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

Exercises
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