Acid Dissociation Constant (AQA A Level Chemistry): Revision Note

Acid Dissociation Constant

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

• As this 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

pH of Weak Acids

Weak acids

• The pH of weak acids can be calculated when the following is known:

  • The concentration of the acid

  • The K_a value of the acid

• From the K_a expression we can see that there are three variables:

• However, the equilibrium concentration of [H⁺] and [A^-] will be the same since one molecule of HA dissociates into one of each ion

• This means you can simplify and re-arrange the expression to

K_a x [HA] = [H⁺]²

[H⁺]² = K_a x [HA] 

• Taking the square roots of each side

[H⁺] = √(K_a x [HA])

• Then take the negative logs

pH = -log[H⁺] = -log√(K_a x [HA])