Pure Substances
When pure substances (liquids or solids) are involved in equilibria they do not appear in the equilibrium equation
Applying the general formula for an equilibrium constant to the specific case of acetic acid one obtains
It may be assumed that the concentration of water is constant. This assumption will be valid for all but very concentrated solutions. The equilibrium constant expression is therefore usually written as
where now
a constant factor is incorporated into the equilibrium constant.
A particular case is the self-ionization of water itself
The self-ionization constant of water is defined as
It is perfectly legitimate to write for the hydronium ion concentration, since the state of solvation of the proton is constant (in dilute solutions) and so does not affect the equilibrium concentrations. Kw varies with variation in ionic strength and/or temperature.
The concentrations of H+ and OH- are not independent quantities. Most commonly is replaced by Kw−1 in equilibrium constant expressions which would otherwise include hydroxide ion.
Solids also do not appear in the equilibrium equation. An example is the Boudouard reaction:
for which the equation (without solid carbon) is written as:
Read more about this topic: Chemical Equilibrium
Famous quotes containing the word pure:
“White Pond and Walden are great crystals on the surface of the earth, Lakes of Light.... They are too pure to have a market value; they contain no muck. How much more beautiful than our lives, how much more transparent than our characters are they! We never learned meanness of them.”
—Henry David Thoreau (1817–1862)