Ionic product of water (Kw)
Understanding the autoionization of water and its equilibrium constant (Kw), and its dependence on temperature.
What is Ionic product of water (Kw)?
The process where water molecules react with each other to produce hydronium ions (H₃O⁺) and hydroxide ions (OH⁻).
Key formula / rule: Ionic Product of Water
Key points
- Define the ionic product of water (Kw).
- Write the equilibrium expression for the autoionization of water.
- Calculate Kw at 25°C.
- Explain the temperature dependence of Kw.
Common exam trap
Assuming Kw is always 1.0 x 10⁻¹⁴, regardless of temperature.
Definitions
- Term
Autoionization of Water
- Meaning
The process where water molecules react with each other to produce hydronium ions (H₃O⁺) and hydroxide ions (OH⁻).
- Term
Ionic Product of Water (Kw)
- Meaning
The equilibrium constant for the autoionization of water, defined as the product of the molar concentrations of hydronium ions and hydroxide ions.
Learning objectives
Define the ionic product of water (Kw).
Write the equilibrium expression for the autoionization of water.
Calculate Kw at 25°C.
Explain the temperature dependence of Kw.
Relate Kw to the concentrations of H₃O⁺ and OH⁻ ions in any aqueous solution.
Formulae
- Name
Ionic Product of Water
- Note
This expression holds true for all aqueous solutions at a given temperature.
- Expression
Kw = [H₃O⁺][OH⁻]
- Name
Ionic Product of Water at 25°C
- Note
This is the standard value used for calculations at room temperature.
- Expression
Kw = 1.0 x 10⁻¹⁴ M²
Prerequisites
Chemical Equilibrium
Equilibrium Constant (Kc)
Concentration (Molarity)
pH and pOH scales
Common mistakes
Assuming Kw is always 1.0 x 10⁻¹⁴, regardless of temperature.
Forgetting that Kw applies to all aqueous solutions, not just pure water.
Confusing Kw with the acid dissociation constant (Ka) or base dissociation constant (Kb).
Keywords
Ionic product of water
Kw
Autoionization
Water equilibrium
Hydronium ion
Hydroxide ion
pH
pOH
Aqueous solution
Temperature dependence
Practice preview
What is the value of the ionic product of water (Kw) at 25°C?…
easy
In pure water at 25°C, what is the concentration of H+ ions?…
easy
At 60°C, the ionic product of water (Kw) is approximately 9.6 x 10^-14 M^2. What is the pH of pure water at this temperature?…
medium