Explain why a buffer made of equal concentrations of HF and F- does not change in pH when a small amount of HCl (a strong acid) is added to the solution. Does the HCl dissociate into ions when it is added to the solution? What do the hydrogen ions react with? Why don’t these hydrogen ions change the pH?
In order to explain why a buffer made of equal concentrations of HF (hydrofluoric acid) and F- (fluoride ion) does not change in pH when a small amount of HCl (hydrochloric acid, a strong acid) is added, we need to understand the concept of buffers and their properties.
A buffer is a solution that resists changes in its pH when small amounts of acid or base are added to it. It consists of a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal amounts. In this case, HF is a weak acid and F- is its conjugate base.
When HCl is added to the buffer solution, it does indeed dissociate into ions. HCl is a strong acid, meaning it almost completely dissociates into H+ (hydrogen ions) and Cl- (chloride ions) in solution. Therefore, the HCl will ionize into H+ and Cl- ions upon addition to the buffer solution.
In the buffer solution, the hydrogen ions (H+) from the dissociation of HCl will react with the conjugate base F- to form HF. This is an example of a reaction known as neutralization, where the acidic component (H+) reacts with the basic component (F-) to form the weak acid (HF). This reaction helps maintain the pH of the buffer solution.
The reason these hydrogen ions do not significantly change the pH of the buffer is because the buffer system effectively neutralizes them. Since the buffer contains a significant concentration of both the weak acid (HF) and its conjugate base (F-), any added H+ ions will readily react with the F- ions to form more HF. This reaction consumes the excess H+ ions and prevents large changes in the overall concentration of H+ ions.
By maintaining a relatively constant concentration of H+ ions in the solution, the buffer keeps the pH stable. It resists significant changes in pH even if additional acid or base is introduced.
In summary, the buffer made of equal concentrations of HF and F- does not change in pH when a small amount of HCl is added because the H+ ions from the dissociation of HCl react with the F- ions in the buffer solution to form more HF. This reaction effectively neutralizes the added H+ ions, preventing any significant change in the solution's pH.