8.2.7 Strong & Weak Acids & Bases

Strong acids

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

The diagram shows the complete dissociation of a strong acid in aqueous solution

• 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

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

Weak acids

• A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
  • Eg. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H₂S (hydrogen sulfide) and H₂CO₃ (carbonic acid)

• The position of the equilibrium is more over to the left and an equilibrium is established

The diagram shows the partial dissociation of a weak acid in aqueous solution

• The solution is less acidic due to the lower concentration of H⁺/H₃O⁺ ions
• Finding the pH of a weak acid requires using the acid dissociation constant, K_a but this not required at Standard Level, but only at Higher Level and is covered in Topic 18

Acid & Equilibrium Position Table

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

The diagram shows the complete dissociation of a strong base in aqueous solution

• 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
  • NH₃ (ammonia), amines and some hydroxides of transition metals

• The position of the equilibrium is more to the left and an equilibrium is established

The diagram shows the partial dissociation of a weak base in aqueous solution

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

Base & Equilibrium Position Table

• The conjugate base of HCl is the chloride ion, Cl^-, but since the reverse reaction is virtually non-existent the chloride ion must be a very weak conjugate base

HCl (g) → H⁺ (aq)  +   Cl^- (aq)

acid                          conjugate base

• In general strong acids produce weak conjugate bases and weak acids produce strong conjugate bases
• A strong base is also fully ionized and is a good proton acceptor
• For example the hydroxide ion is a strong base and readily accepts protons:

OH^- (aq)+  H⁺ (aq)  ⇌  H₂O (l)

• The conjugate acid of the hydroxide ion is water, which is a weak conjugate acid
• In general strong bases produce weak conjugate acids