Acid-Base Reaction Buffer Steps

Strong Acid and Strong Base

When a strong acid and a strong base are mixed, they react quantitatively in a reaction represented by the equation:

• H⁺ (aq) + OH^– (aq) → H₂O (l)

pH determined by concentration of excess reagent

Weak Base-Strong Acid

• If the weak base is in excess, then a buffer solution is formed, and the pH can be determined from the Henderson–Hasselbalch equation equation:

  pOH = pK_b + log [B]/[BH+]

• If the strong acid is in excess, then the pH can be determined from the moles of excess hydronium ion and the total volume of solution

pH = -log10[H₃O⁺]

• If they are equimolar, then the (slightly acidic) pH can be determined from the equilibrium represented by the equation:

HB⁺ (aq) + H₂O (l) ⇌ B (aq) + H₃O⁺ (aq)

Strong Base-Weak Acid

• If the weak acid is in excess, then a buffer solution is formed, and the pH can be determined from the Henderson-Hasselbalch equation

pH = pK_a + log[A-]/[HA]

• If the strong base is in excess, then the pH can be determined from the moles of excess hydroxide ion and the total volume of solution

• If they are equimolar, then the (slightly basic) pH can be determined from the equilibrium represented by the equation:

A^– (aq) + H₂O (l) ⇌ HA (aq) + OH^– (aq)

Weak Acid-Weak Base

• In an acid base reaction, the equilibrium will favour the side with the weaker acid and weaker base

• If the equilibrium constant for the reaction is greater than one (K >1) the equilibrium will favour the formation of products

  • This means the weaker acid and weaker base are on the product side

• If the equilibrium constant for the reaction is less than one (K <1) the equilibrium will favour the formation of reactants

  • This means the weaker acid and weaker base are on the reactant side