Acid Strength (Edexcel A Level Chemistry)

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Acid Dissociation

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

Equilibria Dissociation of a Strong Acid, downloadable AS & A Level Chemistry revision notes

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

Equilibria Dissociation of a Weak Acid, downloadable AS & A Level Chemistry revision notes

The diagram shows the partial dissociation of a weak acid in aqueous solution

Enthalpy change of neutralisation 

  • The enthlapy change of neutralisation of strong acids and strong bases are very similar, around -57 to -58 kJmol-1
  • This is because the acids and alkalis are fully ionised and the neutralisation reaction between H+ + OH occurs to produce water:
    • H+ (aq) + OH(aq) → H2O (l)
  • The other ions are not involved in the reaction, i.e. are spectator ions, so do not affect neutralisation
  • As this is the reaction that is occurring in each stong acid-strong alkali reaction, then the enthalpy change of neutralisation will be very similar
  • Weak acids and weak alkalis only partially ionise, so energy has to be used to fully ionise them
  • This means that the resulting enthalpy change of neutralisation will be less exothermic , i.e. less negative
    • For example, the standard enthalpy change of neturalisation of ethanoic acid with sodium hydroxide is -55.2 kJ mol-1

Ka Expressions

  • For weak acids as there is an equilibrium we can write an equilibrium constant expression for the reaction

The acid dissociation constant, downloadable AS & A Level Chemistry revision notes

  • 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

pH Calculations for Acids

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]

Worked example

What is the pH of 0.01 mol dm-3 hydrochloric acid?

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

  • The pH of weak acids can be calculated when the following is known:
    • The concentration of the acid
    • The Ka value of the acid
  • From the Ka expression we can see that there are three variables:

The acid dissociation constant, downloadable AS & A Level Chemistry revision notes

  • However, the equilibrium concentration of [H+] and [A-] will be the same since one molecule of HA dissociates into one of each ion
  • This means you can simplify and re-arrange the expression to

Ka x [HA] = [H+]2

[H+]2 Ka x [HA] 

  • Taking the square roots of each side

[H+] = √(Ka x [HA])

  • Then take the negative logs

pH = -log[H+] = -log√(Ka x [HA])

Worked example

pH calculations of weak acids

Calculate the pH of 0.100 mol dm-3 ethanoic acid at 298 k with a Ka value of 1.74 × 10-5 mol dm-3

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

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.