CHEM 163

Chapter 18

Spring 2009Instructor: Alissa Agnelloaagnello@sccd.ctc.edu

1

Acids & Bases in WaterHCl (aq) + NaOH (aq) NaCl (aq) + HOH (l)

H+ (aq) + OH- (aq) H2O (l) Net ionic:

strong acid + strong base water

HA + H2O (l) H3O+ (aq) + A-

Hydronium ion

Neutralization reaction

Acid-Base DefinitionsArrhenius:

what does the substance yield when dissociated in water?• Acid: yields H3O+

• Base: yields OH-

Bronsted-Lowry: Can the substance donate or accept proton(s)?• Acid: donates proton(s)• Base: accepts proton(s) Not all bases contain OH-!

Lewis: • Acid: accepts an e- pair• Base: donates an e- pair

Strong or Weak?• Strong acids and bases dissociate completely in water

HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)

cK OH HCl

Cl OH

2

3

tiny

1Favors products a lot

• Weak acids and bases dissociate only slightly in water

HCN (aq) + H2O (l) H3O+ (aq) + CN- (aq)

cK OH HCN

CN OH

2

3

1 Favors reactants, but both products and

reactants are present

↔

Strong or Weak: Ka

Ka: acid dissociation constant• measure of dissociation• Equals Kc without H2O term ([H2O] changes negligibly)

• Stronger weak acid: high Ka (~10-2)

• Weaker weak acid: low Ka (~10-10)

aK

HA

A OH3

~10% dissociated

~0.001% dissociated

Strong or Weak?

Strong acids:HCl, HBr, HIHNO3, H2SO4, HClO4

Strong bases:Group 1A(1) or 2A(2) ion + OH- or O2-

Oxoacid with most oxygen atoms

LiOHNaOHKOH

Li2O

Na2O

K2OCa(OH)2

Sr(OH)2

Ba(OH)2

CaOSrO

BaO

Group1A(1)

Group2A(2)

Water in Solution?H2O (l) ↔ H3O+ (aq) + OH- (aq)2

cK

22

3

OH

OH OH

wK OH OH314100.1

at 25 °C

Autoionization

↑ [OH-]… [H3O+]? If…

Kw? ↓ no change

↑ [H3O+]… [OH-]? Kw? ↓ no change

14100.1 OH OH3

Calculate the concentration of hydroxide ions in coffee, an aqueous solution containing 1.0 x 10-5 M H3O+ ions

8

Calculate the concentration of hydroxide ions in milk, which contains 5.0 x 10-7 M H3O+ ions

3- minute Practice

pHIn any aqueous solution: [OH-] and [H3O+]

[H3O+] > [OH-]

[H3O+] = [OH-]

[H3O+] < [OH-]

acidic

no change

basic

-pH3 10OH

pH < 7

pH > 7

pH = 7

-pOH10OH

00.14pOH pH

OHlogpH 3 OHlogpOH

3-minute Practice

Calculate the pH of a solution that has…

• [H3O+] = 1.0 x 10-8 M

• [OH-] = 4.0 x 10-2 M

10

pKa

(Low pKa high Ka)

aa KlogpK

pOHpHpKw 00.14

at any T for any (aq) solution

Only at 25 °C

Bronsted-Lowry Acids & Bases• Acids: donate a proton

HCl + H2O H3O+ + Cl-

• Bases: accept a protonNH3 + H2O NH4

+ + OH-

Conjugate acid-base pairs:one loses a proton, other gains a proton

12

A- is the conjugate base of HA BH+ is the conjugate acid of B

HA + B +A- BH+

H+ donor H+ donorH+ acceptor

H+ acceptor

Weak-Acid Calculations• H3O+ from two sources: HA and H2O– Assume H2O portion is negligible– All H3O+ comes from HA

• Weak acid = small Ka

– Assume dissociation is negligible• Make I.C.E. table

initHAHA

HA + H2O (l) H3O+ + A-

Starting with 0.10 M HA….

Polyprotic Acids• Acids with multiple protons to lose

H2SO3 (aq) + H2O (l) H3O+ (aq) + HSO3- (aq)

1aK

32

33

SOH

HSO OH

HSO3- (aq) + H2O (l) H3O+ (aq) + SO3

2- (aq)

3

233

HSO

SO OH2aK

Ka1 > Ka2 > Ka3

Weak BasesA (aq) + H2O (l) ↔ BH+ (aq) + OH- (aq)

cK

A

OH BH

bK

base dissociation constant(base isn’t really dissociating)

bb KlogpK (Low pKb Stronger weak base)high Kb

Proton acceptors: • amines (and NH3)• anions of weak acids wba KKK

For conjugate acid-base pairs:

What is the [OH-] concentration of 2.0 M NaF (aq)? The Kb of F- = 1.5 x 10-11

1.Write reaction2.Set up table

• OH- from water is negligible• Kb is small; base reacted is negligible

3.Solve for x 4.Plug in 5.Check assumptions

Sample Calculation

Acid Strength Trends • Nonmetal hydrides– Across a period, strength ↑ • electronegativity

– Down a group, strength ↑• Size, bond strength

• Oxoacids– More O atoms, strength ↑– Same # O atoms? ↑ electronegativity of

nonmetal, ↑ strength• Hydrated metal ions– High charge density metal ion

Acid-Base Properties of Salt Solutions• Salts dissolve in water• Which salts form…• Neutral Solutions?

• Basic Solutions?• Acidic Solutions?

18

cation from strong base and anion from strong acid

NaNO3

NaNO3 (s) Na+ (aq) + NO3 - (aq)

• Basic solutions form from salts that contain:

• Acidic solutions form from salts that contain:

19

cation from strong base and anion from weak acid

NaF

NaF (s) Na+ (aq) + F - (aq)

cation from weak base and anion from strong acid

NH4Cl

NH4Cl (s) NH4+ (aq) + Cl - (aq)

OH2

OH2

• Weak acid anions + weak base cations?

20

cation from weak base and anion from weak acid

NH4F

NH4F (s) NH4+ (aq) + F - (aq)

NH4+ (aq) + H2O (l)

F- (aq) + H2O (l)

OH2

NH3 (aq) + H3O+ (aq)

HF (aq) + OH- (aq)

Which reaction goes further to the right?

If Kb > Ka : basic If Ka > Kb : acidic

Acid-Base DefinitionsArrhenius: • Acid: yields H3O+ in water

• Base: yields OH- in water

Bronsted-Lowry: • Acid: donates proton(s)• Base: accepts proton(s) Not all bases contain OH-!

Lewis: • Acid: accepts an e- pair• Base: donates an e- pair

Lewis Acids & Bases• Lewis Bases– Contain lone pair(s) of e-

• Lewis Acids– Do not need to contain H

A—BA + B ↔ adduct

H—BH+ + B ↔

Examples of Lewis Acids

• Molecules with e- deficient central atom– Compounds of B or Al

• Molecules with polar multiple bonds

• Metal cations– When they dissolve in water