Definitions of
Acids and Bases
Green & Damjii – Chapter 8 – Section 1
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There are multiple definitions
for the terms ‘acid’ and ‘base’.
You will need to be familiar
with three (3) of them:
• Arrhenius acids and bases
• Bronsted-Lowry acids and bases
• Lewis acids and bases
4.3
An Arrhenius acid is a substance that produces H+ (H3O+) in water
Complete these reactions:
• Nitric acid plus water yields
• Acetic acid plus water yields
4.3
An Arrhenius base is a substance that produces OH- in water
Complete these reactions:
• sodium hydroxide plus water yields
• ammonia plus water yields
• baking soda plus water yields
4.3
Why didn’t chemists just stick with the Arrhenius definitions of
acids and bases?
• not all acid base reactions occur in an aqueous solvent
A Brønsted-Lowry acid is a proton donor
• H atom attached by a bond that is easily broken
A Brønsted-Lowry base is a proton acceptor
• contains a non-bonding electron pair – usually the
lone pair on an O or N atom
base
acid
acid
base
base
acid
conjugate
acid
conjugate
base
15.1
15.4
Conjugate acid-base pairs:
•
The conjugate base of a strong acid is weak (has no measurable strength).
ex: HCl (strong acid): Cl− (weak conjugate base)
•
The conjugate acid of a strong base is weak (has no measurable strength)..
ex: NaOH (strong base): H2O (weak conjugate acid)
•
The conjugate base of a weak acid is strong.
ex: CH3COOH (weak acid): CH3COO − (strong conjugate base)
•
The conjugate acid of a weak base is strong.
ex: NH3 (weak base): NH4 + (strong conjugate acid)
15.4
Acid-Base Properties of Water
H+ (aq) + OH- (aq)
H2O (l)
referred to as the autoionization of water
H
O
H
+ H
[H
O
H
]
H
+ H
O
-
H
base
H2O + H2O
acid
O
+
conjugate
acid
H3O+ + OHconjugate
base
15.2
Conjugate acid-base pairs:
•
H3O+ is the strongest acid that can exist in aqueous
solution.
ex: H2O (weak base): H3O+ (strong conjugate acid)
•
The OH- ion is the strongest base that can exist in aqueous
solution.
ex: H2O (weak acid): OH- (strong conjugate base)
NOTE: Substances – like water – that can both accept and
donate a proton are called amphiprotic.
15.4
Identify the Bronsted-Lowry conjugate acid base pairs in
each reaction:
CH3COOH(aq) + H2O (l) ⇌ H3O+(aq)
+ CH3COO−(aq)
NH4+ + NH2−  2 NH3 (l)
H2SO4 (aq)  H+(aq) + HSO4− (aq) ⇌ 2 H+(aq) + SO42− (aq)
Why didn’t chemists just stick with the Bronsted-Lowry
definitions of acids and bases?
 Lewis recognized the special nature of the bond between a
base and the proton it accepts…
• dative bond (aka coordinate covalent bond)
• both of the electrons come from the base
• proton is not contributing any electrons to the bond
(it doesn’t have any!)
• these bonds are indicated by an arrow rather than a line
arrow points in the direction the electrons are donated
• these covalent bonds are identical to other covalent bonds
So – acid base reactions now include reactions that involve the
formation of a dative covalent bond.
A Lewis acid is a substance that can accept a pair of electrons
A Lewis base is a substance that can donate a pair of electrons
••
••
H+ + OH••
acid base
H+ +
acid
••
H
N H
H
base
••
H O H
••
H
+
H N H
H
15.12
 Lewis acids include all Bronsted-Lowry acids (which include
all Arrhenius acids)… but the term is typically used just for
acids that are not already included in the other definitions.
EXs:
• boron trifluoride (see next slide)
• aluminum chloride
• any species that can accept an electron pair into its
incomplete valence shell …
Lewis Acids and Bases
F B
+
••
H
F
N H
F
H
acid
base
F
H
F B N H
F
H
No protons donated or accepted – just electron pairs !
Identify the dative bond ! Mark it with an arrow!
15.12
 Lewis bases are any species with a non-bonding electron pair
EXs:
•
molecules that are NOT hydrides of group 3 &/or 4
elements – generally containing an element with lone pair
electrons
• anions
• includes ligands that combine with transition metal ions to
form complex ions (we learned about this in Unit 2)
There are several metal hydroxides that are amphoteric –
• they behave as both acids and bases.
Show how zinc hydroxide and/or aluminum hydroxide
• can act as a base (what type? Lewis)
• can act as an acid (what type? Lewis )