Syllabus Edition

First teaching 2023

First exams 2025

|

Acid & Base Dissociation (CIE AS Chemistry)

Revision Note

Test yourself
Caroline

Author

Caroline

Last updated

Acid & Base Dissociation

Strong acids

  • A strong acid is an acid that dissociates almost completely in aqueous solutions
    • E.g. 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

 Diagram showing the dissociation of a strong acid in aqueous solution

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

In an aqueous solution, a strong acid almost completely dissociates

  • 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
    • The concentration of H+ / H3O+ ions can be written as [H+ (aq)]
  • 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

pH = –log10 [H+ (aq)]

Weak acids

  • A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
    • E.g. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
  • The position of the equilibrium is more to the left and an equilibrium is established

Diagram showing the dissociation of a weak acid in aqueous solution

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

In an aqueous solution, a weak acid does not fully dissociate

  • 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

  Strong acid Weak acid
Position of equilibrium Right Left
Dissociation Completely (→) Partially (rightwards harpoon over leftwards harpoon)
H+ concentration  High Low
pH Use [strong acid] for [H+] Use Ka to find [H+]
Examples HCl
HNO3 
H2SO4 (first ionisation)
Organic acids, e.g. ethanoic acid
HCN
H2S
H2CO3

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

 Diagram showing the dissociation of a strong base in aqueous solution

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

In an aqueous solution, a strong base almost completely dissociates

  • 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
    • E.g. NH3 (ammonia), amines and some hydroxides of transition metals
  • The position of the equilibrium is more to the left and an equilibrium is established

Diagram showing the dissociation of a weak base in aqueous solution

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

In an aqueous solution, a weak base does not fully dissociate

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

Base & equilibrium position table

  Strong base Weak base
Position of equilibrium Right Left
Dissociation Completely (→) Partially (begin mathsize 16px style rightwards harpoon over leftwards harpoon end style)
OH concentration  High Low
Examples Group 1 metal hydroxides NH3 
Amines
Some transition metal hydroxides

Examiner 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.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.