4.5.3 Acid Strength

Strong acids

• A strong acid is an acid that dissociates almost completely in aqueous solutions
  • HCl (hydrochloric acid), HNO₃ (nitric acid) and H₂SO₄ (sulfuric acid)
• The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

The diagram shows the complete dissociation of a strong acid in aqueous solution

Weak acids

• A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
  • Eg. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H₂S (hydrogen sulfide) and H₂CO₃ (carbonic acid)
• The position of the equilibrium is more over to the left and an equilibrium is established

The diagram shows the partial dissociation of a weak acid in aqueous solution

• For weak acids as there is an equilibrium we can write an equilibrium constant expression for the reaction

• This constant is called the acid dissociation constant, K_a, and has the units mol dm^-3
• Values of K_a are very small, for example for ethanoic acid K_a = 1.74 x 10^-5 mol dm^-3 
• When writing the equilibrium expression for weak acids, the following assumptions are made:
  • The concentration of hydrogen ions due to the ionisation of water is negligible

• The value of K_a indicates the extent of dissociation
  • The higher the value of K_a the more dissociated the acid and the stronger it is
  • The lower the value of K_a the weaker the acid

pK_a

• The range of values of K_a is very large and for weak acids, the values themselves are very small numbers

Table of K_a values

• For this reason it is easier to work with another term called pK_a
• The pK_a  is the negative log of the K_a value, so the concept is analogous to converting [H⁺] into pH values

pK_a = -logK_a

• Looking at the pK_a values for the same acids:

Table of pK_a values

• The range of pK_a values for most weak acids lies between 3 and 7