Acid Strength (Edexcel International A Level Chemistry)

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Acids - 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

Acids - 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

pKa

  • The range of values of Ka is very large and for weak acids, the values themselves are very small numbers

Table of Ka values

Table of Ka values, downloadable AS & A Level Chemistry revision notes

  • For this reason it is easier to work with another term called pKa
  • The pKa  is the negative log of the Ka value, so the concept is analogous to converting [H+] into pH values

pKa = -logKa

  • Looking at the pKa values for the same acids:

Table of pKvalues

Table of pKa values, downloadable AS & A Level Chemistry revision notes

  • The range of pKa values for most weak acids lies between 3 and 7

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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.