Autoionization of Water

Core Concept

Autoionization (or self-ionization) of water is a reaction in which two water molecules interact to produce a hydronium ion (H3O+) and a hydroxide ion (OH−).

Reaction: 2H2O(l)⇌H3O+(aq)+OH−(aq)

Alternatively: H2O(l)⇌H+(aq)+OH−(aq)
This equilibrium is fundamental to the pH scale and acid-base chemistry.

Key Tips

Always convert minutes or hours to seconds before multiplying by current.
Water is Always Ionized: Even pure water contains $H_3O^+$ and $OH^-$ due to autoionization.
pH and Kw: The pH scale is derived from the autoionization of water, with $K_w$ determining the relationship between [$H_3O^+$] and [$OH^-$].
Neutrality is Temperature-Dependent: At higher temperatures, water is neutral at a pH lower than 7 because $K_w$ increases.

Test Yourself

Assorted Multiple Choice

At $25^\circ\text{C}$, the ion-product constant of water ($K_w$) is $1.0 \times 10^{-14}$. If the temperature of pure water is increased to $50^\circ\text{C}$, the auto-ionization process (which is endothermic) shifts forward, raising $K_w$ to approximately $5.5 \times 10^{-14}$. Which of the following is true about pure water at $50^\circ\text{C}$?

Podcast Episode

Episode

Autoionization of Water

Companion Guides

Coming soon

Student Guide Teacher Guide

Practice Problems & Worked Out Examples 🔒

Browse by category

01

The Equilibrium Expression and Ion Product (Kw)

→ 02

Temperature Dependence of Auto-ionization

→ 03

Qualitative Interpreting of "Neutrality"

→ 06

Other / Uncategorized

→ 07

Assorted Multiple Choice

→

Calculations Involving Kw

Calculating Ion Concentrations

Example: In pure water at 25°C, calculate the concentrations of $H_3O^+$ and $OH^-$.
- Given: $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$
- Since $[H_3O^+] = [OH^-]$, let x = [$H_3O^+$]: 2x = $1.0 \times 10^{-14}$
- $x = [H_3O^+] = [OH^-] = 1.0 \times 10^{-7} \, \text{M}$

Non-Neutral Solutions

For a solution with $[H_3O^+] = 1.0 \times 10^{-3} \, \text{M}$:
- $[OH^-] = \frac{K_w}{[H_3O^+]} = \frac{1.0 \times 10^{-14}}{1.0 \times 10^{-3}} = 1.0 \times 10^{-11} \, \text{M}$

Key Concepts

Equilibrium Constant ($K_w$)

The equilibrium constant for the autoionization of water is denoted as $K_w$.
At 25°C: $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$
This value varies with temperature, increasing as the temperature rises.

Neutrality of Water

In pure water at $25^\circ\text{C}$: $[\text{H}_3\text{O}^+] = [\text{OH}^-] = 1.0 \times 10^{-7} \, \text{M}$.
- The solution is neutral because the concentrations of $\text{H}_3\text{O}^+$ and $\text{OH}^-$ are equal.

Temperature Dependence

The autoionization constant $K_w$ increases with temperature, meaning water becomes slightly more ionized at higher temperatures.
- At $50^\circ\text{C}$, $K_w > 1.0 \times 10^{-14}$.

Topic Related Resources

Labs

Demos

Activities

Simulations

LAB — 01

Lab

Coming Soon...