Acid–Base Equilibria
Objectives
To use the ionic product constant of water in calculations
Defines pH, pOH, pKaand pKb
Discusses salt hydrolysis
Looks at calculations involving weak acids and bases
Explains the use of buffers and acid–base indicators
Ionic equilibria in water
Ionization of water
Pure water is a very poor conductor of electricity. This shows that there are virtually
no ions present. However, there are some, and these play a very important role in the
equilibria of aqueous solutions.
At room temperature about one water molecule in every 1 000 000 000 is ionized:
H 2 O(l)H 2 O(l)\===\H 3 O(aq)OH(aq) (16.1)In this reaction, a proton is being transferred from one water molecule to another.
The equilibrium expression for this reaction is
Kc(T)[H 3 O(aq)][OH(aq)]
[H 2 O(l)]^2But [H 2 O(l)], the concentration of a pure liquid, is constant. This gives
Kw(T)[H 3 O(aq)][OH(aq)] (16.2)whereKw(T), which equals Kc(T)[H 2 O(l)]^2 , is known as the ionic product constant
(orautoionization constant) of water.
We can express equation (16.2) in words:
When the concentration of hydroxide and hydronium ions in water are
multiplied together, the product is fixed at that temperature.Experiments show that equation (16.2) applies to water and also to allaqueous solutions,
such as sodium chloride solution, hydrochloric acid and sodium hydroxide solution.
16.1
Contents
16.1Ionic equilibria in
water 28516.2Acids and bases in
aqueous solution 28916.3Hydrolysis of salts 29516.4Buffer solutions 29716.5Acid–base
indicators 30116.6Variation of pH
during an acid–base
titration 30316.7Buffering action of
carbon dioxide in
water 30516.7Revision questions 30616
UNIT