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,?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

Strong acids

• Strong acids?are completely?ionised?in solution

HA (aq) → H+?(aq) + A-?(aq)

• Therefore, the concentration of hydrogen ions, H+, is?equal?to the concentration of acid, HA
• The number of hydrogen ions formed from the ionisation of water is?very small?relative to the [H+] due to ionisation of the strong acid and can therefore be?neglected
• The?total?[H+] is therefore the same as the [HA]

Answer

• • [HCl] = [H+] = 0.01 mol dm-3
  • pH = - log[H+]
  • pH = - log[0.01] = 2.00

The pH of dibasic acids

• Dibasic or diprotic acids have two replaceable protons and will react in a 1:2 ratio with bases
• Sulfuric acid is an example

H2SO4?(aq)??+ 2NaOH?(aq)?→ Na2SO4?(aq) + 2H2O?(l)

• You might think that being a strong acid it is fully ionised so the concentration of the hydrogen is double the concentration of the acid
• This would mean that 0.1 mol dm-3?would be 0.2 mol dm-3?in [H+] and have a pH of 0.69
• However, measurements of the pH of? 0.1 mol dm-3?sulfuric acid show that it is actually about pH 0.98, which indicates it is not fully ionised
• The ionisation of sulfuric acid occurs in two steps

H2SO4?→ HSO4-?+ H+

HSO4-?? SO42-?+ H+

• Although the first step is thought to be fully ionised, the second step is suppressed by the abundance of hydrogen ions from the first step creating an equilibrium
• The result is that the hydrogen ion concentration is less than double the acid concentration

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