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Ionic Product of Water

Chemistry: IB Diploma Higher Level, topic 6.2.2

The Ionic Product of Water

Water is very slightly dissociated into its ions.

Dissociation of water

Dissociation of water

  • Water very slightly dissociates into it's ions according to the following equation:
  • 2H2O(l) ⇋ H3O+(aq) + OH-(aq)
Equilibrium constant

Equilibrium constant

  • The equilibrium constant for the dissociation of water is:
  • Kc = [H3O+][OH−][H2O]\frac{[H_3O^+][OH^-]}{[H_2O]}
Ionic product of water

Ionic product of water

  • The equilibrium constant can be simplified because the concentration of water is pretty much constant.
  • Water only dissociates slightly, so the concentration doesn’t really change.
  • We write a new equation: Kw = [H3O+(aq)][OH-(aq)]
Temperature dependence of Kw

Temperature dependence of Kw

  • Kw is just an equilibrium constant.
  • Because of this, it behaves the same as any other equilibrium constant, Kc.
  • Importantly, this means it has a temperature dependence. The hotter the water is, the more dissociated it is and the larger its Kw value.

Test yourself on Ionic Product of Water

  • What is the best equation to describe the dissociation of water?

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