100 ACID BASE THEORY / Bronsted-Lowery
XI. ACID-BASE THEORY
A. Bronsted-Lowery Theory
1. Definitions
a) Acid: proton (H+) donor
b) Base: proton (H+) acceptor
Example(1):
HCl
+
H2O
H 3 O+
+ Cl-
Example(2):
NH3
+ H2 O
NH4+
+ OH-
2. Dot formulas
3. Reverse reaction
Example(3):
HF
+
H 2O
Example(4):
H2SO4 + F-
H3 O+
+
F-
HSO4- + HF
101 ACID BASE THEORY / Bronsted-Lowery
4. Conjugate pair: two species that differ by a H+
a) Identifying the conjugate pairs
Example(5): What are the two conjugate pairs in the reaction below?
HF
+
H3 O+
H 2O
+
F-
Example(6): What are the two conjugate pairs in the reaction below?
H2SO4 + F-
HSO4- + HF
Example(7): Identify the acid and base in the conjugate pair below.
NH3
and
NH4+
Example(8): Identify the acid and base in the conjugate pair below.
HSO4-
and
SO42-
102 ACID BASE THEORY / Bronsted-Lowery
b) Strength of members of a conjugate pair
Example(9): Label the HCl and its conjugate in the reaction below as acid or base, and strong or weak.
HCl
+
H3 O+
H 2O
+
Cl-
Example(10): Label the HF and its conjugate in the reaction below as acid or base, and strong or weak.
HF
+
H3 O+
H 2O
+
F-
Example(11): Label the NH3 and its conjugate in the reaction below as acid or base, and strong or weak.
NH3
+
NH4+
H2 O
+
OH-
NOTE: When we first discussed acid and bases, we said that HF was a weak acid in
water, and NH3 was a weak base in water. Water is the usual standard by which we
judge the strength of acids and bases. If you put an acid or a base with something
other than water its strength will be different.
Example(12):
HF
+
NaOH
Example(13):
NH3
+
HCl
Example(13):
NH3
+
O2–
NaF
+
NH4+
NH2–
+
H 2O
+
OH-
Cl-
103 ACID BASE THEORY / Theory of Strength
B. Theory of Strength
1. Binary acids (hydrogen and one nonmetal)
a) Within a period the main factor determining acid strength is EN.
The more EN the parent atom, the stronger the acid.
Example(1): How do the following compounds compare in acid strength?
NH3
H2 O
HF
Example(2): Write an equation showing each of the above compounds reacting with a generic base (B:).
Indicate the relative strength of the acids by using different arrow sizes.
Example(3): How do the following compounds compare in base strength?
NH3
H2 O
HF
Example(4): Write an equation showing each of the above compounds reacting with a generic acid(HX).
Indicate the relative strength of the bases by using different arrow sizes.
104 ACID BASE THEORY / Theory of Strength
b) Within a group the main factor determining acid strength is bond strength.
The weaker the bond between the hydrogen and the parent atom, the stronger
the acid.
Example(5): How do the following compounds compare in acid strength?
HF
HCl
HBr
HI
Example(6): Which acid is the strongest in each of the following pairs:
H2 S
or
HCl
H2 O
or
H2S
H2 S
c) Strength of conjugates
Example(7): How do the following ions compare in base strength?
FClBrI-
Example(8): How do the following ions compare in base strength?
NH2-
OH-
F-
or
HBr
105 ACID BASE THEORY / Theory of Strength
2. Oxyacids (hydrogen, oxygen and a nonmetal)
a) Almost all oxyanions have a corresponding acid
Example(9): What are the corresponding acid of the ions below?
SO42-
ClO4-
PO43-
b) Structure: In all oxyacids the acidic hydrogen(s) are bonded directly to an
oxygen atom(s)
Example(10): Draw the structure of the acids below?
H2SO4
H3PO4
HClO4
c) The greater the EN of the group of atoms, the stronger the acid
Example(11): Which is the stronger acid H–O–H or H–O–Cl? Why?
i) the more oxygen on the central atom, the greater the EN of the group.
Example(12): Which is the strongest acid, HClO4, HClO3, HClO2, or HClO? Why?
Example(13): Which is a stronger acid, H2SO4 or H2SO3? Why?
Example(14): Which is a stronger acid, HNO3 or HNO2? Why?
ii) the higher the EN of the central atom, the greater the EN of the group.
Example(15): Which is the strongest acid, HClO3, HBrO3, or HIO3? Why?
106 ACID BASE THEORY / Lewis Theory
C. Lewis Theory
1. Definitions
a) Acid: an electron pair acceptor
b) Base: an electron pair donor
Example(1): H+
Example(2): NH3
OH-
+
+
BF3
 H2 O
 NH3BF3
2. Strength
a) The lower the EN, the greater the Lewis base strength
Example(3): Do the dot formulas of NH3, H2O and HF. Which is the strongest Lewis base?
Example(4): Why can’t CH4 act as a Lewis base, but CH3- can?
Example(5): How does the base strength of CH3- compare to NH3?