CH106 - Acid-base equilibria

Auto ionization of water
Water 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