H2O can act as both an acid and a base, and the proton transfer reaction shown below occurs to a very small extent in pure water. The presence of the two ions can be demonstrated by very sensitive conductivity experiments.
H2O(l) + H-OH(l)
H3O+(aq) + –OH(aq)
The equilibrium expression does not show the concentration of water because it is constant.
Proton transfer reactions are fast, and pure water or any aqueous solution is at equilibrium and the product of [H3O+] and [OH–] is equal Kw.
Kw can be used to calculate [OH–] if [H3O+] is known and vice versa. Recall that pH and [H3O+] are related by: pH = -log [H3O+].
Example:
If pH = 10
[H3O+] = 1 × 10–10
THEN at 25°C
Kw = 1 × 10–14 = 10–10 × [OH–]
THUS [OH–] = 1 × 10–4
If pH is 9.46.
[H3O+] = 3.45 × 10–10
[OH–] = 2.89 × 10–5
Note that the sum of the exponents is -15 in this case because 3.45 × 2.89 is equal to 10.
Try this on your calculator to practice!!
H2O(l) + H-OH(l)
H3O+(aq) + –OH(aq)Kw = [H3O+][OH–]
Kw = 1 × 10–14 at 25 °C
Kw = 1 × 10–14 at 25 °C
The equilibrium expression does not show the concentration of water because it is constant.
The proton transfer reaction above is endothermic; therefore Kw is larger at temperatures higher than 25 °C.
Proton transfer reactions are fast, and pure water or any aqueous solution is at equilibrium and the product of [H3O+] and [OH–] is equal Kw.
In pure water at 25 °C the reaction above is the only source of these ions
[H3O+] = [OH–] = 10–7
In a solution where an acid has been dissolved in water
[H3O+] > [OH–] BUT the product of the concentrations is equal to Kw
These solutions are referred to as being acidic, and they turn blue litmus red.
In a solution where a base has been dissolved in water
[H3O+] < [OH–] BUT the product of the concentrations is equal to Kw
These solutions are referred to as being alkaline, and they turn red litmus blue.
[H3O+] = [OH–] = 10–7
In a solution where an acid has been dissolved in water
[H3O+] > [OH–] BUT the product of the concentrations is equal to Kw
These solutions are referred to as being acidic, and they turn blue litmus red.
In a solution where a base has been dissolved in water
[H3O+] < [OH–] BUT the product of the concentrations is equal to Kw
These solutions are referred to as being alkaline, and they turn red litmus blue.
Kw can be used to calculate [OH–] if [H3O+] is known and vice versa. Recall that pH and [H3O+] are related by: pH = -log [H3O+].
| Useful maths: | |
| 10a × 10b = 10a+b | and 10a ÷ 10b = 10a-b |
Example:
If pH = 10
[H3O+] = 1 × 10–10
THEN at 25°C
Kw = 1 × 10–14 = 10–10 × [OH–]
THUS [OH–] = 1 × 10–4
If pH is 9.46.
[H3O+] = 3.45 × 10–10
[OH–] = 2.89 × 10–5
Note that the sum of the exponents is -15 in this case because 3.45 × 2.89 is equal to 10.
Try this on your calculator to practice!!