Acid & Base Dissociation (Cambridge (CIE) AS Chemistry): Revision Note

Exam code: 9701

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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

Diagram of a strong acid dissociating irreversibly into H⁺ and A⁻ ions, highlighting the nearly complete dissociation process.
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

Diagram showing weak acid dissociation into H⁺ and A⁻ ions in equilibrium. Text explains weak acid partially dissociates, with more undissociated molecules.
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 summary

  • Position of equilibrium

    • Strong acid; right

    • Weak acid; left

  • Dissociation

    • Strong acid; fully dissociated (→)

    • Weak acid; partially dissociated (rightwards harpoon over leftwards harpoon)

  • H+ concentration 

    • Strong acid; high concentration

    • Weak acid; low concentration

  • pH

    • Strong acid; use [strong acid] for [H+]

    • Weak acid; use Ka to find [H+]

  • Examples

    • Strong acid; HCl, HNO3, H2SO4 (first ionisation)

    • Weak acid; 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

Diagram showing strong base dissociation in an irreversible reaction, forming hydroxide ions (OH-) and cations (B+), with explanatory labels.
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

Diagram showing an equilibrium reaction of a weak base, forming OH⁻ and B⁺ ions. Text explains weak base is only slightly dissociated.
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 summary

  • Position of equilibrium

    • Strong base; right

    • Weak base; left

  • Dissociation

    • Strong base; fully dissociated (→)

    • Weak base; partially dissociated (rightwards harpoon over leftwards harpoon)

  • OH- concentration 

    • Strong base; high concentration

    • Weak base; low concentration

  • Examples

    • Strong base; Group 1 metal hydroxides

    • Weak base; NH3 amines, some transition metal hydroxides

Examiner Tips and Tricks

  • 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+ + HSO4-

    • 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.

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