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