Acid-Base Equilibria -I

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Lecture 9
Acid-Base Equilibria -I
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We titrate with strong acids ONLY!
Strong acids
HCl HBr HI HNO3 H2SO4
HClO4
(HO)nEO
(HO)nEO2
Weak acid
Strong acid
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Concentrations below 10-8 M do not affect the pH
value
10-6 -10-8 require special treatment (in
practice, this will be still affected mostly by
CO2 and NH3 from air)
Only concentrations above 10-6 M may change the
pH value and only in absence of other acids or
bases
Conclusion we cannot titrate low
concentrations of acids or bases in water
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Weak acids and bases Ka is a small number (ltlt
1)
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acid
base
HA ? A H
acid
base
Ka?Kb
A H2O ?HA OH-
HA ? A H

A H2O ? HA OH-
HOH-
H2O ? H OH-
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pH value of weak acid solution (Cgt 10-3 M)
The main source of H is HA, not H2O
HA ? A H
A H
HA C - H
A small number
H2 KaC
Check the resulting H and correct if necessary
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Any acid is strong at very low concentration
but it does not change the pH!
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pH value of weak base solution (Cgt 10-3 M)
The main source of OH- is A, not H2O
A H2O ?HA OH-
HA OH-
A C - OH-
A small number
Ka?Kb HOH-10-14
OH-2 KbC
Check the resulting OH- and correct if
necessary
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pH value of mixture of a weak base and weak acid
(Cgt 10-3 M)
HA ? A H
At high concentrations,
ACA and HACHA
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pH value of mixture of a weak base and weak acid
(Cgt 10-3 M)
buffer

• pH does not change when you vary
• concentration a little

In practice, one buffer pair (acid conjugate
base) determines the pH value of the whole
multicomponent mixture
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A complex mixture
Harris, p. AP12 pKa9.24
0.1 M
0.1 M
pH 9.24
All other species in the system (indicator, any
admixtures, ) will follow the pH determined by
the buffer
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Enzyme activity as a function of pH
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or
Effect of A/HA on pH
Effect of pH on A/HA
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pK5
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pH calculations
pH -logH
Ka?Kb HOH-10-14
Kgt1 Strong acid or strong base
HCacid
OH-Cbase
Kltlt1 Weak acid or weak base
A mixture of a weak base and weak acid