Acid & Base Dissociation (CIE AS Chemistry)

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

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 table

	Strong acid	Weak acid
Position of equilibrium	Right	Left
Dissociation	Completely (→)	Partially ()
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

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 table

	Strong base	Weak base
Position of equilibrium	Right	Left
Dissociation	Completely (→)	Partially ()
OH– concentration	High	Low
Examples	Group 1 metal hydroxides	NH3  Amines Some transition metal hydroxides