Transcript Buffers Made Easy

Buffers Made Easy
Common ion effect
Common Ion: the presence of an ion which
appears in both the acid (or base) and a salt
in the solution
Common ion effect: The shift that occurs
because of the addition of an ion already
involved in the equilibrium reaction.
Buffered Solutions
• A solution that resists a change in it’s pH
when either hydroxide ions or
protons(hydrogen) ions are added.
• Consists of a WEAK acid and it’s SALT
• Weak acids or bases and a common ion.
How does it work
• If OH- are added to the system it moves right.
Not allowing OH- to accumulate not changing
the pH
• If H+ are added it will proceed in the reverse to
produce HA not allowing the pH to change
Look at the Eq expression
• The Eq concentration of H+ is determined by
the ratio of [HA]/[A-].
Bases are the opposite
• You will use Kb
Hendersen- Hasselbach
Simpler method
• One equation: Work buffers as an equilbrium
problem
Go back to the reaction HA H+ + A• If acid is added to the buffer, simply add acid to
the numerator AND subtract the same quantity
from the base since it was self-sacrificing and
neutralized the acid. If base is added, simply add
the base to the denominator and subtract from the
numerator. Add or subtract in moles NOT
molarity. Moles = molarity x volume
• When equal concentrations (or moles) of Acid
and Base are present [which occurs at the .
equivalence point of a titration] the ratio of
acid to base equals ONE and therefore, the pH
= pKa. IF you are asked to construct a buffer
of a specific pH and given a table of Ka’s,
choose a Ka with an exponent close to the
desired pH and use equal concentrations of the
acid and base
RECAP
• Go to MOLES. Moles = M x V
• Add to what ever is being added and subtract
from the other.
The titration curve of a strong acid with a strong base.
16.5
Strong Acid-Strong Base Titrations
100% ionization!
NaOH (aq) + HCl (aq)
OH- (aq) + H+ (aq)
H2O (l) + NaCl (aq)
No equilibrium
H2O (l)
16.4
Weak Acid-Strong Base Titrations
CH3COOH (aq) + NaOH (aq)
CH3COOH (aq) + OH- (aq)
CH3COONa (aq) + H2O (l)
CH3COO- (aq) + H2O (l)
At equivalence point (pH > 7):
CH3COO- (aq) + H2O (l)
OH- (aq) + CH3COOH (aq)
16.4
Strong Acid-Weak Base Titrations
HCl (aq) + NH3 (aq)
H+ (aq) + NH3 (aq)
NH4Cl (aq)
NH4Cl (aq)
At equivalence point (pH < 7):
NH4+ (aq) + H2O (l)
NH3 (aq) + H+ (aq)
16.4
Acid-Base Indicators
16.5
pH
16.5
Complex Ion Formation
• These are usually formed from a transition
metal surrounded by ligands (polar
molecules or negative ions).
• As a "rule of thumb" you place twice the
number of ligands around an ion as the
charge on the ion... example: the dark blue
Cu(NH3)42+ (ammonia is used as a test for
Cu2+ ions), and Ag(NH3)2+.
• Memorize the common ligands.
Common Ligands
Ligands
Names used in the ion
H2O
NH3
aqua
ammine
OHClBrCNSCN-
hydroxy
chloro
bromo
cyano
thiocyanato (bonded through
sulphur)
isothiocyanato (bonded through
nitrogen)
Names
• Names: ligand first, then cation
Examples:
– tetraamminecopper(II) ion: Cu(NH3)42+
– diamminesilver(I) ion: Ag(NH3)2+.
– tetrahydroxyzinc(II) ion: Zn(OH)4 2-
• The charge is the sum of the parts
(2+) + 4(-1)= -2.
Coordination Number
• Total number of bonds from the ligands to
the metal atom.
• Coordination numbers generally range
between 2 and 12, with 4 (tetracoordinate)
and 6 (hexacoordinate) being the most
common.
Some Coordination Complexes
molecular
formula
Lewis
base/ligand
Lewis acid donor
atom
coordination
number
Ag(NH3)2+
NH3
Ag+
N
2
[Zn(CN)4]2- CN-
Zn2+
C
4
[Ni(CN)4]2-
CN-
Ni2+
C
4
[PtCl6] 2-
Cl-
Pt4+
Cl
6
Ni2+
N
6
[Ni(NH3)6]2+ NH3
When Complexes Form
• Aluminum also forms complex ions as do some post
transitions metals. Ex: Al(H2O)63+
• Transitional metals, such as Iron, Zinc and Chromium,
can form complex ions.
• The odd complex ion, FeSCN2+, shows up once in a
while
• Acid-base reactions may change NH3 into NH4+ (or vice
versa) which will alter its ability to act as a ligand.
• Visually, a precipitate may go back into solution as a
complex ion is formed. For example, Cu2+ + a little
NH4OH will form the light blue precipitate, Cu(OH)2. With
excess ammonia, the complex, Cu(NH3)42+, forms.
• Keywords such as "excess" and "concentrated" of any
solution may indicate complex ions. AgNO3 + HCl forms
the white precipitate, AgCl. With excess, concentrated
HCl, the complex ion, AgCl2-, forms and the solution
clears.