Ionic product of water (Kw)
Water, the universal solvent, exhibits a remarkable property: even in its pure state, a small fraction of water
molecules dissociate into hydrogen ions (H+) and hydroxide ions (OH−).
This phenomenon is described by the equilibrium expression for water ionization:
H2O (l) ⇌ H+(aq) + OH−(aq)
The extent of this ionization is quantified by the ionic product of water, Kw, which plays a fundamental role in
defining the properties of acids, bases, and neutral solutions. At room temperature (25∘C), Kw is
1.0×10−14 mol/L.
This equilibrium constant is crucial in understanding the pH scale, the behavior of acids and bases, and
their classification as acidic, neutral, or basic solutions.
Moreover, Kw is temperature-dependent. As temperature increases, the extent of water ionization
increases, altering the concentrations of H+and OH−. Despite this, water remains neutral at all temperatures
because [H+]=[OH−].
Understanding the relationship between [H+], [OH−], and Kw is essential for interpreting acid-base
chemistry, solving pH problems, and appreciating the dynamic nature of chemical equilibria. This handout
will guide you through the principles of Kw, its dependence on temperature, and its implications in aqueous
solutions.
Learning Objectives
By the end of this reading, you will be able to:
1. Understand the concept of the ionic product constant of water (Kw) and the inverse relationship
between [H+] and [OH−].
2. Identify solutions as acidic, neutral, or basic based on the relative values of [H+] and [OH−].
3. Explain why the extent of ionization of water increases as temperature increases.
4. Analyze graphs representing the relationship between temperature, [H+], [OH−], and Kw.
Auto-ionization of water
Pure water undergoes a small degree of ionization according to the following equilibrium:
H2O (l)⇌H+(aq)+OH−(aq)
Kw=[H+][OH−]
−14
● At 25∘C, Kw=1.0×10 .
● In pure water, [H+]=[OH−]=1.0×10−7 mol/L, making the solution neutral.
Developed by Arindam Banerjee, Head of Science & IB Chemistry Teacher
American International School in Abu Dhabi, United Arab Emirates
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+ −
Relationship between [H ] and [OH ]
a. The graph is showing the relationship between [H+], [OH−], and Kw at three different temperatures. It
illustrates how [H+] and [OH−] are inversely proportional for a given Kw
b. It also illustrates Kw increases with temperature, leading to higher concentrations of both [H+] and
[OH−].
Temperature Dependence of Kwand Ionization of Water
As temperature increases, water molecules gain more kinetic energy, causing a higher extent of ionization.
a. At 25∘C, Kw= 1.0×10−14
b. At 50∘C, Kw= 5.5×10−14
This means:
a. [H+] and [OH−] both increase, but their product (Kw) remains consistent at each temperature.
b. Even though pH changes with temperature, pure water remains neutral because [H+]=[OH−].
Developed by Arindam Banerjee, Head of Science & IB Chemistry Teacher
American International School in Abu Dhabi, United Arab Emirates
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Graph showing temperature dependence of Kw
a. The graph shows the effect of temperature on the ionic product constant of water (Kw). As the
temperature increases, Kw rises exponentially, indicating that water ionizes more extensively at
higher temperatures.
b. This explains why both [H+] and [OH−] increase with temperature, even though their product (Kw)
remains temperature-dependent.
Developed by Arindam Banerjee, Head of Science & IB Chemistry Teacher
American International School in Abu Dhabi, United Arab Emirates
