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?
Buffers work because of the way they are constructed. Buffers consist of a weak acid and a salt of the weak acid. For example, HF for the weak acid and NaF as the salt.
When a strong acid, such as HCl, is added, the multiple H^+ are used up by the salt to form more of the weak acid HF.
F^- + H^+ ==> HF. Effectively you are taking a strong acid and making it into a weak acid which ionizes only partially. When we add a base, such as NaOH, to the solution, it is used up by the acid. HF + NaOH ==> NaF + H2O. So the base is neutralized by the weak acid to form the salt + water.
A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added to it. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. In the case of a buffer made of equal concentrations of HF (hydrofluoric acid) and F- (fluoride ions), we can examine why the pH remains relatively unchanged when a small amount of HCl (hydrochloric acid) is added.
When HCl is added to the buffer solution, it does indeed dissociate into its ions, H+ (hydrogen ions) and Cl- (chloride ions). The presence of hydrogen ions raises the concentration of H+ in the solution. Now, these hydrogen ions can potentially react with the fluoride ions, as they are a weak base and can accept hydrogen ions to form HF. This reaction is described as follows:
H+ (aq) + F- (aq) → HF (aq)
It is important to note that HF is a weak acid. Weak acids only partially dissociate in water, and their dissociation equilibrium lies towards the undissociated form. Therefore, when a small amount of HCl is added to the solution, the reaction between H+ and F- leads to the formation of HF, but only a fraction of the fluoride ions react with the hydrogen ions.
At the same time, the reaction also generates chloride ions, Cl-. These chloride ions do not significantly affect the pH of the solution since they are the conjugate base of a strong acid (HCl), which means they are highly stable and do not contribute to the acidity of the solution.
The reason why the pH of the buffer solution remains relatively unchanged when HCl is added is due to the principle of Le Chatelier's principle. The buffer system, composed of HF and F-, is able to resist changes in pH by effectively neutralizing the added hydrogen ions. Even though some hydrogen ions react with fluoride ions to form HF, the equilibrium shifts back towards the undissociated HF form, maintaining the initial concentration of HF and F- and thus keeping the pH stable.
Overall, it's the presence of a weak acid and its conjugate base in equal concentrations that allows the buffer solution to absorb or release hydrogen ions, preventing significant changes in pH when small amounts of strong acid (in this case, HCl) are added.