Acid & Base Dissociation (Cambridge International AS Chemistry)

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

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 (A Level only)

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

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 (K_a) 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 ($\rightleftharpoons$)

• H⁺ concentration

  • Strong acid; high concentration
  • Weak acid; low concentration

• pH

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

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

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 ($\rightleftharpoons$)

• 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

Exam Hint

• 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⁺ + SO₄^2-

• 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.