11801 1 Acid Base Concepts

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Acid-Base ConceptsEdward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology



3/22/2012

Acid-Base Concepts

The definitions of an acid and a baserange from proton and hydroxide donors

to electron pair donors and acceptorsto specialized definitions for non-solvent

systems.



3/22/2012

Acid-Base Definitions

Arrhenius - Acids are proton donors,bases are hydroxide donors.

Bronsted-Lowry - Acids are proton donors,bases are proton acceptors.

Lux-Flood - Acids are oxide acceptors,bases are oxide donors.



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Lux-Flood ReactionsAcids are oxide acceptors, bases are oxide donors

CaO + SiO2 CaSiO3

CaO + H2O Ca2+ + 2 OH-

base acid

basicanhydride

SiO2 + H2O H2SiO3

acidicanhydride



Acid-Base Definitions

Don’t Have To Be Limited to H & O

(CH3)4N+ Cl- bases are chloride donors

FeCl3 acids are chloride acceptors

OPCl3 or

OP(OEt)3

(CH3)4N+ Cl- + FeCl3

(CH3)4N+ + FeCl4

-



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Solvent SystemsAutoionization

2 H2O H3O+ + OH-

2 NH3 NH4+ + NH2-

2 OPCl3 OPCl2+ + OPCl4

-

strongest acidallowed in

solvent

strongest baseallowed in

solvent



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Solvent SystemsAcid Leveling

2 H2O H3O+

+ OH-

acids stronger than H3O+

are leveled to H3O+

bases stronger than OH- are leveled to OH-

HNO3 + H2O H3O+ + NO3

-

CaO + H2O Ca2+ + 2 OH-



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Solvent SystemsAcid Leveling

3 HF H2F+ + HF2

-

What is the strongest acid andthe strongest base allowed in

liquid hydrofluoric acid?



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Other Acid-Base Definitions

Lewis - Acids are electron pair acceptor,bases are electron pair donors.

Usanovich - Acids are negative charge acceptors,bases are negative charge donors.



metalloids areamphoteric

General Acid-Base Concepts

m o r e b a

s i c

metal oxides are basic

nonmetal oxidesare acidic



General Acid-Base Concepts

Beryllium oxide is amphoteric

high charge density causes polarization

of any electron source

BeO is more acidic and less basic thanother alkaline earth oxides.

BeO + 2 H+ Be2+ + H2O

BeO + H2O + 2 OH- Be(OH)42-



General Acid-Base ConceptsHydration and Hydrolysis

coordination of the solvent (solvation)releases energy and stabilizes the product

high charge density ions have large

hydration (solvation) energies

Na+ + n H2O [Na(H2O)n]+



General Acid-Base ConceptsHydration and Hydrolysis

Al3+ + 6 H2O [Al(H2O)6]3+

If the cation is acidic enough, it canpolarize the oxygen of water away fromthe hydrogen.

[Al(H2O)5 (OH)]2+ + H3O+

H2O

the pH of0.1 M Al(NO3)3

is 3.07

Why is 0.1 M iron(III) ion even more acidic?



General Acid-Base ConceptsOxyacids

HmXOn The acidity of oxyacids depends on

formal charge on the central atom

inductive effects of substituentscharge on the complex




HmXOn if n-m 2, strong acid pKa << 0

= 1, weak acid pKa 2.1 ± 0.9

= 0, very weak acid pKa 8.5 ± 1

sequential proton loss leads topK2 values of +5 greater

Examples: HNO3

HNO2

HSO4-

HClO2

H3PO4

H2PO4-

HClO3

HClO



HNO3

HNO2

HSO4-

HClO2

H3PO4

H2PO4-

HClO3

HClO

<< 0

3.3

1.9

HClO2

2.1

7.2

HClO3

7.5




HmXOn The acidity of oxyacids depends on

formal charge on the central atom

inductive effects of substituentscharge on the complex

pKa = 10.5 - 5.0 n - x for X(OH)mOn




Special Cases

H2SO3 pKa = 1.81

H3PO3 pKa = 2.00

H2CO3 pKa = 6.37

H3BO3 pKa = 9.1



General Acid-Base ConceptsSubstituted Amines

NR3 The basicity of amines depends on

inductive effects of substituents

steric effects

NF3

no basiccharacter

NH3 NH2CH3 NH(CH3)2 N(CH3)3

4.74

pKb

3.36 3.29 4.28



3/22/2012

11801 1 Acid Base Concepts

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