Acid-Base Properties of Salts

Acid-Base Properties of Salt Solutions Because ions can exhibit acid or base properties (i.e., donate or accept protons), salt solutions can be acidic or basic. -- hydrolysis: occurs when ions “react” w/H2O; the H2O is split into H+ and OH– Hydrolysis occurs when the acetate ion in sodium acetate reacts with water to produce acetic acid and hydroxide ion. OAc– + H2O HOAc + OH–

Anions of weak acids react with water to form OH–, and are thus basic. Example: Consider the weak acid HX(aq) H+(aq) + X–(aq) By the very definition of a weak acid, X– does NOT like to walk alone down the Boulevard of Broken Dreams. Now, say we have X–(aq) in solution from a salt. We’ll get... X– + H2O HX + OH– i.e., X– accepts protons, acting like a base

Ka Kb Anions of strong acids do NOT influence pH. e.g., NO3– (from, say, KNO3) wants NOTHING to do with H+. Anions that still have ionizable protons (e.g.,HSO3–) are amphoteric. -- For such substances, the larger of Ka or Kb determines the pH. Equation for Ka: HSO3– H+ + SO32– Equation for Kb: HSO3– + H2O H2SO3 +OH– Ka Kb ACID Ka Kb BASE For “amphoterics…”

Does Na2HPO4 form an acidic or a basic solution in water? Real question is: “How does HPO42– behave?” Like an acid... HPO42– H+ + PO43– ...or a base? HPO42– + H2O H2PO4– + OH– From a reference table, we find... (a) H3PO4 H+ + H2PO4– Ka1 = 7.5 x 10–3 (b) H2PO4– H+ + HPO42– Ka2 = 6.2 x 10–8 (c) HPO42– H+ + PO43– Ka3 = 4.2 x 10–13

BIGGER. Kb2 Ka1 = 7.5 x 10–3 (a) H3PO4 H+ + H2PO4– Ka2 = 6.2 x 10–8 (b) H2PO4– H+ + HPO42– Ka3 = 4.2 x 10–13 (c) HPO42– H+ + PO43– Compare (c) going forward with (b) going in reverse. Ka3 “conjugatized” Kb2 Kb2 Ka3 4.2 x 10–13 1.6 x 10–7 BIGGER. smaller. Na2HPO4 acts as a base.

Acid-Base Properties of Salts Type of Salt Examples Comment pH of solution Cation is from a strong base, anion from a strong acid KCl, KNO3 NaCl NaNO3 Both ions are neutral Neutral These salts simply dissociate in water: KCl(s)  K+(aq) + Cl-(aq)

Acid-Base Properties of Salts Type of Salt Examples Comment pH of solution Cation is from a strong base, anion from a weak acid NaC2H3O2 KCN, NaF Cation is neutral, Anion is basic Basic The basic anion can accept a proton from water: C2H3O2- + H2O  HC2H3O2 + OH- base acid acid base

Acid-Base Properties of Salts Type of Salt Examples Comment pH of solution Cation is the conjugate acid of a weak base, anion is from a strong acid NH4Cl, NH4NO3 Cation is acidic, Anion is neutral Acidic The acidic cation can act as a proton donor: NH4+(aq)  NH3(aq) + H+(aq) Acid Conjugate Proton base

Acid-Base Properties of Salts Type of Salt Examples Comment pH of solution Cation is the conjugate acid of a weak base, anion is conjugate base of a weak acid NH4C2H3O2 NH4CN Cation is acidic, Anion is basic See below IF Ka for the acidic ion is greater than Kb for the basic ion, the solution is acidic IF Kb for the basic ion is greater than Ka for the acidic ion, the solution is basic IF Kb for the basic ion is equal to Ka for the acidic ion, the solution is neutral

Acid-Base Properties of Salts Type of Salt Examples Comment pH of solution Cation is a highly charged metal ion; Anion is from strong acid Al(NO3)2 FeCl3 Hydrated cation acts as an acid; Anion is neutral Acidic Step #1: AlCl3(s) + 6H2O  Al(H2O)63+(aq) + Cl-(aq) Salt water Complex ion anion Step #2: Al(H2O)63+(aq)  Al(OH)(H2O)52+(aq) + H+(aq) Acid Conjugate base Proton