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Chapter 16

CHEM 105bL Chapter 16: Chapter 16 - Aqueous Ionic Equilibrium


Department
Chemistry
Course Code
CHEM 105bL
Professor
Parr Jessica
Chapter
16

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Chapter XVI. Aqueous Ionic Equilibrium
I. Buffers: Solutions that Resist pH Change
- A buffer resists pH change by neutralizing added
acid or added base
- A buffer contains significant amounts of weak
acid and its conjugate base or significant amounts
of weak base and its conjugate acid
- The common ion effect occurs when a solution
contains two substances that share a common
ion; thus the initial concentration must be
included in the initial amounts
- The Henderson-Hasselbalch equation allows us
to calculate the pH of a buffer solution from the
initial concentrations of the buffer components:
   

- The equation is only valid if the change (x) is too
small: 1) the initial concentrations of acids or
bases are too dilute; 2) the equilibrium constant is
very small
- Calculate in two parts: 1) use stoichiometry to
neutralize the equation; 2) use an ice table or
Henderson-Hassebalch to find the new
equilibrium
II. Buffer Effectiveness: Buffer Range and Capacity
- The capacity of a buffer is how much acid or base
it can effectively neutralize
- The range of a buffer is the pH range over which a
particular acid and its conjugate base can be
effective
- A buffer is most effective when the
[acid]=[conjugate base] and when these
concentrations are high
- The effective range for a buffering system is one
pH unit to either side of pKa
III. Titration and pH Curves
- In an acid-base titration, a basic (or acidic)
solution of unknown concentration reacts with an
acidic (or basic) solution of a known
concentration
- The pH is monitored by a pH meter or an indicator
- An indicator is a substance whose color depends
on the pH
- As the acid and base combine, they neutralize
each other
- The equivalence point is the point in the titration
when the # of moles of base is stoichiometrically
equal to the # of moles of acid
- At the equivalence point, the titration is complete
(neither reactant is in excess)
- Strong acid & strong base:
1. The initial pH is the pH of the strong acid to
be titrated
2. Before the equivalence point, H+ is in
excess;  
 [H+]
3. At the equivalence point, the pH=7 and
neither reactant is in excess
4. Beyond the equivalence point, OH- is in
excess;   

- Weak acid & strong base
1. The initial pH is the pH of the weak acid to
be titrated (equilibrium problem)
2. Between the initial pH and the equivalence
point, the solution becomes a buffer (use
Henderson-Hasselbalch)
3. Halfway to the equivalence point, the buffer
components are equal and pH=pKa
4. At the equivalence point, the acid has been
converted into its conjugate base (pH is
equilibrium w/ weak base)
5. Beyond the equivalence point, OH- is in
excess; 


- Weak base & strong acid: similar to weak acid
and strong base
- When a diprotic acid is titrated with a strong
base and Ka1 and Ka2 are sufficiently different,
the pH curve will have two equivalence points
- The 1st equivalence point represents the
titration of the 1st proton while the 2nd
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