Chapter 21 Notes, part II

Salt HydrolysisSalt HydrolysisBuffer SolutionsBuffer Solutions

Salt Hydrolysis• Remember,

Acid+BaseSalt+Water• Some salts are neutral, but

others are acidic or basic.• Why? Because some salts

promote hydrolysis.• Hydrolysis—the reaction of an

ion with water

Salt Hydrolysis• The cations or anions from the

dissociated (dissolved in water) salt remove hydrogen ions from or donate hydrogen ions to water.

• Depending on the direction of the hydrogen ion transfer, the salt can be acidic or basic.

• Strong Acid+Strong BaseNeutral Solution

• Strong Acid + Weak Base Acidic Solution

• Strong Base + Weak Acid Basic Solution

•Why is this?

Are the following salts going Are the following salts going to be acidic, basic or neutral?to be acidic, basic or neutral?

NaCl

NH4NO3

KC2H3O2

Ca(ClO3)2

FeBr2

KMnO4

MgSO4

LiNO3

ZnCl2

AgBr

Buffers• Buffer—A solution in which the

pH remains relatively constant when a small amount of acid or base is added.

• A buffer is made by making a solution of a weak acid and its salt or a weak base and its salt.

How does a buffer work?• EX: HC2H3O2 and NaC2H3O2

• In soln, the following reactions take placeHC2H3O2 H+ + C2H3O2

-

NaC2H3O2 Na+ + C2H3O2-

• If acid is added, the acetate ion acts like a H+ sponge. C2H3O2

- + H+ HC2H3O2

• If a base is added, the acetic acid will nneutralize it! HC2H3O2+OH-C2H3O2

-+H2O

Buffer Capacity• The buffer capacity is the

amount of acid or base that a buffer can hold before significant change in pH occurs.