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ASChemistryCambridge (CIE)Revision Notes7. EquilibriaBrønsted–Lowry Theory of Acids & BasesAcid & Base Dissociation

Acid & Base Dissociation (Cambridge (CIE) AS Chemistry)

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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

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 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+ + 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.

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