12-7 Buffers (Section 16.6)   And you!!!!

Buffers = Solutions that resist changes to pH Buffers = Solutions that resist changes to pH. That is to say the [H+] remains constant. The addition of an acid or a base to water can cause a drastic change in pH. Many natural systems, like the human body (pH of blood is 7.35), need a stable pH to function efficiently. Buffers are chemicals that maintain a fairly stable pH, even with the addition of small amounts of acid or base.

A buffer is composed of a conjugate acid-base pair of a weak acid or a weak base. A common method of creating a buffer is to take a weak acid, like acetic acid (HC2H3O2), and mix it with an equal amount of the salt of the conjugate base, in this case sodium acetate (NaC2H3O2).

HC2H3O2(aq) + H2O(l)  H3O+ + C2H3O2 - + Na+ aweak b ca cb The salt dissociates in the aqueous solution, yielding the spectator ion Na+ and the acetate ion C2H3O2-. In essence, you now have an equilibrium with large amounts of weak acid and large amounts of the conjugate base

HC2H3O2(aq) + H2O(l)  H3O+ + C2H3O2 - + Na+ aweak b ca cb Or simplify to: HC2H3O2 ↔ H+ + C2H3O2- a lot little a lot

HC2H3O2 ↔ H+ + C2H3O2- a lot little a lot In order for the pH to remain constant, [H+] must remain constant. If acid is added, it will react with C2H3O2- and the equilibrium will shift left. This uses up C2H3O2 - – to maintain equilibrium- [H+] new ~ [H+] old

HC2H3O2 ↔ H+ + C2H3O2- a lot little a lot If base (OH-) is added, the reaction shifts right – as H+ is removed from the product side of the reaction – more is produced from HC2H3O2 and equilibrium is maintained. [H+] new ~ [H+] old In order for the pH to remain constant, H+ must remain constant!!!!!!!

Practice: What could you add to ammonia to create a buffer system? Write the equation for the buffer system and describe how the addition of acid or base shifts the equilibrium.

How about like this NH3 + H2O  NH4+ + OH- a lot little little bweak ca cb What salt would you add to make a buffer system from this weak base and it’s conjugate acid? Any salt that has an ammonium ion (like NH4Cl)… After the buffer system is established and you have a lot of NH4+ :

NH3 + H2O  NH4+ + OH- + Cl- a lot a lot little bweak ca cb If you add acid (H+) the reaction shifts right as the OH- bonds with H+ to form water. The OH- remains the same, so the pH does not change! If you add a base the reaction shifts left and the concentration of the OH- remains about the same – so once again the pH does not change!!!