Acid Dissociation Constant (AQA A Level Chemistry)

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

Answer

Weak acids

Answer

Ethanoic acid is a weak acid which ionises as follows:

CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO^- (aq)

Step 1: Write down the equilibrium expression to find K_a

Step 2: Simplify the expression

The ratio of H⁺ to CH₃COO^- ions is 1:1

The concentration of H⁺ and CH₃COO^- ions are therefore the same

The expression can be simplified to:

Step 3: Rearrange the expression to find [H⁺]

Step 4: Substitute the values into the expression to find [H⁺]

= 1.32 x 10^-3 mol dm^-3

Step 5: Find the pH

pH = -log[H⁺]

= -log(1.32 x 10^-3)

= 2.88