Acid-Base Concepts

1
Acid-Base Concepts

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

2
Acid-Base Concepts
The definitions of an acid and a base range from
proton and hydroxide donors to electron pair
donors and acceptors to specialized definitions
for non-solvent systems.
3
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.
4
Lux-Flood ReactionsAcids are oxide acceptors,
bases are oxide donors
CaO SiO2 ?? CaSiO3
base
acid
CaO H2O ?? Ca2 2 OH-
basic anhydride
SiO2 H2O ?? H2SiO3
acidic anhydride
5
Acid-Base DefinitionsDont Have To Be Limited to
H O
(CH3)4N Cl-
bases are chloride donors
FeCl3
acids are chloride acceptors
6
Solvent SystemsAutoionization
7
Solvent SystemsAcid Leveling
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-
8
Solvent SystemsAcid Leveling
What is the strongest acid and the strongest base
allowed in liquid hydrofluoric acid?
9
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.
10
General Acid-Base Concepts
?more basic
11
General Acid-Base Concepts
Beryllium oxide is amphoteric
high charge density causes polarization of any
electron source
BeO is more acidic and less basic than other
alkaline earth oxides.
12
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
13
General Acid-Base ConceptsHydration and
Hydrolysis
If the cation is acidic enough, it can polarize
the oxygen of water away from the hydrogen.
the pH of 0.1 M Al(NO3)3 is 3.07
Al(H2O)5 (OH)2 H3O
Why is 0.1 M iron(III) ion even more acidic?
14
General Acid-Base ConceptsOxyacids
HmXOn
The acidity of oxyacids depends on
formal charge on the central atom
charge on the complex
inductive effects of substituents
15
General Acid-Base ConceptsOxyacids
HmXOn
if n-m ? 2, strong acid pKa ltlt 0
1, weak acid pKa ? 2.1 0.9
0, very weak acid pKa ? 8.5 1
sequential proton loss leads to pK2 values of 5
greater
Examples
16
(No Transcript)
17
General Acid-Base ConceptsOxyacids
HmXOn
The acidity of oxyacids depends on
formal charge on the central atom
charge on the complex
inductive effects of substituents
pKa 10.5 - 5.0 n - ?x for X(OH)mOn
18
General Acid-Base ConceptsOxyacids
Special Cases
H2SO3 pKa 1.81
H3PO3 pKa 2.00
H2CO3 pKa 6.37
H3BO3 pKa 9.1
19
General Acid-Base ConceptsSubstituted Amines
NR3
The basicity of amines depends on
inductive effects of substituents
steric effects
pKb
NF3
NH3
NH2CH3
NH(CH3)2
N(CH3)3
no basic character
4.74
3.36
3.29
4.28
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)