Question

1) How do the effects of a small amount of strong acid or strong base on water vs those on a buffer differ? Explain what happens on a molecular level in a diluted buffer, undiluted buffer and water solution when a small amount of strong acid or base is added.

2)Explain the term buffer capacity and what observations would indicate that the buffer capacity has been exceeded,

Answer 1

1) Addition of small amount of, strong acid or strong base on water makes it (water) acidic or basic/alkaline respectively. However, addition of a small amount of strong acid or strong base on buffer does not make any effect on the pH of the buffer solution.
Addition of a strong base to a buffer solution, the hydrogen ion concentration decreases. The acid and its conjugate base consume the hydroxide ions. This happens as per Le Chatelier's Principle, the position of equilibrium moves to the right to make up for the loss of hydrogen ion in the reaction with the base. Similarly, addition of strong acid increase the hydrogen ion concentration. And as per  Le Chatelier's Principle, the position of equilibrium moves to the left to make up for the gain of hydrogen ion in the reaction with the acid.

2) Buffer capacity is defined as the measure of the efficiency of a buffer in resisting changes in pH. The buffer capacity depends essentially on the: Ratio of the salt to the acid or base. The ratio of [A^-]/[HA] influences the pH of a solution, the actual concentrations of A^- and HA influence the effectiveness of a buffer.

Addition of a strong acid such as HCl.
A^- + HCl → HA + Cl^-.

This changes the pH by lowering the ratio [A^-]/[HA], but as long as there is still a lot of A^- present, the change in pH will be small. But if we keep adding HCl, the weak base A^- will eventually run out. Once the A^- is gone, any additional HCl will donate its proton to water (HCl + H₂O → H₃O⁺ + Cl^-). This will dramatically increase the concentration [H⁺] and so the pH drops.

This is called "buffer solution breaking", and the amount of acid a buffer can absorb before it breaks the "buffer capacity for addition of strong acid". A solution with more weak base, [A^-], has a higher buffer capacity for addition of strong acid.

Similarly, a buffer breaks when the amount of strong base added is so large it consumes all the weak acid,
HA + OH^- → A^-+ H₂O.
A solution with more weak acid, [HA], has a higher buffer capacity for addition of strong base.

So, the pH of a buffer is determined by only the ratio [A^-]/[HA], the ability of the buffer to absorb strong acid or base is determined by the individual concentrations of [A^-] and [HA].