Proton transfer reactions - the ion product of water

Question 1

What is autoionisation?

Answer 1

The spontaneous formation of low concentrations of [H+] and [OH-] ions by proton transfer from one molecule to another.

Question 2

Ionisation of water formula

Answer 2

H2O (l) ⇌ H+ (aq) + OH– (aq)

Question 3

Ionic product constant of water formula

Answer 3

Kw = [H+] [OH-]

Question 4

Concentrations of H+ and OH- in pure water?

Answer 4

[H+] = [OH-]

Question 5

How can the formula to find Kw or concentrations be simplified for water?

Answer 5

Kw = [H+][OH-] = 1.00 x 10^-14
Kw = [H+]^2
[H+] = √Kw

Question 6

How does temperature affect Kw and pH?

Answer 6

Increasing temperature increases Kw.
Increasing temperature decreases pH of water (becomes more acidic).

Question 7

What is Le Chatelier’s explanation for how temperature affects Kw?

Answer 7

Increase in temperature results in the forward reaction being favoured, the ionisation of water is an endothermic process therefore the concentrations of the hydrogen and hydroxide ions will increase. This leads to the magnitude of Kw increasing, therefore pH will decrease.

Question 8

Draw a graph to show how Kw changes with temperature

Answer 8

Y-axis: Kw(10^-14)
X-axis: Temperature (°C)
Curve starts on x-axis at 10°C, exponential curve?