H + ][A - ] [HA]

Weak Acids and Acid Ionization Constants What is the pH of a 0.5 M HF solution (at 25 0 C)? HF (aq) H + (aq) + F - (aq) K a = [H + ][F - ] [HF] = 7.1 x ICE

Weak Acids and Acid Ionization Constants HF (aq) H + (aq) + F - (aq) Initial (M) Change (M) Equilibrium (M) x-x+x+x+x+x xxx

Weak Acids and Acid Ionization Constants K a = x2x x = 7.1 x K a << – x  0.50 Ka  Ka  x2x = 7.1 x x 2 = 3.55 x x = M [H + ] = [F - ] = M [HF] = 0.50 – x = 0.48 M pH = -log [H + ] = 1.72

When can we use the Approximation Method? When can I use the approximation Method? When x is less than 5% of the value from which it is subtracted. x = M 0.50 M x 100% = 3.8% Less than 5% Approximation ok.

The Quadratic Equation If approximation method does not work, use the quadratic equation. If approximation method does not work, use the quadratic equation. –Write the ionization expression in terms of x. -b ± b 2 – 4ac  2a2a x =

Solving Weak Acid Ionization Problems Solving weak acid ionization problems: Identify the major species that can affect the pH. – –In most cases, you can ignore the autoionization of water. – –Ignore [OH - ] because it is determined by [H + ]. Use ICE to express the equilibrium concentrations in terms of single unknown x. Write K a in terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly. Calculate concentrations of all species and/or pH of the solution.

Percent Ionization Percent Ionization- measure of the strength of an acid. Percent Ionization- measure of the strength of an acid. percent ionization = Ionized acid concentration at equilibrium Initial concentration of acid x 100% For a monoprotic acid HA: Percent ionization = [H + ] [HA] 0 x 100%

Percent Ionization

Weak Bases and Base Ionization Constants Base Ionization Constant- the equilibrium constant for the ionization of a base. Base Ionization Constant- the equilibrium constant for the ionization of a base. NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) K b = [NH 4 + ][OH - ] [NH 3 ]

Weak Bases and Base Ionization Constants

Relationship between Ionization Constants and their Conjugate Bases K a K b = K w Weak Acid and Its Conjugate Base Ka =Ka = KwKw KbKb Kb =Kb = KwKw KaKa

Diprotic and Polyprotic Acids An ionization constant expression can be written for each ionizable proton. An ionization constant expression can be written for each ionizable proton. Two or more expressions may be necessary to calculate the concentrations of all species present in the acid solution. Two or more expressions may be necessary to calculate the concentrations of all species present in the acid solution.

Diprotic and Polyprotic Acids

Molecular Structure and Strength of Acids Strength of an acid depends on: Strength of an acid depends on: –Structure of the acid –Strength of the bond –Polarity of bond H X H + + X - The stronger the bond The weaker the acid

Molecular Structure and Strength of Acids

Hydrohalic Acids Order of relative acid strength: Order of relative acid strength: HF << HCl < HBr < HI H X H + + X -

Acid-Base Properties of Salts Salt- an ionic compound formed by the reaction between an acid and a base. They are string electrolytes that completely dissociate into ions in water. Salt- an ionic compound formed by the reaction between an acid and a base. They are string electrolytes that completely dissociate into ions in water. Salt Hydrolysis- the reaction of an anion or a cation of a salt, or both, in water. Usually affects the pH of a solution. Salt Hydrolysis- the reaction of an anion or a cation of a salt, or both, in water. Usually affects the pH of a solution.

Salts that produce Neutral Solutions Neutral Solutions: Salts containing an alkali metal or alkaline earth metal ion (except Be 2+ ) and the conjugate base of a strong acid (e.g. Cl -, Br -, and NO 3 - ). NaCl (s) Na + (aq) + Cl - (aq) H2OH2O

Salts that produce Basic Solutions Basic Solutions: Salts derived from a strong base and a weak acid. NaCH 3 COOH (s) Na + (aq) + CH 3 COO - (aq) H2OH2O CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq)

Salts that produce Acidic Solutions Acid Solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 3+2+

Lewis Acids and Bases Lewis Acid- a substance that can accept a pair of electrons. Lewis Base- a substance that can donate a pair of electrons. H+H+ + OH - H O H acidbase H+H+ + N H H H N H H H H + acid base

Lewis Acids and Bases B(OH) 3 (aq) + H 2 O (l) ↔ B(OH 4 ) - (aq) + H + (aq) CO 2 (g) + H 2 O (l) ↔ H 2 CO 3 (aq)