John E. McMurry Robert C. Fay C H E M I S T R Y Chapter 14 Aqueous Equilibria: Acids and Bases

Acid-Base Concepts: The Brønsted-Lowry Theory Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H + Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH 1  M + (aq) + OH  (aq)MOH(aq) H + (aq) + A  (aq)HA(aq)

Acid-Base Concepts: The Brønsted-Lowry Theory Conjugate Acid-Base Pairs: Chemical species whose formulas differ only by one hydrogen ion, H + Brønsted-Lowry Acid: A substance that can transfer hydrogen ions, H +. In other words, a proton donor Brønsted-Lowry Base: A substance that can accept hydrogen ions, H +. In other words, a proton acceptor

Acid-Base Concepts: The Brønsted- Lowry Theory Acid-Dissociation Equilibrium

Acid-Base Concepts: The Brønsted- Lowry Theory Base-Dissociation Equilibrium

Examples Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water H 2 CO 3 (aq) HSO 3 - (aq)

Examples In the following equation, label each species as an acid or a base. Identify the conjugated acid-base pair a.NO 2 - (aq) + HF(aq) HNO 2 (aq) + F - (aq) b.HClO 4 (aq) + H 2 O(l) H 3 O + (aq) + ClO 4 - (aq)

14.2 Acid Strength and Base Strength H 3 O + (aq) + A  (aq)HA(aq) + H 2 O(l) BaseAcidBaseAcid Weaker acid + Weaker baseStronger acid + Stronger base With equal concentrations of reactants and products, what will be the direction of reaction? The reaction almost goes completely to the right, to the formation of a weaker acid. Why?

Acid Strength and Base Strength Weak Acid: An acid that is only partially dissociated in water and is thus a weak electrolyte.

Examples If you mix equal amount concentrations of reactants and products, decide which species (reactants or products) are favored at the completion of the reaction ◦ H 2 SO 4 (aq) + NH 3 (aq) NH 4 + (aq) + HSO 4 - (aq) ◦ HF(aq) + NO 3 -(aq) HNO 3 (aq) + F - (aq) ◦ H 2 S(aq) + F-(aq) HF(aq) + HS - (aq)

14.3 Hydrated Protons and Hydronium Ions H + (aq) + A  (aq)HA(aq) [H(H 2 O) n ] + For our purposes, H + is equivalent to H 3 O +. n = 4H 9 O 4 + n = 1H 3 O + n = 2H 5 O 2 + n = 3H 7 O 3 + Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules.

14.4 Dissociation of Water

Dissociation of Water H 3 O + (aq) + OH  (aq)2H 2 O(l) K w = [H 3 O + ][OH  ] Ion-Product Constant for Water: Dissociation of Water: K w = (1.0 x 10  7 )(1.0 x 10  7 ) = 1.0 x 10  14 at 25 o C: [H 3 O + ] = [OH  ] = 1.0 x 10  7 M [OH  ] = [H 3 O + ] =

Dissociation of Water Acidic: [H 3 O + ] > [OH  ] Neutral: [H 3 O + ] = [OH  ] Basic: [H 3 O + ] < [OH  ]

The pH Scale © 2012 Pearson Education, Inc. pH=  log[H 3 O + ] [H 3 O + ] = 10  pH Acidic:pH < 7 Neutral:pH = 7 Basic:pH > 7

The pH Scale The hydronium ion concentration for lemon juice is approximately M. What is the pH when [H 3 O + ] = M? pH =  log(0.0025) = significant figures 2 decimal places

Examples At 50oC the value of Kw is 5.5 x What are the concentration of H 3 O + and – OH in a neutral solution at 50 o C? Calculate the [ - OH] in a solution with [H 3 O + ] = 7.5 x M. Is this solution basic, acidic or neutral? Calculate the pH of a solution with [ - OH] = 8.2 x M Calculate the concentration of H 3 O + and – OH for a solution with a pH of 8.37

14.6 Measuring pH Acid-Base Indicator: A substance that changes color in a specific pH range. Indicators exhibit pH-dependent color changes because they are weak acids and have different colors in their acid (H I n) and conjugate base ( I n  ) forms. H 3 O + (aq) + I n  (aq)H I n(aq) + H 2 O(l) Color BColor A

Measuring pH

14.7 pH of strong acids and strong bases A strong monoprotic acds – 100% dissociated in aqueous solution (HClO 4, HCl, HBr, HI, H2SO4 HNO 3 etc…) ◦ Contains a single dissociable proton HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq) ◦ pH = - log H 3 O + ◦ [H 3 O + ] = [A - ] = initial concentration of the acid ◦ Undissociated [HA] = 0

Strong Bases Alkali metal hydroxide, MOH ◦ Water-soluble ionic solids ◦ Exits in aqueous solution as alkali metal cations and hydroxide anions ◦ Calculate pH from [ - OH] Alkaline earth metal hydroxide, M(OH) 2 where M= Mg, Ca, Sr, Ba ◦ Less soluble than alkali hydroxide, therefore lower [ - OH]

The pH in Solutions of Strong Acids and Strong Bases Since HNO 3 is a strong acid, [H 3 O + ] = [HNO 3 ]. H 3 O + (aq) + NO 3  (aq)HNO 3 (aq) + H 2 O(l) pH =  log([H 3 O + ]) =  log(0.025) = % What is the pH of a M solution of HNO 3 ?

Example What is the pH of a M solution of NaOH? Calculate the pH of a solution by dissolving 0.25 g of CaO in enough water to make 0.500L of limewater Ca(OH) 2 Calculate the [ - OH] and pH of 0.30 M HCl

14.8 Equilibria in Solutions of Weak Acids Acid-Dissociation Constant: H 3 O + (aq) + A  (aq)HA(aq) + H 2 O(l) [H 3 O + ][A  ] [HA] K a = pK a = - log K a

Calculating Equilibrium Concentrations in Solution of Weak acids Step 1: Write the balance equation for all possible proton transfer (both acids and water) Step 2: Identify the principle reaction (the reaction that has larger Ka) Step 3: Generate an ICE table Step 4: Solve for x Step 5: Calculate pH and all other concentrations (HA, H 3 O + and A-)

Example Determine the concentration of all species present (H 3 O +, CH 3 CO 2 -, CH 3 CO 2 H) and pH of a M CH 3 CO 2 H K a = 1.8 x 10 -5

Example Determine the concentration of all species present (H 3 O +, CH 3 CO 2 -, CH 3 CO 2 H) and pH of a 1.00 M CH 3 CO 2 H K a = 1.8 x 10 -5

Example Find the pH and [ - OH] of a M HClO 2 solution K a = 1.1 x 10 -2

Equilibria in Solutions of Weak Acids The pH of M HF is What are the values of K a and pK a for hydrofluoric acid? H 3 O + (aq) + F  (aq)HF(aq) + H 2 O(l)

Percent Dissociation in Solutions of Weak Acids Percent dissociation = [HA] initial [HA] dissociated x 100%

Example Find the percent ionization of M HClO 2 solution (previous example) Nicotine acid (niacin) is a monoprotic acid with the formula HC 6 H 4 NO 2. A solution that is 0.012M in nicotine acid has a pH of 3.49 at 25 o C. What is the degree of ionization of nicotine acid in this solution?