8.4 Strong and weak acids and bases

Strengths of acids and bases

  • Strength of an acid or base depends on the degree to which it ionizes (dissociates) in water
  • In the following reactions, an acid or a base reacts with water to produce conjugates
    • Strong acid is an effective proton donor that is assumed to completely dissociate in water.
      • Examples: HCl, H2SO4 and HNO3
      • Conjugate base of strong acid is weak base
    • Weak acid only partially dissociates in water; it is a poor proton donor.
      • Examples: CH3COOH and H2CO3
      • The dissociation of a weak acid is a reversible reaction that reaches equilibrium
      • Only small portion of acid molecules have dissociated at equilibrium
      • Conjugate base of weak acid is strong base
    • Strong base completely dissociates in water
      • Examples: KOH, NaOH (Any group 1 metal hydroxides)
      • Note: Metal hydroxides don’t act as Bronsted-Lowry bases, however in solution the hydroxide ion OH– acts as a base
    • Weak base partially dissociates in water
      • Example: NH3
  • Words strong and concentrated, and weak and diluted have very different meanings.
    • Strong: completely dissociated into ions
    • Concentrated: a high number of moles of solute per litre of solution
    • Weak: slightly dissociated
    • Diluted: a low number of moles of solute per litre of solution

Experimental determination of the strength of acids and bases

  • Techniques to compare strength of acids and bases of equal concentration
  • Conductivity
    • All acids and bases dissociate in water to form ions. The conductivity of an aqueous solution depends on the concentration of ions present. Voltage is applied to different solutions (of equal concentration), and the voltage reflects on concentration of ions, and thus strength as well.
    • Strong acids and bases have higher conductivities
    • Weak acids and bases have lower conductivities

Energy changes on neutralization

  • Neutralization: occurs when an acid and base react together
  • This reaction is exothermic, ΔH < 0
  • The enthalpy change of neutralization for a strong acid is almost identical to that of weak acid
    • For strong acid or base:
      • They are completely dissociated in water, thus only enthalpy change to consider is exothermic formation of water from H+ and OH– ions
    • For weak acid or base:
      • Dissociation of weak acid or base is mildly endothermic
      • Enthalpy change for strong base-weak acid is less than strong base-strong acid
      • The weaker the acid, the more endothermic, and thus lower enthalpy change of neutralization

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