The Ionic Product of Water
- In all aqueous solutions, an equilibrium exists in water where a few water molecules dissociate into protons and hydroxide ions
- We can derive an equilibrium constant for the reaction:
- This is a specific equilibrium constant called the ionic product for water
- The product of the two ion concentrations is always 1 x 10-14 mol2 dm-6
- This makes it straightforward to see the relationship between the two concentrations and the nature of the solution:
[H+] & [OH–] Table
The effect of temperature on Kw
- The dissociation of water to form hydrogen and hydroxide ions is an endothermic process so absorbs heat energy
H2O (l) ⇌ H+ (aq) + OH- (aq)
- If temperature is increased, the forward reaction will be favoured to counteract the change and lower the temperature
- Th equilibrium will shift to the right and more H+ and OH- ions will be formed causing the value of Kw to increase
- So, as temperature increases the value of Kw increase
- If the value of Kw increases, the pH decreases as shown below:
T (°C) |
Kw (mol2 dm-6) |
pH |
0 |
0.114 x 10-14 |
7.47 |
10 |
0.293 x 10-14 |
7.27 |
20 |
0.681 x 10-14 |
7.08 |
25 |
1.008 x 10-14 |
7.00 |