Intermediate Topic 509 of 1102 Kinetics & Equilibrium
Equilibrium Constant of Water
Kw equals 1.0 x 10^-14 at 25 degrees Celsius for the autoionization of water.
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In-depth explanation of Equilibrium Constant of Water
The ionic product of water, Kw = [H+][OH-], is the equilibrium constant for the autoionization of water, H2O <-> H+ + OH-. At 25 degrees C its value is 1.0 x 10^-14, which underlies the pH scale because pH + pOH = 14 at that temperature. Kw increases with temperature, since autoionization is endothermic, so neutral water has a higher hydrogen ion concentration when hot and its pH falls below 7 while remaining neutral. Water is neutral whenever [H+] equals [OH-], irrespective of the actual pH value. The relationship links the self-ionization of water to the behaviour of all acids and bases in aqueous solution.
Going deeper: Equilibrium Constant of Water
Key ideas: Relations: pKw = pH + pOH = 14.00 at 25 degrees C, Ka x Kb = Kw for a conjugate pair, and pKa + pKb = pKw. Kw values: 1.0 x 10^-14 at 25 degrees C, 5.5 x 10^-13 at 100 degrees C, and 1.1 x 10^-15 at 0 degrees C. Pure water at 25 degrees C has [H+] = 1.0 x 10^-7 M.
Common mistakes and connections: Students insist that neutral always means pH 7, or that pH cannot be negative or above 14. Concentrated acids can reach negative pH. Another mistake is applying 14 to the sum of pH and pOH at other temperatures. Also the concentration of water is built into Kw, so [H2O] is not shown. Connects to acid-base equilibria, temperature effects on equilibrium and buffer calculations.
Visualization of Equilibrium Constant of Water
Kw increases with temperature because autoionisation is endothermic, so pKw falls from 14.0 at 25 C to 12.26 at 100 C and neutral pH drops to about 6.1. The interactive lesson shows this as a live animation or chart.
Worked example and use case
At 100 degrees C the value of Kw is about 5.5 x 10^-13, so neutral water has a pH near 6.1.
Frequently asked questions about Equilibrium Constant of Water
What is Equilibrium Constant of Water?
Kw equals 1.0 x 10^-14 at 25 degrees Celsius for the autoionization of water.
Is Equilibrium Constant of Water a beginner, intermediate or advanced topic?
Equilibrium Constant of Water is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Equilibrium Constant of Water?
Study these earlier topics in Kinetics & Equilibrium first: Heterogeneous Equilibrium Vapor, Partition Coefficient, Distribution Law.
What comes after Equilibrium Constant of Water?
The next topics in Kinetics & Equilibrium are Steady State vs Equilibrium.
Can I practise Equilibrium Constant of Water with quizzes?
Yes. The interactive lesson includes a self-check question on Equilibrium Constant of Water, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Equilibrium Constant of Water fits in Kinetics & Equilibrium
Equilibrium Constant of Water is topic 90 of 91 in Kinetics & Equilibrium.