Why is water sometimes omitted from the equilibrium constant? To answer this equation, we begin by noting that the equilibrium constants
are approximations of the thermodynamic definition of the equilibrium constant, which is:
where ai is the activity of species i.
For a dilute solution,
where [i]o is the concentration of the pure species at standard conditions of 1 bar and 298.15K.
For an ideal gas,
where pio is the pressure of the pure species at standard conditions of 1 bar and 298.15K.
Combining eq3 through eq6,
Consider the following reversible reaction:
with
Water, being in excess, is assumed to have a constant concentration throughout the reaction, approximately equal to that of its pure state. Hence and eq8 becomes
Since the standard state of a solute is defined as 1 mol dm-3, eq9 approximates to
Therefore, water is excluded from the equilibrium constant if it acts as a solvent. However, for the reactions
water is not a solvent but a reactant, and the respective equilibrium constants are
In the case of a reversible reaction containing one or more solid species, e.g.,
the concentrations of the solid compounds are assumed to be the same as that of their respective pure states. Hence,
Question
Write the equilibrium constants for the following reactions:
a)
b)
c)
Answer
a)
b)
H2O is not a solvent in the hydrolysis reaction but a reactant. A typical ester hydrolysis reaction involves adding, for example, 0.10 M of CH3COOC2H5, 0.10 M of H2O and a catalyst in an inert organic solvent.
c)
The reaction occurs in an aqueous solution, i.e., the solvent is water.