Acidity constants
Weak acids in equilibrium.
Acid/base reactions in water are also examples of equilibrium reactions. Consider the ionisation of the weak acid ethanoic acid.
The equilibrium expression for this above reaction is
The concentration of water is
considered to be constant and is not shown in the expression. It is part
of the equilibrium constant.
The acidity
constants are an indication of the strength of the acid, the higher the
acidity constant the greater the strength of the acid.
Ethanoic acid is a weak acid and has an acidity constant of 
HCl is a strong acid and its equilibrium constant is given below.
The equilibrium constants of ethanoic and hydrochloric acids tell us that
ethanoic acid will ionise to a negligible extent in water(low Ka value)
while hydrochloric acid will completely ionise(high Kat value). We can
use the acidity constant to label acids as strong or weak.
Lets try some
examples involving the Ka.
Calculate the pH of a 0.4M acid solution. The ka of this particular
acid is given below.
.
Step 1 Notice
that the Ka is very low indicating a weak acid. When an acid is weak
we assume negligible ionisation. We assume that the concentration of
acid present at equilibrium is 0.4M.
Step 2 According to the equation below, the
amount of hydronium ions formed is equal to the amount of acetate ions
formed
We can therefore write the equilibrium expression as
The concentration of acetic acid at equilibrium is 0.4M
We now find the concentration of hydronium ions.
The pH is therefore
