Weak Acid and Base Equilibria

Weak Acid and Base Equilibria

Conceptual Understanding

  • In solutions of weak acids and weak bases, an equilibrium occurs between the weak acid or base and the ions it forms when it donates or accepts a proton, also known as the dissociation process.

  • A weak acid (HA) partially dissociates into ions: HA <=> H+ + A-, whilst a weak base (B) partially reacts with water to form OH- ions: B + H2O <=> BH+ + OH-

  • The double arrow indicates the reversible nature of these reactions and that at equilibrium, both reactant and product are present.

  • The extent of this dissociation is expressed by the acid’s acid dissociation constant (Ka) or a base’s base dissociation constant (Kb).

Understanding Ka and Kb

  • Ka (Acid dissociation constant) is the measure of the strength of an acid in solution, defined as the equilibrium constant for the reaction of the acid dissociating in water to give H⁺ and A⁻ ions.

  • Similarly, Kb (Base dissociation constant) measures the strength of a base in solution. It is defined as the equilibrium constant for a base’s reaction with water to give BH⁺ and OH⁻ ions.

  • The weaker an acid or base, the smaller the value of Ka or Kb, indicating less ionisation.

Important Equations

  • The expression for Ka in terms of concentrations is Ka = [H+][A-]/[HA], hence pKa = -log(Ka). A lower pKa corresponds to a stronger acid.

  • In a similar vein, the expression for Kb is Kb = [BH+][OH-]/[B], so pKb = -log(Kb). A lower pKb corresponds to a stronger base.

pH and pOH

  • pH is a measure of the hydrogen ion concentration in a solution. It is defined as pH = -log [H+]. For a given weak acid or base equilibrium, using the concentrations and the Ka or Kb, one can calculate the pH or pOH of the solution.

  • pOH measures the hydroxide ion concentration in a solution. It is defined as pOH = -log [OH-].

Constraints and Limitations

  • The values of Ka and Kb are temperature dependant; changing temperature changes the equilibrium position.

  • These equations assume that water’s ionisation (which also occurs to a small extent in any solution) is negligible, which might not hold true in all scenarios.

  • Additionally, concentrations usually change during a reaction, which can impact the predicted pH.

Remember: Understanding weak acid and base equilibria depends on a good grasp of equilibrium principles. Practice with calculating pH, pOH, Ka, and Kb will enhance your proficiency in handling such equations.