Chapter 17 Additional Aspects of
Aqueous Equilibria
Aqueous
Equilibria
The Common Ion Effect
Aqueous
Equilibria
The common ion effect is an event that occurs when
an ionic equilibrium shifts due to the addition of a
solute that contains an ion that is the same as an
existing ion in the equilibrium.
Aqueous
Equilibria
An Example of the Common-Ion Effect
HC2H3O2(aq) + H2O(l)
H3O+(aq) + C2H3O2−(aq)
Aqueous
Equilibria
Example Problem The Common-Ion Effect
Calculate the fluoride ion concentration and pH of a
solution that is 0.20 M in HF and 0.10 M in HCl.
Ka for HF is 6.8  10−4.
[H3O+] [F−]
Ka =
= 6.8  10-4
[HF]
Aqueous
Equilibria
The Common-Ion Effect
H3O+(aq) + F−(aq)
HF(aq) + H2O(l)
Because HCl, a strong acid, is also present,
the initial [H3O+] is not 0, but rather 0.10 M.
Initially
Change
At Equilibrium
[HF], M
[H3O+], M
[F−], M
0.20
0.10
0
−x
0.20 − x  0.20
+x
0.10 + x  0.10
+x
x
Aqueous
Equilibria
The Common-Ion Effect
6.8 
10−4
(0.10) (x)
=
(0.20)
(0.20) (6.8  10−4)
=x
(0.10)
1.4  10−3 = x
Aqueous
Equilibria
The Common-Ion Effect
• Therefore, [F−] = x = 1.4  10−3
[H3O+] = 0.10 + x = 0.10 + 1.4  10−3 = 0.10 M
• So,
pH = −log (0.10)
pH = 1.00
Aqueous
Equilibria
Buffers
Aqueous
Equilibria
A buffer is an acid-base solution that resist changes in pH by
neutralizing the effects of external acids and bases.
NH4Cl and NH3 is a buffer system.
Aqueous
Equilibria
Examples of five buffers.
Three solutions and two
powders.
Aqueous
Equilibria
SEM of a red blood cells
passing through a small artery
Blood is a suspension
and the serum of part
of blood is buffered
at 7.35-7.45
Aqueous
Equilibria
Buffers
If a small amount of hydroxide is added to an
equimolar solution of HF in NaF, for example, the
Aqueous
HF reacts with the OH− to make F− and water. Equilibria
Buffers
If acid is added, the F− reacts to form HF and water.
Aqueous
Equilibria
Buffers
If a small amount of hydroxide is added to an
equimolar solution of HF in NaF, for example, the HF
Aqueous
reacts with the OH− to make F− and water.
Equilibria
Derivation of the Henderson-Hasselbalch
Equation
Aqueous
Equilibria
A specific equation exists for calculating the pH of a
buffer ..i.e. the Henderson-Hasselbalch (H-H) equation.
Aqueous
Equilibria
Buffer Calculations
Consider the equilibrium constant
expression for the dissociation of a generic
acid, HA:
HA + H2O
H 3O + + A −
[H3O+] [A−]
Ka =
[HA]
Aqueous
Equilibria
Buffer Calculations
Rearranging slightly, this becomes
−]
[A
Ka = [H3O+]
[HA]
Taking the negative log of both side, we get
−]
[A
−log Ka = −log [H3O+] + −log
[HA]
pKa
pH
base
acid
Aqueous
Equilibria
Buffer Calculations
• So
[base]
pKa = pH − log
[acid]
• Rearranging, this becomes
[base]
pH = pKa + log
[acid]
• This is the Henderson–Hasselbalch equation.
Aqueous
Equilibria
Practice Problem: Henderson–Hasselbalch Equation
What is the pH of a buffer that is 0.12 M in lactic
acid, HC3H5O3, and 0.10 M in sodium lactate? Ka
for lactic acid is
1.4  10−4.
Aqueous
Equilibria
Henderson–Hasselbalch
Equation
[base]
pH = pKa + log
[acid]
pH = −log (1.4 
10−4)
(0.10)
+ log (0.12)
pH = 3.85 + (−0.08)
pH = 3.77
Aqueous
Equilibria
pH Range
• The pH range is the range of pH values over
which a buffer system works effectively.
• It is best to choose an acid with a pKa close to the
desired pH.
Aqueous
Equilibria
When Strong Acids or Bases
Are Added to a Buffer…
…it is safe to assume that all of the strong acid
or base is consumed in the reaction.
Aqueous
Equilibria
Addition of Strong Acid or
Base to a Buffer
1. Determine how the neutralization
reaction affects the amounts of
the weak acid and its conjugate
base in solution.
2. Use the Henderson–Hasselbalch
equation to determine the new
pH of the solution.
Aqueous
Equilibria
Calculating pH Changes in Buffers
A buffer is made by adding 0.300 mol
HC2H3O2 and 0.300 mol NaC2H3O2 to
enough water to make 1.00 L of
solution. The pH of the buffer is 4.74.
Calculate the pH of this solution after
0.020 mol of NaOH is added.
Aqueous
Equilibria
Calculating pH Changes in Buffers
Before the reaction, since
mol HC2H3O2 = mol C2H3O2−
pH = pKa = −log (1.8  10−5) = 4.74
Aqueous
Equilibria
Calculating pH Changes in Buffers
The 0.020 mol NaOH will react with 0.020 mol of the
acetic acid:
HC2H3O2(aq) + OH−(aq)  C2H3O2−(aq) + H2O(l)
HC2H3O2
C2H3O2−
OH−
Before reaction
0.300 mol
0.300 mol 0.020 mol
After reaction
0.280 mol
0.320 mol 0.000 mol
Aqueous
Equilibria
Calculating pH Changes in Buffers
Now use the Henderson–Hasselbalch equation to
calculate the new pH:
(0.320)
pH = 4.74 + log
(0. 200)
pH = 4.74 + 0.06
pH = 4.80
Aqueous
Equilibria
Acid-Base Titrations
Aqueous
Equilibria
Titration
A lab method used to determine
the concentration of unknown
solutions.
Aqueous
Equilibria
Titration
For an acid-base titration the equivalence point is
the state where the moles of acid in the solution is equal to
the moles base in the solution.
Aqueous
Equilibria
Titration
For an acid-base titration the endpoint is when the indicator
changes color.
Aqueous
Equilibria
Strong Acid-Strong Base Titration
Aqueous
Equilibria
Titration of a Strong Acid with a Strong
Base
From the start of the
titration to near the
equivalence point,
the pH goes up slowly.
Aqueous
Equilibria
Titration of a Strong Acid with a Strong
Base
Just before and after the
equivalence point, the
pH increases rapidly.
Aqueous
Equilibria
Titration of a Strong Acid with a Strong
Base
At the equivalence point,
moles acid = moles base
Aqueous
Equilibria
Titration of a Strong Acid with a Strong Base
As more base is
added, the increase in
pH again levels off.
Aqueous
Equilibria
Weak Acid-Strong Base Titration
Aqueous
Equilibria
Titration of a Weak Acid with a Strong
Base
• Unlike in the previous
case, the conjugate base
of the acid affects the pH
when it is formed.
• The pH at the
equivalence point will be
>7.
• Phenolphthalein is
commonly used as an
indicator in these
titrations.
Aqueous
Equilibria
Titration of a Weak Acid with a Strong
Base
With weaker acids, the
initial pH is higher and
pH changes near the
equivalence point are
more subtle.
Aqueous
Equilibria
Strong Acid-Weak Base Titration
Aqueous
Equilibria
Titration of a Weak Base with a Strong
Acid
• The pH at the
equivalence point in
these titrations is < 7.
• Methyl red is the
indicator of choice.
Aqueous
Equilibria
Titration of Polyprotic Acids
Aqueous
Equilibria
A polyprotic acid is a Bronsted acid with two or more
ionizable protons.
H2SO4 is the only polyprotic acid that is a strong acid.
Aqueous
Equilibria
Titrations of Polyprotic Acids
Polyprotic acids will have
An equivalence point for
each ionizable proton.
Aqueous
Equilibria
Solubility Product Constant (Ksp)
Aqueous
Equilibria
The solubility product constant (Ksp) is an equilibrium
constant for a slightly soluble salt.
BaSO4(s)
Ba2+(aq) + SO42−(aq)
Ksp = [Ba2+] [SO42−]
Aqueous
Equilibria
Ksp is used in calculating the solubility of a slightly
soluble salt.
Solubility is given as g/L which can be converted
to molar solubility mol/L (M).
Aqueous
Equilibria
Relationship Between Solubility and Ksp
Aqueous
Equilibria
Practice Exercise: Calculating Molar Solubility
Calculate the molar solubility of Al(OH)3 if the Ksp is
1.8 x 10-33 .
Aqueous
Equilibria
Factors Affecting Solubility
Aqueous
Equilibria
Three factors affect the solubility of a slightly soluble
salt.
(1) A Common Ion (solubility decreases)
(2) pH (solubility increases for a basic anion in an acidic
solution and for an acidic cation in a basic solution)
(3) Complex Ion Formation (solubility increases)
Aqueous
Equilibria
Effect of a Common Ion on Solubility
An ion that is already
dissolved in a solution
will shift the equilibrium to
the left which decreases the
solubility of the salt.
Aqueous
Equilibria
Practice Exercise: Calculating Molar Solubility in the
Presence of a Common Ion
a.Calculate the molar solubility of AgBr.
b. Calculate the molar solubility of AgBr in a 0.030
M AgNO3 solution.
c. Calculate the molar solubility of AgBr in a 0.10 M
NaBr solution.
.
Aqueous
Equilibria
Effect of pH on Solubility
Aqueous
Equilibria
Explain how decreasing the pH increases the solubility
of this precipitate.
Basic anions react with acid shifting the equilibrium
to the right.
Aqueous
Equilibria
Explain how lower pH can cause sinkholes?
Limestone is
CaCO3.
Rainwater has a pH
~ 5-6
Aqueous
Equilibria
Effect of Complex Ions on Solubility
Recall metal ions are Lewis acids and can react with Lewis
bases to form complex ions.
The rxn is an equilibrium and is described by the
equilibrium constant Kf.
Aqueous
Equilibria
Effect of Complex Ions on Solubility
Write a chemical rxn and give an explanation that
accounts for the dissolution of the solid.
Aqueous
Equilibria
Factors Affecting Solubility
• Amphoterism
 Amphoteric metal
oxides and hydroxides
are soluble in strong
acid or base, because
they can act either as
acids or bases.
 Examples of such
cations are Al3+, Zn2+,
and Sn2+.
Aqueous
Equilibria
Predicting the Formation of a Precipitate
Q = Ksp, no change
Q < Ksp, more solid dissolves until Q = Ksp.
Q > Ksp, the salt will precipitate until Q = Ksp.
Aqueous
Equilibria
Qualitative Analysis of Ions
by Selective Precipitation
Aqueous
Equilibria
What is selective precipitation (ppt) ?
Selective ppt is a qualitative analysis method where ions
are separated and identified using a series of precipitation
rxns.
Aqueous
Equilibria
General Flowchart for an Analysis
of an Unknown Aqueous Solution
Aqueous
Equilibria
Qualitative Analysis of Ions by Selective Precipitation
Aqueous
Equilibria
Practice Problems
Aqueous
Equilibria
Equation I 1-3 refer to Equation I and Figure I
Questions
Equation I
Figure I
Figure I
Graph A
Graph B
Graph C
Graph A
Graph B
Graph C
1. Which graph represents the change in solubility of BaCO3 if HBr
is added to a saturated solution of BaCO3.
a. Graph A
b. Graph B
c. Graph C
Aqueous
Equilibria
Equation I 1-3 refer to Equation I and Figure I
Questions
Equation I
Figure I
Figure I
Graph A
Graph B
Graph C
Graph A
Graph B
Graph C
2. Which graph represents the change in solubility of BaCO3 if
K2SO4 is added to a saturated solution of BaCO3.
a. Graph A
b. Graph B
c. Graph C
Aqueous
Equilibria
Equation I 1-3 refer to Equation I and Figure I
Questions
Equation I
Figure I
Figure I
Graph A
Graph B
Graph C
Graph A
Graph B
Graph C
3. Which graph represents the change in solubility of BaCO3 if
Li2CO3 is added to a saturated solution of BaCO3.
a. Graph A
b. Graph B
c. Graph C
Aqueous
Equilibria
The beaker below contains an aqueous solution of potassium
ions and barium ions. Recommend a qualitative analysis
strategy that will separate the ions.
Aqueous
Equilibria
Circle the salts that are more soluble in an acidic solution
than in pure water.
BiI3
CdS
CaCO3
AgCN
Ba3(PO4)2
Strategy
Write the chemical equation and determine if the conjugate
acid of the anion is weak.
Aqueous
Equilibria
Titration of a Weak Acid with a
Strong Base
At each point below the equivalence point, the
pH of the solution during titration is determined
from the amounts of the acid and its conjugate
base present at that particular time.
Aqueous
Equilibria
The beaker below contains an aqueous solution of chromium (III) ions
and iron (III) ions. Recommend a qualitative analysis strategy that
will separate the ions.
Aqueous
Equilibria