This document discusses various topics relating to aqueous equilibria, including the common ion effect, buffers, titrations, solubility products, and factors that affect solubility. It provides examples of calculations for concentrations and pH involving these concepts and explains how precipitation of ions from solution can be used to separate mixtures.
This is useful to the chemical analysis persons. Tittration is one of the basic and standard method for quantitative chemical analysis. This describs the principles of titration, function of indicators, calculation of errors etc.
This is useful to the chemical analysis persons. Tittration is one of the basic and standard method for quantitative chemical analysis. This describs the principles of titration, function of indicators, calculation of errors etc.
Acids are divided into two categories based on the ease with which they can donate protons to the solvent: i) strong acids and ii) weak acids
Strong acids are acids that completely dissociate in water. The reaction of an acid with its solvent (typically H2O) is called an acid dissociation reaction.
Weak acids are acids that dissociate partially in water. The extent of dissociation is given by the equilibrium constant.
Note:
A measure of the relative strength of an acid is: i) the equilibrium constant ka of the dissociation reaction of the acid in water (depends on temperature) ii) the degree of dissociation α of the acid in water (depends on the concentration of the acid an on temperature).
In chemistry, acids and bases have been defined differently by three sets of theories. One is the Arrhenius definition, which revolves around the idea that acids are substances that ionize (break off) in an aqueous solution to produce hydrogen (H+) ions while bases produce hydroxide (OH-) ions in solution.
In chemistry, pH (potential of hydrogen) is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions.
Buffers are compounds or mixtures
of compounds that by their presence
in the solution resist changes in the
pH upon the addition of small
quantities of acid or alkali.
Acids are divided into two categories based on the ease with which they can donate protons to the solvent: i) strong acids and ii) weak acids
Strong acids are acids that completely dissociate in water. The reaction of an acid with its solvent (typically H2O) is called an acid dissociation reaction.
Weak acids are acids that dissociate partially in water. The extent of dissociation is given by the equilibrium constant.
Note:
A measure of the relative strength of an acid is: i) the equilibrium constant ka of the dissociation reaction of the acid in water (depends on temperature) ii) the degree of dissociation α of the acid in water (depends on the concentration of the acid an on temperature).
In chemistry, acids and bases have been defined differently by three sets of theories. One is the Arrhenius definition, which revolves around the idea that acids are substances that ionize (break off) in an aqueous solution to produce hydrogen (H+) ions while bases produce hydroxide (OH-) ions in solution.
In chemistry, pH (potential of hydrogen) is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions.
Buffers are compounds or mixtures
of compounds that by their presence
in the solution resist changes in the
pH upon the addition of small
quantities of acid or alkali.
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2. Aqueous
Equilibria
The Common-Ion Effect
• Consider a solution of acetic acid:
• If acetate ion is added to the solution,
Le Châtelier says the equilibrium will
shift to the left.
HC2H3O2(aq) + H2O(l) H3O+
(aq) + C2H3O2
−
(aq)
3. Aqueous
Equilibria
The Common-Ion Effect
“The extent of ionization of a weak
electrolyte is decreased by adding to
the solution a strong electrolyte that has
an ion in common with the weak
electrolyte.”
4. Aqueous
Equilibria
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−
]
[HF]
Ka = = 6.8 × 10-4
5. Aqueous
Equilibria
The Common-Ion Effect
Because HCl, a strong acid, is also present,
the initial [H3O+
] is not 0, but rather 0.10 M.
[HF], M [H3O+
], M [F−
], M
Initially 0.20 0.10 0
Change −x +x +x
At Equilibrium 0.20 − x ≈ 0.20 0.10 + x ≈ 0.10 x
HF(aq) + H2O(l) H3O+
(aq) + F−
(aq)
9. Aqueous
Equilibria
Buffers
If a small amount of hydroxide is added to an
equimolar solution of HF in NaF, for example, the HF
reacts with the OH−
to make F−
and water.
16. 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.
17. 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.
18. 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.
19. 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.
21. 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
22. Aqueous
Equilibria
Calculating pH Changes in Buffers
Now use the Henderson–Hasselbalch equation to
calculate the new pH:
pH = 4.74 + log
(0.320)
(0. 200)
pH = 4.74 + 0.06
pH = 4.80
24. Aqueous
Equilibria
Titration
A pH meter or
indicators are used to
determine when the
solution has reached
the equivalence point,
at which the
stoichiometric amount
of acid equals that of
base.
25. 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.
26. Aqueous
Equilibria
Titration of a Strong Acid with
a Strong Base
Just before and after
the equivalence point,
the pH increases
rapidly.
27. Aqueous
Equilibria
Titration of a Strong Acid with
a Strong Base
At the equivalence
point, moles acid =
moles base, and the
solution contains only
water and the salt from
the cation of the base
and the anion of the
acid.
29. 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.
30. 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.
31. 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.
32. 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.
36. Aqueous
Equilibria
Solubility Products
• Ksp is not the same as solubility.
• Solubility is generally expressed as the mass
of solute dissolved in 1 L (g/L) or 100 mL
(g/mL) of solution, or in mol/L (M).
37. Aqueous
Equilibria
Factors Affecting Solubility
• The Common-Ion Effect
If one of the ions in a solution equilibrium
is already dissolved in the solution, the
equilibrium will shift to the left and the
solubility of the salt will decrease.
BaSO4(s) Ba2+
(aq) + SO4
2−
(aq)
38. Aqueous
Equilibria
Factors Affecting Solubility
• pH
If a substance has a
basic anion, it will be
more soluble in an
acidic solution.
Substances with
acidic cations are
more soluble in
basic solutions.
41. 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+
.
42. Aqueous
Equilibria
Will a Precipitate Form?
• In a solution,
If Q = Ksp, the system is at equilibrium
and the solution is saturated.
If Q < Ksp, more solid will dissolve until
Q = Ksp.
If Q > Ksp, the salt will precipitate until
Q = Ksp.