Acid-Base Reactions (College Board AP® Chemistry): Revision Note

Weak base–strong acid

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

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

• For example if NH₃ and HCl react

NH₃ (aq) + HCl (aq) → NH₄Cl (aq)

NH₃ (aq) + H₃O⁺ (aq) ⇌ NH₄⁺ (aq) + H₂O

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

pH = pK_a + log $\frac{\mathbf{\left[}\mathbf{B}\mathbf{\right]}}{\mathbf{\left[}{\mathbf{HB}}^{\mathbf{+}}\mathbf{\right]}}$

• Where pK_a is for the conjugate acid HB⁺ (pK_a = 14.00 − pK_b at 25 °C)

  • Alternatively, using pK_b: pOH = pK_b + log $\frac{[\mathrm{BH}⁺]}{\left[\mathrm{B}\right]}$, then pH = 14.00 − pOH

• This is because the remaining solution contains both members of a conjugate pair: the excess weak base (B) and its conjugate acid (HB⁺)

• 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⁺] 

OR

pH = -log10[H⁺] 

• 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)

• Acid and base reactions will favor the side with the weakest acids and bases

NH₄⁺ (aq) + H₂O (l) ⇌ NH₃ (aq) + H₃O⁺ (aq)

• NH₄⁺ is a weak acid, so will have a K_a value < 1

• Therefore the equilibrium lies to the left: most NH₄⁺ stays un-ionized

Weak acid–weak base

• When a weak acid and a weak base are mixed, they will react to an equilibrium state whose reaction may be represented by the equation:

HA (aq) + B (aq) ⇌ A^– (aq) + HB⁺ (aq)

• In an acid base reaction, the equilibrium will favor 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 favor 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 favor the formation of reactants

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

• The reaction between HSO₄^- and CO₃^2- is an example of a weak acid - weak base reaction

• The products are weaker acids and bases than the reactants, so the equilibrium will favor the formation of the products

Weak acid–weak base reaction

• For the following reaction:

HSO₄^– (aq) + CO₃^2– (aq) ⇌ HCO₃^– (aq) + SO₄^2– (aq)

• HSO₄^–/SO₄^2– is one conjugate pair and CO₃^2–/HCO₃^– is the other

  • HSO₄^– is a stronger acid than HCO₃^–, so K > 1 and the products are favored

• If the solutions are equimolar, compare the K_a and K_b values to determine whether the final solution will be acidic, neutral or basic

• This can be shown by the reaction between HF and NH₃

Reaction between HF and NH₃

• The K_a and K_b values are greater for the reactants than the products

  • This means they are stronger acids and bases

  • Therefore the equilibrium will favor the products and K will be greater than 1

• The K_a value for NH₄⁺ is greater than the K_b value for F^–, therefore it is better at forming  H₃O⁺ ions than the F^– ion is at forming OH^– ions

• Therefore the solution will be acidic