Autoionization of Water: Exploring the Role of H2O in Chemical Equilibrium

[lha08]

Homework Statement

i was wondering if anyone knows why water is ignored in the autoionization of water? thanks

Homework Equations

H2O+H2O= H3O+OH
Kc=[H][OH]

The Attempt at a Solution

 

[Ygggdrasil]

In an aqueous solution (i.e. a solution where water is the solvent), the concentration of water is about 55M. Any reactions that occur in water that use water as a reactant or product will not significantly change the concentration of water in the solution. Therefore, one can treat [H₂O] as a constant that gets added into the equilibrium constant.

 

[Blueshift5]

The autoionization of water is a process in which water molecules, in a pure liquid state, can self-ionize to form hydronium (H3O+) and hydroxide (OH-) ions. This process occurs due to the presence of a small amount of impurities or ions in the water, which act as catalysts for the ionization reaction.

In most cases, water is not explicitly included in the equilibrium expression for the autoionization of water (H2O + H2O ⇌ H3O+ + OH-). This is because the concentration of water remains constant throughout the reaction and does not affect the equilibrium constant (Kc). Therefore, it is often omitted for simplicity in calculations.

However, it is important to note that water molecules play a crucial role in the autoionization process. Without water, the reaction would not occur as it requires the breaking and formation of hydrogen bonds between water molecules. Additionally, the concentration of water can indirectly affect the equilibrium constant through changes in temperature or pressure.

In conclusion, while water may not be explicitly included in the equilibrium expression, it is still a vital component in the autoionization of water and should not be ignored.