Chapter 16Aqueous Ionic Equilibrium
Buffer Solutions•Resist changes in pH when an acid or base is added.•Act by neutralizing acid or base that is added to the buffered solution.•Contain either•Significant amounts of a weak acid and its conjugate base•Significant amounts of a weak base and its conjugate acid•Blood has a mixture of H2CO3and HCO3−
Making an Acidic Buffer SolutionIt must contain significant amounts of both a weak acid and its conjugate base.
An Acidic Buffer Solution: How it works•If a strong base is added, it is neutralized by the weak acid (HC2H3O2) in the buffer.NaOH(aq) + HC2H3O2(aq)H2O(l) + NaC2H3O2(aq) •If the amount of NaOH added is less than the amount of acetic acid present, the pH change is small.•If a strong acid is added, it is neutralized by the conjugate base (NaC2H3O2) in the buffer.HCl(aq) + NaC2H3O2 HC2H3O2(aq) + NaCl(aq) •If the amount of HCl is less than the amount of NaC2H3O2 present, the pH change is small.
How Acid Buffers Work: Addition of Base HA(aq)+ H2O(l)A−(aq)+ H3O+(aq)•Buffers work by applying Le Châtelier’sprinciple to weak acid equilibrium.•Buffer solutions contain significant amounts of the weak acid molecules, HA .•These molecules react with added base to neutralize it.HA(aq) + OH−(aq) →A−(aq) + H2O(l)•You can also think of the H3O+combining with the OH−to make H2O; the H3O+is then replaced by the shifting equilibrium.
H2OHow Buffers WorkHA+H3O+AddedHO−newA−A−
How Acid Buffers Work: Addition of AcidHA(aq)+ H2O(l)A−(aq)+ H3O+(aq)•The buffer solution also contains significant amounts of the conjugate base anion, A−. •These ions combine with added acid to make more HA.H+(aq) + A−(aq) →HA(aq)•After the equilibrium shifts, the concentration of H3O+is kept constant.
H2OHow Buffers Work+H3O+A−AddedH3O+newHAHA
Common Ion EffectHA(aq)+ H2O(l)A−(aq)+ H3O+(aq)•Adding a salt containing the anion NaA, which is the conjugate base of the acid (the common ion), shifts the position of equilibrium to the left.•This causes the pH to be higher than the pH of the acid solution, lowering the H3O+ion concentration.
Common Ion EffectHC2H3O2+ H2O H3O++ C2H3O2-
What is the pH of a buffer that is 0.100 M HC2H3O2and 0.100 M NaC2H3O2?HC2H3O2+ H2O C2H3O2+ H3O+Kafor HC2H3O2= 1.8 X 10-5
Calculate the pH of a buffer solution that is 0.200 M in HC2H3O2and 0.100 M in NaC2H3O2.HC2H3O2+ H2O C2H3O2+ H3O+Kafor HC2H3O2= 1.8 X 10-5
Henderson–Hasselbalch Equation•An equation derived from the Kaexpression that allows us to calculate the pH of a buffer solution.