Lecture notes
CH106 - Acid-base equilibria
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- The University Of Strathclyde (UOS)
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- Chemistry(3)
Following document contains everything covered in Dr. Moore's Acid-base equilibria - Example calculations - Equations
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Auto ionization of waterWater can undergo acidbase reactions with itself
H2O 1 t
Hill Hyo Cag OHCag
Reaction which has an equilibrium constant
Kw Hoot OH 1 1 10 at 298K
Kw is called the water ion constant
pKw logo Kw 14 and pKw pHtpOH 14
Definitions of Acids and Bases
Acid Base
Ht donor OH donor
Arrhenius
Bronsted fit donor Ht acceptor
Lowry
Electron pair Electron pair
Lewis acceptor donor
Bransted Lowry conjugate acidsbases
Ci is a conjugate
HCl H2O Hft CL base of HCl
A Clos
X BASES
tabioatisfaegnosugate
These pairs are conjugate
acid base pairs
, Some substances such as HSG and H2O are
amphiprotic meaning they can act as an acid OR as an
base
Strong acids are completely ionized in water whereas
weak acids are dissociated only partly
consider HA H2O FA 1 t
Hyot
HAT A Ht
The equilibrium constant for this reaction is called
acid dissociation acid ionisation constant
lH7
KaCHA HA
From which we can deduct that the stronger the
acid the higher ka value
So for 0.05M benzoic acid which in water has pH2.75
2 75 2 75 Ht BE
10 10
µ 0.05 10 2.75
HB z 0.05 10
2 75
Ka 6.56 10
5 9
Mass balance
If we have a ka 1.4 10 for 0.05M lactic acid pH
z
4
1.4 10 0.05 x I 0.05 O O
E 0.05 X TX TX
If very weak we can
assume 0.05 X t 0.05 E 0.05 X X X
3
Then X 2.65 10 3
Thus pH log o 2.65 10
pH 2.58
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