Acids: Strong vs Weak

Acids

• Acids are proton (H+) donors
• Acid molecules release hydrogen ions when they dissociate in solution
  • e.g. HCl(aq) → H+(aq) + Cl-(aq)
• Acids are classified as strong or weak based on their dissociation level in solution

Strong Acids

• Strong acids fully dissociate in solution
• Meaning every molecule releases its H+ ions
• Hydrochloric acid (HCl(aq) → H+(aq) + Cl-(aq)) is a strong acid
• Nitric acid (HNO3(aq) → H+(aq) + NO3-(aq)) is a strong acid
• Sulfuric acid (H2SO4(aq) → 2H+(aq) + SO4-(aq)) is a strong acid
• Monoprotic acids release only 1 H+ ion per molecule
  • Hydrochloric and nitric acids are monoprotic
• Diprotic acids release 2 H+ ions
  • Sulfuric acid is diprotic

Weak Acids

• Weak acids only partially dissociate in solution
• Meaning only some molecules release H+ ions
• They exist in equilibrium
• Ethanoic acid (CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)) is a weak acid

Important Considerations for Acids

• "Strong" and "weak" do not refer to concentration
• Concentration refers to the number of molecules per unit volume
• A concentrated weak acid solution and a dilute strong acid solution is possible
• A strong acid, at equal concentration to a weak acid, will have a lower pH
• Strong acids fully dissociate, so the concentration of H+ ions is known if the concentration of acid is known
  • This is needed to calculate pH
  • e.g. a 0.02 mol.dm-3 solution of HCl(aq) has an H+ concentration of 0.02 mol.dm-3
• Diprotic strong acids must be accounted for
  • e.g. a 0.02 mol.dm-3 solution of H2SO4(aq) has an H+ concentration of 0.04 mol.dm-3
• Weak acids exist in equilibrium
  • Le Chatelier’s Principle applies

Bases

• Bases are proton (H+) acceptors
• Bases accept protons (H+) from acids
• Soluble bases are classified as "alkalis"
  • Metal oxides, carbonates, and hydroxides are bases
  • Group 1 Hydroxides are alkalis because they are soluble

Strong Bases

• Strong bases fully dissociate in solution
• Every molecule releases its OH- ions
  • Which accept protons
• Common strong bases include:
  • Sodium hydroxide (NaOH(aq) → Na+(aq) + OH-(aq))
  • Potassium hydroxide (KOH(aq) → K+(aq) + OH-(aq))
  • Barium hydroxide (Ba(OH)2(aq) → Ba2+(aq) + 2OH-(aq))

Weak Bases

• Weak bases only partially dissociate in solution
• Only some of the molecules accept H+ ions, existing in an equilibrium
• Ammonia (NH3(aq) + H+(aq) ⇌ NH4+(aq)) is a weak base

Important Considerations for Bases

• A strong base, at equal concentration to a weak base, has a higher pH as it accepts more H+ ions
• It is not necessary to calculate the pH of a base/alkali
• Diprotic strong bases must be accounted for
  • e.g. Group 2 hydroxides
  • A 0.02 mol.dm-3 solution of Ba(OH)2(aq) has an OH- concentration of 0.04 mol.dm-3
• Weak bases exist in equilibrium, so Le Chatelier’s Principle applies

Neutralisation

• Neutralisation occurs between an acid and an alkali (base)
• In its simplest form: H+(aq) + OH-(aq) → H2O(l)

Neutralisation Reactions

• Acids react with bases & alkalis to form a salt & water:
  • Acid + Metal Oxide → Salt + Water
  • Acid + Metal Hydroxide → Salt + Water
  • Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
• The name of the salt is determined by the type of acid used and the metal in the base:
  • Hydrochloric acid gives chloride
  • Nitric acid gives nitrate
  • Sulfuric acid gives sulfate
  • Ethanoic acid gives ethanoate