AP Chemistry--Chapter 15: Applications of Aqueous Equilibria
I.
Solutions of Acids or Bases Containing a Common Ion
A. The common ion effect occurs when the dissociation of a weak electrolyte is
decreased by adding to the solution a strong electrolyte that has an ion in
common with the weak electrolyte.
B. The common ion effect is when a solution containing a weak acid (for
example) and a soluble salt of that weak acid is formed.
C. Think of Le Chatelier’s Principle when deciding how the pH will be affected.
Example Reactions of the Common-Ion Effect
II.
Buffered Solutions
A. Composition and Action of Buffered Solutions
1. A buffer is a solution that resists changes in pH when small amounts of
acid or base are added.
2. Buffers contain a weak conjugate acid-base pair.
3. Buffers resist changes in pH because they contain both an acidic species to
neutralize OH‾ ions and a basic one to neutralize H+ ions.
4. Buffers most effectively resist a change in pH in either direction when the
concentrations of HX and X‾ are about the same. When this is true, [H+]
is approximately equal to Ka.
5. So when choosing a buffer, select one in which the acid form has a pKa
close to the pH you want.
Buffer Capacity
A. Buffer Capacity and pH
1. Buffer capacity is the amount of acid or base the buffer can neutralize
before the pH begins to change to an appreciable degree.
2. The Henderson-Hasselbalch equation is used to calculate buffer pH:
[X ]
pH pK a log
[ HX ]
B. Addition of Strong Acids or Bases to Buffers--these calculations are easily
performed by adjusting amounts of X– and HX in the Henderson-Hasselbalch
equation
Titrations and titration Curves
A. Definitions
1. Titration is a method of determining the concentration of a solution with
the acid-base reaction with a solution of known concentration.
2. Equivalence point is the point in a titration at which stoichiometrically
equivalent quantities of acid and base have been brought together.
3. A titration curve is a graph of the pH as a function of the volume of the
added titrant (see last 2 pages of notes)
III.
IV.
AP Chemistry--Chapter 15: Applications of Aqueous Equilibria
V.
VI.
VII.
B. Strong Acid-Strong Base Titrations--reactions go to completion (see sample
problems)
C. The reaction between a weak acid and a strong base (or between a weak base
and a strong acid) proceeds to completion--keep in mind when working with
these types of titrations
Acid-Base Indicators
A. Most acid-base indicators are weak acids that are one color in the HIn form
and another color in the In– form
B. Indicators are chosen that have a pKa close to the equivalence point of the
titration
C. The human eye can detect a color change when 1/10th of the initial form has
been converted, so most acid-base indicators have a useful range of about two
pH units
Solubility Equilibria and Solubility Product
A. The Solubility-Product Constant, Ksp
1. In addition to acid-base equilibria, there is an equilibrium that exists
between the precipitated and dissolved state of an ionic substance.
2. An example would be BaSO4(s)
Ba2+(aq) + SO42‾(aq)
3. The solubility product constant (Ksp ) for this reaction is simply the K
expression (recall that pure substances are not written in the equilibrium
expression and that coefficients become exponents).
B. Common Ion Effect--if a common ion is present with the initial salt, use Le
Chaterlier’s principle to see how the equilibrium is changed
C. Solubility and pH--pH changes will affect the solubility of any substance that
has a basic anion. Basic ions are OH‾, but also F‾, CO32‾, PO43‾, CN‾ for
example
Precipitation and Qualitative Analysis
A. Determination of Q
1. As before, Q (called the reaction quotient or ion product) can be calculated
to determine the direction in which a reaction must proceed to reach
equilibrium.
2. Q is found by taking given concentration, regardless of whether they are at
equilibrium or not, and using them in the Ksp expression.
3. Values obtained for Q
a. If Q > Ksp, precipitation occurs until Q = Ksp.
b. If Q = Ksp, equilibrium exists (saturated solution).
c. If Q < Ksp, solid dissolves until Q = Ksp.
B. Selective precipitation of ions can be performed from a solution containing
different ions since the Ksp of various salts differs.
C. This leads to the process of qualitative analysis
1. Mixtures of common cations can be separated into five major groups
based on solubilities
2. Each group is then further treated to separate and identify the individual
ions
AP Chemistry--Chapter 15: Applications of Aqueous Equilibria
Stong and Weak acid titrated with Strong Base
Weak Acid (acetic acid) titrated with Strong Base
AP Chemistry--Chapter 15: Applications of Aqueous Equilibria
Bases being titrated with Acids
Polyprotic Weak Acid being titrated
with Strong Base