Self-Ionization of Water — Ion-Product Constant (Kw)

(Video) The Self-ionization of Water and Kw

by Science Beetle (TM)

In this video, you will learn about how water dissociates into ions and also forms back into water at a constant rate. This ionization constant is important in making pH calculations.

What is Self-Ionization?

Self-ionization occurs when a molecule or compound forms positive and negative ions of itself. This can happen if the substance is in a liquid state. Keep in mind that a solution with ions is not self-ionization unless the solvent is the ionized material. The substance must be able to form ions without the assistance of a solvent.

Water Self-Ionization

Water is able to self-ionize or form ions. When this happens two water molecules interact and one water molecule removes a proton or hydrogen from the other. The molecule with the additional proton is a positive cation and the molecule with one less hydrogen is a negative anion. The cation, $H_3O^+$, is called hydronium. The anion, $OH^-$, is called hydroxide. This interaction can be represented by the chemical equation: $$2\ H_2O (l)\ \rightarrow\ H_3O^+(aq)\ +\ OH^-(aq)$$ Another way to represent the water ionization is to think of one water molecule deprotonating, or losing a proton. This forms a free proton and hydroxide. The ionization of one water molecule is represented by: $$H_2O (l)\ \rightarrow\ H^+(aq)\ +\ OH^-(aq)$$

Equilibrium Constant of Water - $K_w$

Since water can self-ionize, it creates an equilibrium between the ionized and non-ionized water. The equilibrium of water is described with the equation: $$K= \frac{[H_3O^+][OH^-]}{[H_2O]}$$ Where $K$ is the equilibrium constant equal to the concentration of products over the concentration of reactants. The ion concentrations of hydronium and hydroxide are equal to each other at standard temperature, 25°C, [$H_3O^+$]=[$OH^-$]= 1.00 x 10$^{-7}$. The concentration of water stays virtually the same and is represented by the constant $K_w$. The relationship between water and the self-ionization is: $$K_w\ =\ [OH^-][H_3O^+]\\ K_w\ =\ [1.00 \times 10^{-7}][1.00 \times 10^{-7}]\ =\ 1.01 \times 10^{-14}$$