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CHAPTER 15

Acids and Bases

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•  Sour taste •  React with “active” metals

 i.e., Al, Zn, Fe, but not Cu, Ag, or Au 2 Al + 6 HCl → 2 AlCl3 + 3 H2

 corrosive •  React with carbonates, producing CO2

 marble, baking soda, chalk, limestone CaCO3 + 2 HCl → CaCl2 + CO2 + H2O

•  Change color of vegetable dyes  blue litmus turns red

•  React with bases to form ionic salts

Properties of Acids!

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Table 18.1

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Structures of Acids

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•  Binary acids have acid hydrogens attached to a nonmetal atom  HCl, HF

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Structure of Acids

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•  Oxy acids have acid hydrogens attached to an oxygen atom  H2SO4, HNO3

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Structure of Acids

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•  Carboxylic acids have COOH group  HC2H3O2, H3C6H5O7

•  Only the first H in the formula is acidic  the H is on the COOH

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Properties of Bases

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•  Also known as alkalis •  Taste bitter

 alkaloids = plant product that is alkaline  often poisonous

•  Solutions feel slippery •  Change color of vegetable dyes

 different color than acid  red litmus turns blue

•  React with acids to form ionic salts  neutralization

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Common Bases

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Structure of Bases

•  Most ionic bases contain OH− ions – NaOH, Ca(OH)2

•  Some contain CO32− ions

– CaCO3 NaHCO3 •  Molecular bases contain structures

that react with H+

– mostly amine groups

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Arrhenius Theory •  Bases dissociate in water to produce OH− ions and cations

–  ionic substances dissociate in water NaOH(aq) → Na+(aq) + OH−(aq)

•  Acids ionize in water to produce H+ ions and anions –  because molecular acids are not made of ions, they cannot

dissociate –  they must be pulled apart, or ionized, by the water

HCl(aq) → H+(aq) + Cl−(aq) –  in formula, ionizable H written in front

HC2H3O2(aq) → H+(aq) + C2H3O2−(aq)

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Arrhenius Theory

HCl ionizes in water, producing H+ and Cl– ions

NaOH dissociates in water, producing Na+ and OH– ions

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Hydronium Ion •  The H+ ions produced by the acid are so reactive they cannot exist in

water –  H+ ions are protons!!

•  Instead, they react with water molecules to produce complex ions, mainly hydronium ion, H3O+

H+ + H2O → H3O+

–  there are also minor amounts of H+ with multiple water molecules, H(H2O)n

+

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Arrhenius Acid–Base Reactions

•  The H+ from the acid combines with the OH− from the base to make a molecule of H2O –  it is often helpful to think of H2O as H-OH

•  The cation from the base combines with the anion from the acid to make a salt

acid + base → salt + water HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

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Typical Arrhenius Acids, Bases

An acid is a substance that has H in its formula and dissociates in water to yield H+ (H3O+).

A base is a substance that has OH in its formula and dissociates in water to yield OH-.

HX + H2O H3O+ + X-

YOH OH- + Y+

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Acid-Base Reactions: Neutralization

When an acid and a base react, the H+ ion from the acid reacts with the OH- ion from the base to form water and a salt.

HCl + KOH H2O + KCl

When the acid and base are strong, the reaction is complete and fully go to the products; thus, all strong a/b reactions are similar.

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Figure 18.1

Strong acid: HA(g or l) + H2O(l) H3O+(aq) + A-(aq)

Strong Acids

100% dissociation: [H3O+] ~ [HA]initial

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HNO3 is a Strong Acid

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Figure 18.2

Weak acid: HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Weak Acids.

€

Ka =[H3O

+][A−][HA]

Ka << 1

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Acetic Acid is A Weak Acid

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Ka and Acids

Stronger acid higher [H3O+] larger Ka

Ka % Dissoc. Acid (1M) [H3O+] ~10-2 ~10% HClO2 0.1

~10-5 ~0.3% CH3COOH 0.0042

~10-10 ~0.0025% HCN 0.000025

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AC

ID S

TREN

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Some Guidelines Strong Acids:

1.  Hydrohalic acids: HCl, HBr, HI (not HF)

2.  Oxoacids: HNO3, H2SO4, HClO4.

Weak Acids: 1.  HF

2.  H is not bonded to O or a halogen : HCN, H2S.

3.  Oxoacids where #O =, or exceeds by 1, the number of inoizable protons: H3PO4, HNO2, HClO.

4.  Carboxylic acids (RCOOH).

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Strong Base

100% dissociation: [OH-] ~ [YOH]initial

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Weak Base

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Some Guidelines Strong Bases:

1.  Group IA oxides or hydroxides (M2O, MOH).

2.  Oxides (MO) or hydroxides (M(OH)2) of Ca, Sr, Ba.

Weak Bases: 1.  NH3

2.  Amines in general (RNH2, R2NH, R3N): CH3Ch2NH2, (CH3)2NH, (C3H7)3N.

3.  Note all the above has an N with a lone electron pair.

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H2O(l) H2O(l)

H3O+(aq) OH-(aq)

+

+

Autoionization of Water and the pH Scale

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Kc = [H3O+][OH-]

[H2O]2

Kc[H2O]2 = [H3O+][OH-]

The Ion-Product Constant for Water

Kw =

A change in [H3O+] causes an inverse change in [OH-].

= 1.0 x 10-14 at 25oC

H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

In an acidic solution, [H3O+] > [OH-]

In a basic solution, [H3O+] < [OH-]

In a neutral solution, [H3O+] = [OH-]

Autoionization of Water and Kw

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Figure 18.4

[H3O+] [OH-] Divide into Kw

ACIDIC SOLUTION

BASIC SOLUTION

[H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-]

NEUTRAL SOLUTION

The relationship between [H3O+] and [OH-] and the relative acidity of solutions.

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Figure 18.5

pH = -log [H3O+]

The pH values of some familiar aqueous

solutions.

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Acid Name (Formula) Ka at 25oC pKa

Hydrogen sulfate ion (HSO4-) 1.0x10-2

Nitrous acid (HNO2)

Acetic acid (CH3COOH)

Hypobromous acid (HBrO)

Phenol (C6H5OH)

7.1x10-4

1.8x10-5

2.3x10-9

1.0x10-10

1.99

3.15

4.74

8.64

10.00

Table 18.3 The Relationship Between Ka and pKa

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Figure 18.6 The relations among [H3O+], pH, [OH-], and pOH.

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Figure 18.7

pH (indicator) paper

pH meter

Methods for measuring the pH of an aqueous solution.

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Figure 18.8

(acid, H+ donor) (base, H+ acceptor)

Lone pair binds H+

(base, H+ acceptor) (acid, H+ donor)

Lone pair binds H+

Proton transfer as the essential feature of a Brønsted-Lowry acid-base reaction.

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An acid-base reaction can now be viewed from the standpoint of the reactants AND the products.

An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair.

An acid is a proton donor, any species which donates a H+.

A base is a proton acceptor, any species which accepts a H+.

Brønsted-Lowry Acid-Base Definition

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Base Acid + Acid Base +

Conjugate Pair

Conjugate Pair

Reaction 4 H2PO4- OH- +

Reaction 5 H2SO4 N2H5+ +

Reaction 6 HPO42- SO3

2- +

Reaction 1 HF H2O + F- H3O+ +

Reaction 3 NH4+ CO3

2- +

Reaction 2 HCOOH CN- + HCOO- HCN +

NH3 HCO3- +

HPO42- H2O +

HSO4- N2H6

2+ +

PO43- HSO3

- +

Table 18.4 The Conjugate Pairs in Some Acid-Base Reactions

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Sample Problem 18.5

PROBLEM: Predict the net direction and whether Kc is greater or less than 1 for each of the following reactions (assume equal initial concentrations of all species):

(b) H2O(l) + HS-(aq) OH-(aq) + H2S(aq)

(a) H2PO4-(aq) + NH3(aq) HPO4

2-(aq) + NH4+(aq)

SOLUTION:

PLAN: Identify the conjugate acid-base pairs and then consult Figure 18.9 (button) to determine the relative strength of each. The stronger the species, the more preponderant its conjugate.

(a) H2PO4-(aq) + NH3(aq) HPO4

2-(aq) + NH4+(aq)

stronger acid weaker acid stronger base weaker base Net direction is to the right with Kc > 1.

(b) H2O(l) + HS-(aq) OH-(aq) + H2S(aq) stronger base weaker base stronger acid weaker acid

Net direction is to the left with Kc < 1.

Predicting the Net Direction of an Acid-Base Reaction

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Figure 18.9

Strengths of conjugate acid-base

pairs.

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Calculating [H3O+] and [OH-]: ICE Approach

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Using or Finding Ka of Weak Acids and Bases

• Given Ka, find equilibrium concentration

• Given equilibrium concentration, find Ka

Note: Ka is for the following reaction:

AH + H2O H3O+ + A-

If you are given Ka for a base, then you must convert to Kb before using it for the reaction

B + H2O OH- + BH+

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Ka = 1.8 x 10-5 = [H3O+][OAc-][HOAc]

= x2

1.00 - x

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Approximating

Ka = 1.8 x 10-5 = x2

1.00 x = [H3O+] = [Ka • 1.00]1/2

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Example of Non-Compliant Condition

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Kb = 1.8 x 10-5 = [NH4+][OH-]

[NH3] = x2

0.010 - x

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Strong Acid and Strong Base

Mixing equal molar quantities of a strong acid and strong base produces a neutral solution.

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Strong Acid and Weak Base

Mixing equal molar quantities of a strong acid and weak base produces

the bases’s conjugate acid. The solution is acid.

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Strong Base and Weak Acid

Mixing equal molar quantities of a weak acid and strong base produces

the acid’s conjugate base. The solution is basic.

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Weak Acid and Weak Base

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Summary of Acid and Base Reactions

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Percent HA dissociation = [HA]dissociated

[HA]initial x 100

acids with more than one ionizable proton

H3PO4(aq) + H2O(l) H2PO4-(aq) + H3O+(aq)

H2PO4-(aq) + H2O(l) HPO4

2-(aq) + H3O+(aq)

HPO42-(aq) + H2O(l) PO4

3-(aq) + H3O+(aq)

Ka1 = [H3O+][H2PO4-]

[H3PO4]

Ka2 = [H3O+][HPO42-]

[H2PO4-]

Ka3 = [H3O+][PO43-]

[HPO42-]

Ka1 > Ka2 > Ka3

= 7.2x10-3

= 6.3x10-8

= 4.2x10-13

Polyprotic acids

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CID

STR

ENG

TH

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BA

SE S

TREN

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Kb = [BH+][OH-] [B]

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Relationship Between Ka and Kb

HA + H2O H3O+ + A-

€

Ka =[H3O

+][A−][HA]

A- + H2O OH- + HA

€

Kb =[HA][OH −]

[A−]

Acid

Conjugate Base

€

KaKb = Kw

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B + H2O BH+ + OH-

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Kb = (BH+ )(OH − )(B)

Since Kw = KaKb or Ka = Kw/Kb

or Ka = Kw/Kb

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Ka = (H +)(B)(BH+)

BH+ + H2O H3O+ + B Conjugate acid to B

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Figure 18.11

6A(16)

H2O

H2S

H2Se

H2Te

7A(17)

HF

HCl

HBr

HI

Electronegativity increases, acidity increases

Bon

d st

reng

th d

ecre

ases

, ac

idity

incr

ease

s

The effect of atomic and molecular properties on nonmetal hydride acidity.

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H O I H O Br H O Cl < <

H O Cl

O

O

O <<

Figure 18.12

δ+ δ- δ+ δ- δ+ δ-

H O Cl

δ+ δ- δ+ δ-

The relative strengths of oxoacids.

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Free Ion Hydrated Ion Ka

Fe3+ Fe(H2O)63+(aq) 6 x 10-3

Sn2+ Sn(H2O)62+(aq) 4 x 10-4

Cr3+ Cr(H2O)63+(aq) 1 x 10-4

Al3+ Al(H2O)63+(aq) 1 x 10-5

Cu2+ Cu(H2O)62+(aq) 3 x 10-8

Pb2+ Pb(H2O)62+(aq) 3 x 10-8

Zn2+ Zn(H2O)62+(aq) 1 x 10-9

Co2+ Co(H2O)62+(aq) 2 x 10-10

Ni2+ Ni(H2O)62+(aq) 1 x 10-10

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Table 18.7 Ka Values of Some Hydrated Metal Ions at 25oC

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Al(H2O)5OH2+ Al(H2O)63+

The acidic behavior of the hydrated Al3+ ion.

H2O H3O+

Electron density drawn toward Al3+

Nearby H2O acts as base

Figure 18.13

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F

B

F F

H

N

H H+

F

B

F F

H

N

H H

acid base adduct

An acid is an electron-pair acceptor.

A base is an electron-pair donor.

Molecules as Lewis Acids

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The Mg2+ ion as a Lewis acid in the chlorophyll molecule.

Figure 18.14