CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.7.9 Acid & Base Dissociation

Acid & Base 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 almost complete dissociation of a strong acid in aqueous solution

  • The solution formed is highly acidic due to the high concentration of the H+/H3O+ ions
  • Since the pH depends on the concentration of H+/H3O+ ions, the pH can be calculated if the concentration of the strong acid is known

 

Equilibria pH Formula, downloadable AS & A Level Chemistry revision notes

pH is the negative log of the concentration of H+/H3O+ ions and can be calculated if the concentration of the strong acid is known using the stoichiometry of the reaction

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 almost complete dissociation of a weak acid in aqueous solution

 

  • The solution is less acidic due to the lower concentration of H+/H3O+ ions
  • Finding the pH of a weak acid is a bit more complicated as now the concentration of H+ ions is not equal to the concentration of acid
  • To find the concentration of H+ ions, the acid dissociation constant (Ka) should be used

Acid & equilibrium position table

Equilibria Table 1_Acid & Base Dissociation, downloadable AS & A Level Chemistry revision notes

Strong bases

  • A strong base is a base that dissociates almost completely in aqueous solutions
    E.g. group 1 metal hydroxides such as NaOH (sodium hydroxide)
    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 Base, downloadable AS & A Level Chemistry revision notes

The diagram shows the almost complete dissociation of a strong base in aqueous solution

 

  • The solution formed is highly basic due to the high concentration of the OH ions

Weak bases

  • A weak base is a base that partially (or incompletely) dissociates in aqueous solutions
    • NH3 (ammonia), amines and some hydroxides of transition metals
  • The position of the equilibrium is more to the left and an equilibrium is established

 

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

The diagram shows the almost complete dissociation of a weak base in aqueous solution

 

  • The solution is less basic due to the lower concentration of OH ions

Base & equilibrium position table

Equilibria Table 2_Acid & Base Dissociation, downloadable AS & A Level Chemistry revision notes

Exam Tip

Hydrogen ions in aqueous solutions can be written as either as H3O+ or as H+ however, if H3O+ is used, H2O should be included in the chemical equation:

HCl(g) → H+(aq) + Cl(aq)

or

HCl(g) + H2O(l) → H3O+(aq) + Cl(aq)

 

Remember that some acids are both strong and weak acids – for example, H2SO4 (sulfuric acid) has two hydrogen ions that can ionise.

H2SO4 acts as a strong acid: H2SO4 → H+ + SO4

HSO4 acts as a weak acid: HSO4 ⇌ H+ + SO42-

 

Also, don’t forget that the terms strong and weak acids and bases are related to the degree of dissociation and not the concentration.

The appropriate terms to use when describing concentration are dilute and concentrated.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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