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First teaching 2023

First exams 2025

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The Ion Product of Water (SL IB Chemistry)

Revision Note

Philippa Platt

Last updated

The Ion Product of Water

pH of water

  • An equilibrium exists in water, where a few water molecules dissociate into proton and hydroxide ions

H2O (l) ⇌ H+ (aq) + OH– (aq)

  • The equilibrium constant for this reaction is:

Kc = [H+][OH-][H2O]{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}

Kc x [H2O] = [H+][OH]

  • Since the concentration of the H+ and OH- ions is very small, the concentration of water is considered to be a constant
  • This means that the expression can be rewritten as:

Kw = [H+] [OH-]

    • Where Kw (ion product of water) =  Kc x [H2O] =  1.00 10-14 at 298K
  • The product of the two ion concentrations is always 1.00 x 10–14 
  • This makes it straightforward to see the relationship between the two concentrations and the nature of the solution:

[H+] & [OH] Table

[H+] [OH] Type of solution
0.1 1 x 10–13 acidic
1 x 10–3 1 x 10–11 acidic
1 x 10–5 1 x 10–9 acidic
1 x 10–7 1 x 10–7 neutral
1 x 10–9 1 x 10–5 alkaline
1 x 10–11 1 x 10–3 alkaline
1 x 10–13 0.1 alkaline

Worked example

What is the pH of a solution of potassium hydroxide, KOH (aq) of concentration 1.0 × 10−3 mol dm−3 ?

 Kw = 1.0 × 10−14 at 298 K

   A. 3

   B. 4

   C. 10

   D. 11

 

Answer:

  • The correct option is D.
    • Since Kw = [H+] [OH-] , rearranging gives [H+]  = Kw ÷ [OH-]
    • The concentration of  [H+] is (1.0 × 10−14) ÷ (1.0 × 10−3) = 1.0 × 10−11 mol dm−3
    • So the pH = 11

How does temperature affect the ion product of water, Kw?

  • The ionisation of water is an endothermic process

2H2O (l) ⇌ H3O+  (aq) + OH- (aq) 

  • In accordance with Le Châtelier's principle, an increase in temperature will result in the forward reaction being favoured
    • This causes an increase in the concentration of the hydrogen and hydroxide ions
    • This leads to the magnitude of Kw increasing
    • Therefore, the pH will decrease
  • Increasing the temperature decreases the pH of water (becomes more acidic)
  • Decreasing the temperature increases the pH of water (becomes more basic)

Graph to show how Kw changes with temperature

Graph to show how Kw changes with temperature

As temperature increases, Kw increases so pH decreases

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.