ACID - BASE CHEMISTRY
hydrogen ion donor
• What is an acid?
• What is a base?
hydrogen ion acceptor
• How does HNO3, a
strong acid, behave
in water?
Strong electrolyte: HNO3 ---> H+ + NO3- 100%
• How does HNO2, a
weak acid, behave
in water?
Weak electrolyte: HNO2 = H+ + NO2- partial
• How does NH3 behave in water?
Weak electrolyte: NH3 + H2O = NH4+ + OH-
• Write a reaction for the general
behavior of any weak acid.
HA + H2O = H3O+ + A-
or
HA = H+ + A-
• Write a reaction for the general
behavior of any weak base.
B + H2O = BH+ + OH-
How about an equilibrium constant
for weak acids?
HA + H2O = H3O+ + A+) (A-)
(H
O
3
K=
(HA) (H2O)
or
HA = H+ + A-
+) (A-)
(H
Ka =
(HA)
acid dissociation constant
For dilute solutions (<1M):
(H2O) = 56 M
and is a constant
How about an equilibrium constant
for weak bases?
B + H2O = BH+ + OH-
Kb =
(BH+) (OH-)
(B)
base dissociation constant
The Bronsted-Lowry Concept
Conjugate pairs
HCl
Cl-
NH4+ NH3
CH3COOH
CH3COO-
HNO3 NO3-
How does a conjugate pair differ?
Behavior of water
at 25oC:
H2 O =
H+
+
OH-
Kw = (H+)(OH-) = 1.0 x 10-14
This reaction is endothermic. Why?
What happens to Kw if temperature
increases?
pH
pH = -log (H+)
(H+) = 10-pH
What is pH of 0.0271M HCl?
If pH = 3.48, what is (H+)?
What is the pH of pure water
at 25oC?
H2O =
H+
+
OH-
Neutrality
Kw = 1.0 x 10-14
Other useful relationships
pX = -log (X)
(X) = 10-pX
_______ = -log (OH-)
pKw = _______
______ = 10-pKa
Kw = (H+)(OH-) convert to log form
How are pH, pOH, (H+), and (OH-)
related at 25oC?
acidic
basic
(H+) > (OH-)
(H+) < (OH-)
neutral
(H+) = (OH-)
acid rain (NOx, SOx)
pH of 4.2 - 4.4 in
Washington DC area
pH
0-14 scale for the chemists
2
3
4
5
acidic
(H+) > (OH-)
normal rain (CO2)
pH = 5.3 – 5.7
6
7
8
neutral @ 25oC
(H+) = (OH-)
distilled water
fish populations
drop off pH < 6
and to zero pH < 5
9
10
11
basic or alkaline
(H+) < (OH-)
natural
waters pH =
6.5 - 8.5
12
Can you have a negative pH?
What is the pH of 6.0M HCl?
What is the pH of 10M NaOH?
What is the pH of 10-8 M HCl?
How about the pH of a weak acid
solution?
To the CHM 102 webpage….
Good pH tutorial:
http://www.chem.ubc.ca/courseware/pH/index.html
practice…
1.
Calculate pH of 0.0016 M NaOH.
2.
What is (H+) of solution with pOH =
2.61.
3.
What is the pH of neutrality at
60oC where Kw = 9.62 x 10-14?
4.
What is the pOH of neutrality at
60oC?
5.
How many times more concentrated
is a pH of 4 compare to a pH of 7?
1.
2.
3.
4.
5.
pH = 11.2
(H+) = 4.1 x 10-12 M
pH = 6.50
pOH = 6.50
1000 x
answers…