Chapter 14 Acids and Bases

Chapter 14 Table of Contents Copyright © Cengage Learning. All rights reserved The Nature of Acids and Bases 14.2Acid Strength 14.3The pH Scale 14.4Calculating the pH of Strong Acid Solutions 14.5 Calculating the pH of Weak Acid Solutions 14.6Bases 14.7Polyprotic Acids 14.8Acid–Base Properties of Salts 14.9The Effect of Structure on Acid–Base Properties 14.10Acid–Base Properties of Oxides 14.11The Lewis Acid–Base Model

Section 14.1 The Nature of Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 3 Models of Acids and Bases Arrhenius: Acids produce H + ions in solution, bases produce OH – ions. Brønsted – Lowry: Acids are proton (H + ) donors, bases are proton acceptors. HCl + H 2 O → Cl – + H 3 O + acid base

Section 14.1 The Nature of Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 4 Acid in Water HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base Conjugate base is everything that remains of the acid molecule after a proton is lost. Conjugate acid is formed when the proton is transferred to the base.

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 5 Strong acid:  Ionization equilibrium lies far to the right.  Yields a weak conjugate base. Weak acid:  Ionization equilibrium lies far to the left.  Weaker the acid, stronger its conjugate base.

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 6

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 7 Various Ways to Describe Acid Strength

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 8 Concept Check HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base What is the equilibrium constant expression for an acid acting in water?

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 9 Concept Check If the equilibrium lies to the right, the value for K a is __________. large (or >1) If the equilibrium lies to the left, the value for K a is ___________. small (or <1)

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 10 Concept Check HA(aq) + H 2 O(l) H 3 O + (aq) + A – (aq) If water is a better base than A –, do products or reactants dominate at equilibrium? Does this mean HA is a strong or weak acid? Is the value for K a greater or less than 1?

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 11 Concept Check Consider a 1.0 M solution of HCl. Order the following from strongest to weakest base and explain: H 2 O(l) A – (aq) (from weak acid HA) Cl – (aq)

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 12 Let’s Think About It… How good is Cl – (aq) as a base? Is A – (aq) a good base? The bases from strongest to weakest are: A –, H 2 O, Cl –

Section 14.2 Atomic MassesAcid Strength Return to TOC Copyright © Cengage Learning. All rights reserved 13 Concept Check Acetic acid (HC 2 H 3 O 2 ) and HCN are both weak acids. Acetic acid is a stronger acid than HCN. Arrange these bases from weakest to strongest and explain your answer: H 2 O Cl – CN – C 2 H 3 O 2 –

Section 14.3 The MoleThe pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 14 pH = –log[H + ] A compact way to represent solution acidity. pH decreases as [H + ] increases. Significant figures:  The number of decimal places in the log is equal to the number of significant figures in the original number.

Section 14.3 The MoleThe pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 15 Exercise Calculate the pH for each of the following solutions. a) 1.0 × 10 –4 M H + pH = 4.00 b) M OH – pH = 12.60

Section 14.3 The MoleThe pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 16 Exercise The pH of a solution is What is the [H + ] for this solution? [H + ] = 1.4 × 10 –6 M

Section 14.3 The MoleThe pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 17 pH and pOH Recall: K w = [H + ][OH – ] –log K w = –log[H + ] – log[OH – ] pK w = pH + pOH = pH + pOH

Section 14.3 The MoleThe pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 18 Exercise Calculate the pOH for each of the following solutions. a) 1.0 × 10 –4 M H + pOH = b) M OH – pOH = 1.40

Section 14.4 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 19 Concept Check Consider an aqueous solution of 2.0 x 10 –3 M HCl. What are the major species in solution? H +, Cl –, H 2 O What is the pH? pH = 2.70

Section 14.4 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 20 Concept Check Calculate the pH of a 1.5 x 10 –11 M solution of HCl. pH = 7.00

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 21 Concept Check Consider a 0.80 M aqueous solution of the weak acid HCN (K a = 6.2 x 10 –10 ). What are the major species in solution? HCN, H 2 O

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 22 Let’s Think About It… see page 637 Why aren’t H + or CN – major species?

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 23 Consider This HCN(aq) + H 2 O(l) H 3 O + (aq) + CN – (aq) K a = 6.2 x H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq) K w = 1.0 x Which reaction controls the pH? Explain.

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 24 Exercise Calculate the pH of a 0.50 M aqueous solution of the weak acid HF. (K a = 7.2 x 10 –4 )

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 25 Let’s Think About It… What are the major species in solution? HF, H 2 O Why aren’t H + and F – major species?

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 26 Let’s Think About It… What are the possibilities for the dominant reaction? HF(aq) + H 2 O(l) H 3 O + (aq) + F – (aq) K a =7.2 x H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq) K w =1.0 x Which reaction controls the pH? Why?

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 27 Steps Toward Solving for pH K a = 7.2 x 10 –4 pH = 1.72 HF(aq) +H2OH2OH 3 O + (aq) + F – (aq) Initial0.50 M~ 0 Change–x+x Equilibrium0.50–xxx

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 28 Percent Dissociation (Ionization) For a given weak acid, the percent dissociation increases as the acid becomes more dilute.

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 29 Example at page 642 A solution of M lactic acid (HC 3 HO 2 ) is 3.7% ionized in water. Calculate the K a value for formic acid.

Section 14.5 Calculating the pH of Weak Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 30 Concept Check The value of K a for a 4.00 M formic acid solution should be: higher than lower thanthe same as the value of K a of an 8.00 M formic acid solution.

Section 14.6 Bases Return to TOC Copyright © Cengage Learning. All rights reserved 31 Arrhenius: bases produce OH – ions. Brønsted–Lowry: bases are proton acceptors. In a basic solution at 25°C, pH > 7. Ionic compounds containing OH – are generally considered strong bases.  LiOH, NaOH, KOH, Ca(OH) 2 pOH = –log[OH – ] pH = – pOH

Section 14.6 Bases Return to TOC Copyright © Cengage Learning. All rights reserved 32 Equilibrium expression for weak bases uses K b. CN – (aq) + H 2 O(l) HCN(aq) + OH – (aq)

Section 14.6 Bases Return to TOC Copyright © Cengage Learning. All rights reserved 33 pH calculations for solutions of weak bases are very similar to those for weak acids. K w = [H + ][OH – ] = 1.0 × 10 –14 pOH = –log[OH – ] pH = – pOH

Section 14.6 Bases Return to TOC Copyright © Cengage Learning. All rights reserved 34 Concept Check Calculate the pH of a 2.0 M solution of ammonia (NH 3 ). (K b = 1.8 × 10 –5 ) pH = 11.78

Section 14.7 Polyprotic Acids Return to TOC Copyright © Cengage Learning. All rights reserved 35 Acids that can furnish more than one proton. Always dissociates in a stepwise manner, one proton at a time. The conjugate base of the first dissociation equilibrium becomes the acid in the second step. For a typical weak polyprotic acid: K a1 > K a2 > K a3 For a typical polyprotic acid in water, only the first dissociation step is important to pH.

Section 14.7 Polyprotic Acids Return to TOC Copyright © Cengage Learning. All rights reserved 36 Exercise Calculate the pH of a 1.00 M solution of H 3 PO 4. K a1 = 7.5 x K a2 = 6.2 x K a3 = 4.8 x pH = 1.08

Section 14.7 Polyprotic Acids Return to TOC Copyright © Cengage Learning. All rights reserved 37 Concept Check Calculate the equilibrium concentration of PO 4 3- in a 1.00 M solution of H 3 PO 4. K a1 = 7.5 x K a2 = 6.2 x K a3 = 4.8 x [PO 4 3- ] = 3.6 x M

Section 14.7 Polyprotic Acids Return to TOC Amino acids are polyprotic Amino acids from which proteins are built have an acidic carboxylic acid group, a basic amino group, and a variable substituent designated R: The resulting structure, with positive and negative sites, is called a zwitterion.

Section 14.7 Polyprotic Acids Return to TOC

Section 14.7 Polyprotic Acids Return to TOC

Section 14.7 Polyprotic Acids Return to TOC Finding the pH 1) Titration of polyprotic acids (ex) H 3 PO 4 (H 3 A)

Section 14.7 Polyprotic Acids Return to TOC Finding the pH 2) solutions containing Amphoteric Anions (H 2 A - ) as the only A-B major species

Section 14.7 Polyprotic Acids Return to TOC Finding the pH .. ‚.‚.

Section 14.7 Polyprotic Acids Return to TOC Finding the pH  & 

Section 14.7 Polyprotic Acids Return to TOC Which is the principle species ?

Section 14.7 Polyprotic Acids Return to TOC For H 2 A Which is the principle species ?

Section 14.7 Polyprotic Acids Return to TOC 3) For H 3 A

Section 14.7 Polyprotic Acids Return to TOC What is the principal form of arginine at pH 10.0? Approximately what fraction is in this form? What is the second most abundant form at this pH?

Section 14.7 Polyprotic Acids Return to TOC Proteins are polyprotic acids and bases myoglobin

Section 14.7 Polyprotic Acids Return to TOC Isoelectric pH (isoionic pH) The pH at which the average charge of the polyprotic acid is zero

Section 14.7 Polyprotic Acids Return to TOC (ex)

Section 14.7 Polyprotic Acids Return to TOC Isoelectric focusing : A technique of protein separation pH gradient design Protein stop migrating in an electric field at isoelectric pH

Section 14.8 Acid–Base Properties of Salts Return to TOC Copyright © Cengage Learning. All rights reserved 53 Qualitative Prediction of pH of Salt Solutions (from Weak Parents)

Section 14.8 Acid–Base Properties of Salts Return to TOC Copyright © Cengage Learning. All rights reserved 54 Concept Check Consider a 0.30 M solution of NaF. The K a for HF is 7.2 x What are the major species? Na +, F -, H 2 O

Section 14.9 The Effect of Structure on Acid–Base Properties Return to TOC Copyright © Cengage Learning. All rights reserved 55 Models of Acids and Bases Bond Polarity (high is good) Bond Strength (low is good)

Section 14.9 The Effect of Structure on Acid–Base Properties Return to TOC Copyright © Cengage Learning. All rights reserved 56 Several Series of Oxyacids and Their K a Values

Section 14.9 The Effect of Structure on Acid–Base Properties Return to TOC Copyright © Cengage Learning. All rights reserved 57 Comparison of Electronegativity of X and K a Value

Section Acid–Base Properties of Oxides Return to TOC Copyright © Cengage Learning. All rights reserved 58 Oxides Acidic Oxides (Acid Anhydrides):  O – X bond is strong and covalent. SO 2, NO 2, CO 2 Basic Oxides (Basic Anhydrides):  O – X bond is ionic. K 2 O, CaO

Section The Lewis Acid–Base Model Return to TOC Copyright © Cengage Learning. All rights reserved 59 Lewis Acids and Bases Lewis acid: electron pair acceptor Lewis base: electron pair donor Lewis acid Lewis base

Section The Lewis Acid–Base Model Return to TOC Copyright © Cengage Learning. All rights reserved 60 Three Models for Acids and Bases