AQA A Level Chemistry复习笔记5.6.1 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, Ka, and has the units mol dm-3
• Values of Ka are very small, for example for ethanoic acid Ka = 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 Ka indicates the extent of dissociation
  • The higher the value of Ka the more dissociated the acid and the stronger it is
  • The lower the value of Ka 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 Ka

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