UNIT IV PPT #3 Ka and Kb KEY.notebook. November 23, WHAT IS Ka? UNIT IV. CALCULATIONS USING Ka. WHAT IS Ka? Nov 10 9:42 PM.

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1 WHAT IS Ka? Recall: Find ph of M HCl. But What is ph of 0.10 M HF? UNIT IV Ka and Kb CALCULATIONS USING Ka I. [H 3O + ] (or ph) from Ka Ex. Find the [H 3O + ] in 0.10 M HF. WHAT IS Ka? Look at equilibrium for weak acid HF: HF (aq) + H 2O (l) H 3O + (aq) + F (aq) Keq = [H 3O + ][F ] [HF] For WA s Keq is called K a (acid ionization constant). Hebden Textbook Questions #74, 75, 79, 81 See acid table for list of Ka s. higher Ka stronger acid lower Ka weaker acid For SA s (eg. HCl) Ka = [H 3O + ] [Cl ] = called very large [HCl] 1

2 CALCULATIONS USING Ka CALCULATIONS USING Ka II. Ka from ph Ex. A M solution of the weak acid HA has a ph of Find the Ka of HA. III. Co from ph and Ka Ex. Find the concentration of HCOOH needed to form a solution with ph = Hebden Textbook Questions #76, 78 Hebden Textbook Questions #77, 80 WHAT IS Kb? Base Ionization: NH 3 is a very common weak base. It partially ionizes in water to form NH 4 + and OH : NH 3(aq) + H 2O (l) NH 4 + (aq) + OH (aq) Kb expression: Kb = [NH 4+ ] [OH ] [NH 3] Equilibrium constant is called base ionization constant (Kb). WHAT IS Kb? Ex. CN (aq) + H 2O (l) HCN (aq) + OH (aq) Kb expression: Kb = [HCN][OH ] [CN ] Ex. N 2H 4 (aq) + H 2O (l) N 2H 5 + (aq) + OH (aq) Kb expression: Kb = [N 2H 5+ ] [OH ] [N 2H 4] Hebden Textbook Page 128 Question #32 2

3 Look at hydrolysis of base F : F + H2O HF + OH Kb (F ) = [HF] [OH ] [F ] Look at ionization of weak acid HF: HF + H2O H3O + + F Ka (HF) = [H3O + ] [F ] [HF] In general: Ka (HF) x Kb (F ) = Kw Or Kb (F ) = Kw Ka (HF) Kb (weak base) = Kw Ka (its conj. acid) Multiply Ka[HF] x Kb[F ] Ka[HF] x Kb[F ] = [H3O + ] [F ] x [HF] [OH ] = [H3O + ] [OH ] [HF] [F ] Using Acid Table: Find base on right side ( if amphiprotic locate base on right side only). Its conjugate acid will be across from it on the left side. The Ka of its conjugate acid is on the far right of the same line. Use equation: Kb (base) = Kw Ka (conj. acid) Ex. Calculate the Kb for HCO 3. Ex. Find Kb of SO

4 CALCULATIONS USING Kb I. [OH ] (or ph) from Kb Ex. Find [OH ] in a 0.20 M solution of KNO 2 (this is a salt, so it must be dissociated into its ions first). Similarly, if Kb (base) given: Ka (weak acid) = Kw Kb(its conj. base) NOTES: Table only states Ka values. For questions like this Kb will have to be calculated if not given. All Ka s on table are 2 SD s limits any calculation using them to 2 SD s maximum. The larger the Kb, the stronger the weak base the more OH produced. The smaller the Ka of an acid, the larger the Kb of its conjugate Base. Weaker acids have stronger conjugate bases. CALCULATIONS USING Kb II. Kb from ph Ex. At a certain temperature, a 0.20 M solution of K 2SO 3 has a ph of Calculate the Kb of SO 3 2 at this temp. WHAT IS HYDROLYSIS? Reaction between a salt (ion or ions in a salt) and water to produce an acidic or basic solution. Net ionic equation for hydrolysis: ion + water molecule or ion + H 3O + or OH Hebden Textbook Page 153 Questions #84, 87, 88, 89 4

5 SPECTATORS IN HYDROLYSIS Spectator Cations (look on Periodic Table): Group 1 (Alkali Metal ions) eg. Li +, Na +, K +, Rb +, Cs +, Fr + Group 2 (Alkaline Earth ions) eg. Be +, Mg 2+, Ca 2+, Ba 2+, Sr 2+, Ra 2+ Spectator Anions (look on Acid Table): Top 5 ions on the right side of table. ClO 4 I Br Cl NO 3 HSO 4 is not a spectator it is amphiprotic will be dealt wit later Spectators are eliminated in net ionic equations for hydrolysis! PROCESS FOR HYDROLYSIS Strategy: (dissociate eliminate evaluate) Write dissociation equation. Eliminate spectators. Remaining ions: left side of table undergo acid hydrolysis produce H 3O + right side of table undergo base hydrolysis produce OH amphiprotic determine K a and K b to find dominant hydrolysis HYDROLYSIS Ex. Is the salt NaF acidic, basic or neutral in water? Ex. Is the salt NH 4NO 3 acidic, basic or neutral in aqueous solution? Ex. Is the salt KCl acidic, basic or neutral? Hydrolyzing Cations: (LEFT SIDE OF ACID TABLE) Fe(H 2O) 6 3+ = Fe 3+ Cr(H 2O) 6 3+ = Cr 3+ undergo acid hydrolysis Al(H 2O) 6 3+ = Al 3+ NH 4 + Hydrolyzing Anions: (RIGHT SIDE OF ACID TABLE) Most of the anions from IO 3 down to PO 4 3 will undergo base hydrolysis. (Amphiprotic anions will be discussed next.) 5

6 Ex. Is the salt ammonium nitrite NH 4NO 2 acidic, basic or neutral? Amphiprotic Anions: Ions which start with H and have a negative charge. Eg. HSO 4, HSO 3, H 2PO 4, HPO 4 2, HS Ex. Determine whether the salt NH 4CN (ammonium cyanide) is acidic, basic or neutral. If Then the predominant And, in aqueous solution, the hydrolysis is: ion: Ka the ion > Kb the ion ACID HYDROLYSIS Acts as an Acid Kb the ion > Ka the ion BASE HYDROLYSIS Acts as a Base HYDROLYSIS...PUTTING IT ALL TOGETHER Ex. Calculate the ph of 0.30 M Na 2CO 3. Ex. Find the predominant hydrolysis of the hydrogen carbonate ion (HCO 3 ) and write the netionic equation for it. Hebden Textbook Page 148 Questions #

7 HYDROLYSIS...PUTTING IT ALL TOGETHER Ex. Calculate the ph of a 0.24 M solution of the salt aluminum nitrate. Nov 23 1:22 PM 7