Buffers Whittier Union High School District Whittier 缓冲区惠蒂尔联合高中学区惠蒂尔.pptVIP

Buffers AP Chemistry Common Ion Effect Adding a common ion to an equilibrium system will shift the equilibrium forward or reverse. For a weak acid equilibrium this can affect the pH of the solution. For example: Adding NaF to a solution of HF. HF H+ + F- Adding F- ions will shift the equilibrium to reactants and produce more HF and reduce [H+] ions in the process. This makes the pH increase. Buffers Buffers are solutions in which the pH remains relatively constant, even when small amounts of acid or base are added Contain a weak acid (HA) and its salt(NaA); H2CO3 NaHCO3 Or a weak base (B) and its (BHCl); (NH3 and NH4Cl) Buffers A buffer system is better able to resist changes in pH than pure water Since it is a pair of chemicals: one chemical neutralizes any acid added, while the other chemical would neutralize any additional base AND, they produce each other in the process!!! How a Buffer Works Consider the following buffer system HCO3- + H+ H2CO3 If you add more H+ this buffer system it will react with the conjugate base HCO3- to produce more H2CO3. If you add base, OH-, it will grab an H+ from H2CO3 to produce more HCO3- ion as follows H2CO3 + OH- H2O + HCO3- Buffer Capacity The buffer capacity is the amount of acid or base that can be added before a significant change in pH This depends on the amounts of HA and A- present in the buffer Most efficient buffer is when [A-] [HA] = 1 Henderson-Hasselbach Equation Derived from the equilibrium expression of a weak acid and the pH equation. This equation allows you to determine the pH of a buffer solution pKa = -log Ka pH = pKa + log [A-] [HA] The Common Buffer Problems 1. Compute pH of a buffer given the actual concentrations of the conjugate acid and conjugate base. The pH is easily calculated with: pH = pKa + log [A-] [HA] Example: Determine the pH in which 1.00 mole of H2CO3 (Ka = 4.2 x 10-7) and 1.00 mole NaHCO3 dissolved i