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UNIT IV Ka and Kb. W HAT IS K A ? Recall: Find pH of 0.100 M HCl. But… What is pH of 0.10 M HF?

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Presentation on theme: "UNIT IV Ka and Kb. W HAT IS K A ? Recall: Find pH of 0.100 M HCl. But… What is pH of 0.10 M HF?"— Presentation transcript:

1 UNIT IV Ka and Kb

2 W HAT IS K A ? Recall: Find pH of M HCl. But… What is pH of 0.10 M HF?

3 W HAT IS K A ? Look at equilibrium for weak acid HF: HF (aq) + H 2 O (l) H 3 O + (aq) + F - (aq) Keq = [H 3 O + ][F - ] [HF] For WAs Keq is called K a ( acid ionization constant). See acid table for list of Kas. higher Ka stronger acid lower Ka weaker acid For SAs (eg. HCl) Ka = [H 3 O + ] [Cl - ] = called very large [HCl]

4 C ALCULATIONS U SING K A I.[H 3 O + ] (or pH) from Ka Ex. Find the [H 3 O + ] in 0.10 M HF. Hebden Textbook Questions #74, 75, 79, 81

5 C ALCULATIONS U SING K A II. Ka from pH Ex. A M solution of the weak acid HA has a pH of Find the Ka of HA. Hebden Textbook Questions #77, 80

6 C ALCULATIONS U SING K A III.Co from pH and Ka Ex. Find the concentration of HCOOH needed to form a solution with pH = Hebden Textbook Questions #76, 78

7 W HAT IS K B ? 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 2 O (l) NH 4 + (aq) + OH - (aq) Kb expression: Kb = [NH 4 + ] [OH - ] [NH 3 ] Equilibrium constant is called base ionization constant (Kb).

8 W HAT IS K B ? Ex. CN - (aq) + H 2 O (l) HCN (aq) + OH - (aq) Kb expression: Kb = [HCN][OH - ] [CN - ] Ex. N 2 H 4 (aq) + H 2 O (l) N 2 H 5 + (aq) + OH - (aq) Kb expression: Kb = [N 2 H 5 + ] [OH - ] [N 2 H 4 ] Hebden Textbook Page 128 Question #32

9 H OW TO D ETERMINE V ALUE OF K B Look at hydrolysis of base F - : F - + H 2 O HF + OH ­ ­ Kb (F-) = [HF] [OH - ] [F - ] Look at ionization of weak acid HF: HF + H 2 O H 3 O + + F - Ka (HF) = [H 3 O + ] [F - ] [HF] Multiply Ka [HF] x Kb [F - ] Ka [HF] x Kb [F - ] = [H 3 O + ] [F - ] x [HF] [OH - ] = [H 3 O + ] [OH - ] [HF] [F - ]

10 H OW TO D ETERMINE V ALUE OF K B Ka (HF) x Kb (F - ) = Kw Or Kb (F - ) = Kw Ka (HF) In general: Kb (weak base) = Kw Ka (its conj. acid)

11 H OW TO D ETERMINE V ALUE OF K B Using Acid Table: 1. Find base on right side ( if amphiprotic -locate base on right side only). 2. Its conjugate acid will be across from it on the left side. 3. The Ka of its conjugate acid is on the far right of the same line. 4. Use equation: Kb (base) = Kw Ka (conj. acid)

12 H OW TO D ETERMINE V ALUE OF K B Ex. Calculate the Kb for HCO 3 -. Ex. Find Kb of SO 3 2-.

13 H OW TO D ETERMINE V ALUE OF K B 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 Kas on table are 2 SDslimits any calculation using them to 2 SDs 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.

14 C ALCULATIONS U SING K B 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).

15 C ALCULATIONS U SING K B II. Kb from pH Ex. At a certain temperature, a 0.20 M solution of K 2 SO 3 has a pH of Calculate the Kb of SO 3 2- at this temp. Hebden Textbook Page 153 Questions #84, 87, 88, 89

16 W HAT IS H YDROLYSIS ? 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 3 O + or OH -

17 S PECTATORS IN H YDROLYSIS 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 with later Spectators are eliminated in net ionic equations for hydrolysis!

18 P ROCESS FOR H YDROLYSIS Strategy: (dissociate eliminate evaluate) 1. Write dissociation equation. 2. Eliminate spectators. 3. Remaining ions: left side of table – undergo acid hydrolysis – produce H 3 O + right side of table – undergo base hydrolysis – produce OH - amphiprotic – determine K a and K b to find dominant hydrolysis

19 H YDROLYSIS Ex. Is the salt NaF acidic, basic or neutral in water? Ex. Is the salt NH 4 NO 3 acidic, basic or neutral in aqueous solution? Ex. Is the salt KCl acidic, basic or neutral?

20 H YDROLYZING I ONS Hydrolyzing Cations: (LEFT SIDE OF ACID TABLE) Fe(H 2 O) 6 3+ = Fe 3+ Cr(H 2 O) 6 3+ = Cr 3+ undergo acid hydrolysis Al(H 2 O) 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.)

21 H YDROLYZING I ONS Ex. Is the salt ammonium nitrite NH 4 NO 2 acidic, basic or neutral? Ex. Determine whether the salt NH 4 CN (ammonium cyanide) is acidic, basic or neutral.

22 H YDROLYZING I ONS IfThen the salt is: Ka (cation) > Kb (anion)Acidic Kb (anion) > Ka (cation)Basic Ka (cation) = Kb (anion)Neutral

23 H YDROLYZING I ONS Amphiprotic Anions: Ions which start with H and have a negative charge. Eg. HSO 4 -, HSO 3 -, H 2 PO 4 -, HPO 4 2-, HS - IfThen the predominant hydrolysis is: And, in aqueous solution, the ion: Ka (the ion) > Kb (the ion)ACID HYDROLYSISActs as an Acid Kb (the ion) > Ka (the ion)BASE HYDROLYSISActs as a Base

24 H YDROLYZING I ONS Ex. Find the predominant hydrolysis of the hydrogen carbonate ion (HCO 3 - ) and write the net-ionic equation for it. Hebden Textbook Page 148 Questions #69-73

25 H YDROLYSIS...P UTTING I T A LL T OGETHER Ex. Calculate the pH of 0.30 M Na 2 CO 3.

26 H YDROLYSIS...P UTTING I T A LL T OGETHER Ex. Calculate the pH of a 0.24 M solution of the salt aluminum nitrate.


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