Key PointsThe self-ionization of water have the right to be express as: H_2O + H_2O \rightleftharpoons H_3O^+ + OH^- .The equilibrium continuous for the self-ionization that water is well-known as KW; it has actually a worth of 1.0\times 10^-14 .The worth of KW leader to the convenient equation relating pH with pOH: pH + pOH = 14.

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TermsionizationAny procedure that leads to the dissociation of a neutral atom or molecule right into charged particles (ions).autoprotolysisThe autoionization that water (or similar compounds) in i m sorry a proton (hydrogen ion) is moved to kind a cation and an anion.hydroniumThe character language hydrogen ion ( H_3O^+ ).

Under typical conditions, water will self-ionize come a very small extent. The self-ionization that water refers to the reaction in which a water molecule donates one of its proton to a surrounding water molecule, either in pure water or in aqueous solution. The an outcome is the formation of a hydroxide ion (OH–) and also a hydronium ion (H3O+). The reaction deserve to be composed as follows:

H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-

This is an example of autoprotolysis (meaning “self-protonating”) and also it exemplifies the amphoteric nature the water (ability come act together both an acid and also a base).

Autoprotolysis of waterThe self-ionization the water produces hydronium and also hydroxide ions in solution.

The Water Ionization Constant, KW

Note that the self-ionization the water is an equilibrium reaction:

H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-\quad\quad\quad K_W=1.0\times10^-14

Like every equilibrium reactions, this reaction has an equilibrium constant. Because this is a unique equilibrium constant, particular to the self-ionization the water, it is denoted KW; it has actually a worth of 1.0 x 10−14. If we write out the really equilibrium expression because that KW, we get the following:

K_W==1.0\times 10^-14

However, since H+ and OH– are created in a 1:1 molar ratio, us have:

==\sqrt1.0\times 10^-14=1.0\times 10^-7\;M

Now, keep in mind the meaning of pH and pOH:



If we plug in the above value into our equation because that pH, we uncover that:

\textpH=-log(1.0\times 10^-7)=7.0

\textpOH=-log(1.0\times 10^-7)=7.0

Here we have the reason why neutral water has a pH that 7.0; it to represent the condition at i m sorry the concentration of H+ and also OH– are precisely equal in solution.

pH, pOH, and also pKW

We have already established that the equilibrium consistent KW have the right to be expressed as:


If us take the an unfavorable logarithm of both sides of this equation, we obtain the following:




However, since we understand that pKW = 14, we can establish the complying with relationship:


This relationship always holds true for any type of aqueous solution, regardless of the level of mountain or alkalinity. Making use of this equation is a convenient method to quickly determine pOH native pH and vice versa, and also to recognize hydroxide concentration provided hydrogen concentration, or angry versa.

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Self-ionization the WaterExplanation the self-ionization of water and the formation of hydronium and also hydroxide ions.

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