Edexcel A Level Chemistry:复习笔记5.2.3 Acid Strength

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