Chapter 9
Acids and Bases
Autoionization of water
• Equation
• Hydronium ion
–No free H+,
Autoionization
• Dynamic equilibrium
•
Problem
• Write the equilibrium expression
for the autoionization of water.
•
• In pure water at 25oC,
Therefore…
• Kw =
• How much ion is present,
relatively speaking? A lot or a
little?
• If [H+] increases… what must
happen to [OH-]?
What are Acids/Bases?
Acids
• Produce H+
•
• Neutralize bases to
produce salt water
•
Bases
• Produce OH•
• Neutralize acids to
produce salt water
•
Indicators
•
•
•
•
•
Litmus, blue and red
Universal
Phenolphthalein
Red cabbage juice
Phenol red
Strong Acids
• No equilibrium
•
•
• HCl
•
• HI
•
•
• HClO4
Strong Bases
• No equilibrium
•
•
•
• Ba(OH)2
•
•
Practice I
• What is the concentration of
hydronium ion in a 0.0400 M
solution of HNO3?
Practice II
• What is the [OH-] in a solution
of KOH that ha a concentration
of 0.320 M?
Practice III
• What is the [H+] in a 0.400 M
solution of H2SO4?
Practice IV
• What is the [OH-] in a solution
of HCl that has a [H+]= 0.00300
M?
Practice V
• What is the [H+] in a 0.0500 M
Ba(OH)2?
pH
• Logarithmic scale
•
•
• pH
pH
• So… if [H+] =
–pH =
–
–Pure water
pH
• To find [H+] given pH, use
+
–[H ]
=
pH scale
1
Acid
7
14
Neutral
Base
pOH
• Calculated the same way, but using
[OH-]
• pH + pOH =
– From Kw =
Weak Acids and Bases
• If the acid/base is not one of the
strong, it is considered weak
• Majority of acids and bases
• React with water,
Practice I
• Write the equilibrium expression for
formic acid reacting with water
• Keq =
Practice II
• Repeat the above for NH3 reacting
with water
• Keq =
• Also called
Conjugate Acids and Bases
• Write the reaction between nitrous
acid, HNO2 and water
–
• Write the reaction between nitrite
ion and water
–
Ka and Kb
• Multiply
• Cancel items that cancel
• What do you get?
•
Practice
• What is Ka for carbonate ion if Kb =
2.19 x 10-4
pKa and pKb
•
•
• Determining Ka from pKa
–
Polyprotic acids
• Write the dissociations for H3PO4, a
weak acid. There are three steps
• Ka1 = 5.93 e-3
• Ka2 = 6.32 e-8
• Ka3 = 4.84 e-13
Salts and Hydrolysis
• Ions of a strong acids or bases
• Ions of weak acids and bases
Practice
• Predict whether each of the
following will make an acidic, basic,
or neutral solution.
– Mg(NO)3
– K2CO3
– NH4NO3
Buffers
• Resist changes in pH
•
• Why do companies make buffered
aspirin?
–
–
Buffer system
• Conjugate acid/base pair
•
•
• Concentrations of each must be
about equal
Sponge factor
• One ion acts as a sponge for
• One ion acts as a sponge for
• Greater concentration of acid and
base, greater the ability to resist pH
changes
Henderson-Hasselbalch
• pH = pKa + log
[Base]
[Acid]
•
• Tells you what changes in pH will
happen with addition of acid or
base
Practice
• Calculate the pH of a solution that
is 0.00600 M acetic acid and
0.0080 M sodium acetate.
• What is the pH if the concentration
of acetic acid is changed to 0.0090
M?
Practice
• Determine the buffer system ions to
use to have a system pH of 6.1 +/0.3
Carbon Dioxide
•
•
•
•
•
CO2(aq) + H2O(l)  H2CO3(aq)
H2CO3(aq) + H2O(l)  H3O+(aq) + HCO3pKa
Used in blood to maintain pH at 7.45
How?
Blood buffer system
• Use Henderson-Hasselbalch
• pH = pKa + log
[HCO3-(aq)]
[CO2(aq) + H2CO3(aq)]
Blood buffer system
• pKa =
• pH-pKa =
• So… the rest of the equation must
equal 1
• Log 10 =
• Therefore the ratio of HCO3/(CO2+H2CO3) =
Blood buffer system
• CO2 is constantly being replaced by
metabolic processes and is present in
the blood
•
•
Blood buffer system
• CO2 is constantly lost
• If CO2 is removed too fast…
problems arise
–
Smokers
• Inhale CO2
•
– Look at Henderson-Hasselbalch
–
Respiratory Acidosis
• When CO2 not removed or is added
from the environment
•
• pH falls
•
• Death
Equations
• CO2(aq) + H2O(l)  H2CO3(aq)
• H2CO3(aq) + H2O(l)  H3O+(aq) + HCO3•
• LeChatlier’s principal
Respiratory Alkalosis
• CO2 removed too fast
•
• Hyperventilation
•
Metabolic Acidosis
•
• Body attempts to compensate by adding
CO2
•
• Help buffer system by adding HCO3-
Metabolic Alkalosis
•
•
• Eating too many Rolaids
•
• Add NH4Cl
Titration
•
•
•
•
Lab experiment
React an acid and base to form water
Measure the endpoint with indicator
Use MaVa = MbVb
Practice
• What volume of 0.200 M NaOH is
needed to completely neutralize 40.00
mL of 0.0855 M HCl?
Practice II
• What volume of 0.250 M KOH is
needed to neutralize 50.0 mL of 0.250
M H2SO4?
Practice III
• What is the concentration of HCl if
35.00 mL of the acid are neutralized by
61.50 mL of 0.500 M NaOH?