Download Water is capable of acting as either an acid or a base

Water is capable of acting as either an acid or a base and can undergo
self­ionization.
LEARNING OBJECTIVE [ edit ]
Explain the amphoteric properties of water.
KEY POINTS [ edit ]
+
The self­ionization of water can be expressed as: H 2 O + H 2 O ⇌ H 3 O + OH −
.
The equilibrium constant for the self­ionization of water is known as KW; it has a value of 1.0 × 10−14 .
The value of KW leads to the convenient equation relating pHwith pOH: pH + pOH = 14.
TERMS [ edit ]
ionization
Any process that leads to the dissociation of a neutral atom ormolecule into charged particles
(ions).
hydronium
+
The hydrated hydrogen ion ( H 3 O ).
autoprotolysis
The autoionization of water (or similar compounds) in which a proton (hydrogen ion) is
transferred to form a cation and an anion.
Give us feedback on this content: FULL TEXT [ edit ]
Under standard conditions, water will self­ionize to a very small extent. The self­ionization of
water refers to the reaction in which a water molecule donates one of its protons to a
neighboring water molecule, either in pure water or inaqueous solution. The result is the
formation of a hydroxide ion (OH­) and a hydronium ion (H3O+). The reaction can be written
as follows:
H 2 O + H 2 O ⇌ H 3 O + + OH −
This is an example of autoprotolysis (meaning "self­protonating") and it exemplifies
the amphoteric nature of water (ability to act as both an acid and a base).
OH ­
H 3O+
2H 2O
+
+
Autoprotolysis of water
The self­ionization of water produces hydronium and hydroxide ions in solution.
The Water Ionization Constant, KW
Note that the self­ionization of water is an equilibrium reaction:
+
H 2 O + H 2 O ⇌ H 3 O + OH −
K W = 1.0 × 10−14
Like all equilibrium reactions, this reaction has anequilibrium constant. Because this is a
special equilibrium constant, specific to the self­ionization of water, it is denoted KW; it has a
value of 1.0 x 10−14. If we write out the actual equilibrium expression for KW, we get the
following:
K W = [H + ][OH − ] = 1.0 × 10−14
However, because H+ and OH­ are formed in a 1:1 molarratio, we have:
−−−−−−−−−
[H + ] = [OH − ] = √ 1.0 × 10−14 = 1.0 × 10−7 M
Now, note the definition of pH and pOH:
pH = −log[H + ]
pOH = −log[OH − ]
If we plug in the above value into our equation for pH, we find that:
pH = −log(1.0 ×
−7
) = 7.0
pH = −log(1.0 × 10−7 ) = 7.0
pOH = −log(1.0 × 10−7 ) = 7.0
Here we have the reason why neutral water has a pH of 7.0; it represents the condition at
which the concentrations of H+and OH­ are exactly equal in solution.
pH, pOH, and pKW
We have already established that the equilibrium constant KW can be expressed as:
K W = [H + ][OH − ]
If we take the negative logarithm of both sides of this equation, we get the following:
−log(K W ) = −log([H + ][OH − ])
−log(K W ) = −log[H + ] + −log[OH − ]
pK W = pH + pOH
However, because we know that pKW = 14, we can establish the following relationship:
pH + pOH = 14
This relationship always holds true for any aqueous solution, regardless of its level
of acidity or alkalinity. Utilizing this equation is a convenient way to quickly determine pOH
from pH and vice versa, as well as to determine hydroxide concentration given hydrogen
concentration, or vice versa.
Self­ionization of Water
Explanation of self­ionization of water and the formation of hydronium and hydroxide ions.