Synthesis of Alum Prem Sattsangi Copyright 2009Rev 2

Synthesis of Alum Prem Sattsangi Copyright 2009(Rev. )

#2 Synthesis of Alum Al KAl(SO 4)2. 12 H 2 O K 2 SO 4 + Al 2(SO 4)3 2 Simple salts 2 KAl(SO 4)2. 12 H 2 O A Complex Salt FW = 474 g/mol Theoretical Yield: (BLB, p. 102) Al (FW = 27. 0) 1 mol Al produces 1 mol KAl(SO 4)2. 12 H 2 O • Limiting reagent (BLB, p. 100) • • • Calculate Theo. yield of Alum (g) from 0. 2345 g Al. [Hint: Al(g) Al (mol) Alum(g)]

#3 Analytical Balance Tare, place foil, close door, record wt. Calculate Theoretical Yield Of Alum from 0. 0566 g Al

#4 Cut Al foil in 16 squares • Length wise • Breadth wise

#5 Plastic Reaction vial with cut Aluminum foil

#6 Mark MP Bulb 1, 2, 3, cm a. b. Fill Micropipet with 1. 5 M KOH to 2 cm mark c. Add to reaction vial

#7 Add more KOH (1 cm) a. Fill KOH to 1 cm mark b. Add to reaction vial

#8 Start reaction, In fume hood, Label 250 m. L beaker with Stn. # (a) Take hot tap water up to 2 cm mark in the coffee cup. (b) Support the coffee cup in 250 m. L beaker (Stn. #__) STN# 4 (c) FLOAT reaction vial in the coffee cup using forceps.

#9 Oxidation Numbers HELP KEEP TRACK OF ELECTRONS • BLB p. 128 -129 • Ox. No. of elements in standard states = 0 • Ox. No. of an ION = Charge on it Ox. No. • • • Aluminum (foil) Aluminum ion (Al 3+) Hydrogen (H 2(g) Hydrogen ion (H+) Hydride ion (H: -) H in water [H+ OH-] LEO the lion says GER Al foil 0 +3 Loss of Electron = Oxidation 0 +1 -1 Gain of electron = Reduction +1

#10 Allow REDOX reaction to finish ELECTRONS GAINED = ELECTRONS LOST 2 Al(s) + 2 KOH(aq) + 6 HOH(l) 2 KAl(OH)4(aq) + 3 H 2(g) 0 +1 +3 0 [Ox. Nos. /atom or ion] 0 6 x +1= +6 2 x+3 = +6 0 • Oxidation: (Loss of Electrons) Each electron lost = 1+ charge 2 Al 3+ + 6 e. Aluminum [0 to +6 ] Reduction: (Gain of Electrons) 6 [H+ OH-] + 6 e- 3 H 2 + 6 OHWater [+6 to 0 ] Hydrogen gas forming (FLAMMABLE)

#11 KAl(OH)4(aq) Complex Finished reaction AMPHOTERISM: (BLB p. 689 -690) KOH(aq) + Al(OH)3(s) KAl(OH)4(aq) Two Simple hydroxides ADD, form a SOLUBLE complex. Black particles are impurities in “Al”

#12 Remove Black Particles • Insert a wad of cotton in a glass pipet. • clamp to a stand, wet, remove water, filter. • May need 2 times filtration on the same pipet to get a filtrate. clear KAl(OH)4(aq)

#13 To KAl(OH)4(aq) colorless, Add Sulfuric acid White ppt. Al(OH)3(s) Add 1/3 MP H 2 SO 4 Stir with a glass -rod Colorless Solution White ppt. Soluble, More reactive REMEMBER: 2 KOH (aq) + 2 Al(OH)3(s) 2 KAl(OH)4 (aq) Colorless #10 Base + Acid Salt + water [2 KOH (aq) + H 2 SO 4(aq) K 2 SO 4 (aq) + 2 HOH] • A. 2 KAl(OH)4 (aq) + H 2 SO 4(aq) K 2 SO 4 (aq) + 2 Al(OH)3(s) + 2 HOH • B. 2 Al(OH)3(s) + 3 H 2 SO 4(aq) Al 2(SO 4)3 (aq) + 6 HOH

#14 White precipitate dissolves. Alum forms. (a)Stir with a glass rod. (b)Heat on hot plate • K 2 SO 4(aq) + Al 2(SO 4)3 (aq) 2 KAl(SO 4)2. 12 H 2 O • Simple salts ADD, form a COMPLEX salt

#15 Crystallization of Alum • Room temperature • (15 minutes or more) SEEDING? • Ice bath • (10 minutes)

#16 Alum crystals Typical result Single

#17 Suction Filter Alum • Water aspirator [Removes Mother liquor] Isolation of Alum: (a)Suction filter. (b)Rinse 2 x with 15 drs 50% ethanol(0 o. C) (c) Rinse 2 x with 95% ethanol(0 o. C) (d)Continue suction ~5 more minutes to dry the crystals.

#18 Suction Filter Alum (a) Water aspirator [Removes Mother liquor] (b) 50% ethanol [Removes Al 3+(aq), K+(aq), SO 42 -(aq)] (c) 95% ethanol. [Removes water] (d) Continue suction ~5 minutes. [To air dry the crystals]

#19 Dry on a pre-weighed paper Weight of paper_____ g Transfer Alum on PAPER. Cover with a Funnel. Allow to dry.

#20 Weigh crystals, Calculate %Yield Wt of crystals + paper = 0. 9802 g Wt of paper = – 0. 4181 g Wt of crystals = 0. 5621 g Exp. Yield = 0. 5621 g • %Yield • = Exp. Yield x 100 Theo. Yield (Slide #2) Please calculate %Yield now: =

#21 Summary of Equations (i) REDOX (Change in oxidation no. ) • 2 Al(s) + 2 KOH(aq) + 6 HOH(l) 2 KAl(OH)4(aq) + 3 H 2(g) (ii) Complex (Add the reagents) • Al(OH)3(s) + KOH(aq) KAl(OH)4(aq) • K 2 SO 4(aq) + Al 2(SO 4)3 (aq) 2 KAl(SO 4)2. 12 H 2 O (iii) Acid-Base (Water is formed) • • 2 KAl(OH)4(aq) + H 2 SO 4(aq) K 2 SO 4(aq) + Al(OH)3(s) + 2 HOH 2 Al(OH)3(s) + 3 H 2 SO 4(aq) Al 2(SO 4)3 (aq) + 3 H 2 O

#22 Qualitative Analysis of Alum KAl(SO 4)2. 12 H 2 O • • • 1. Test for p. H, pink color (Al 3+ ion) 2. Test for SO 42 - ion, [with Ba. Cl 2(aq) white ppt) 3. Flame test for K+ ion (LILAC COLOR) and evolution of H 2 O(g) and SO 3(g) [PUFFS UP]. 4. Test for absence of sulfate, [with Ba. Cl 2(aq)] (after THOROUGHLY heating the alum). • 5. Test for presence of Al 3+, 0. 15 M KOH. gelatinous ppt Al(OH)3(s). • Add 1. 5 M KOH to ppt dissolves, Al(OH)4(aq), • Forms SOLUBLE Complex

#23 Prepare saturated Alum solution. • Take 2 mm mound of Alum, KAl(SO 4)2. 12 H 2 O, with 3 drops of water on the plastic sheet. Crush alum with a glass rod.

#24 p. H Test for Al 3+ • Alum, KAl(SO 4)2. 12 H 2 O, contains Al 3+ ion. • Al 3+ ion has: • a. High charge. • b. Small radius. • Such cations, may rip apart the water molecule to produce H+. • Al 3+ + HOH Al. OH 2+ + H+

#25 Chemical Test for Al 3+ 0. 15 M KOH Gel (soluble in 1. 5 M KOH) • In the 24 well tray, mix 1 dr. 1. 5 M KOH + 9 drs. Distilled water to make 0. 15 M KOH • Alum contains Al 3+. • 1 drop Alum(aq) + 1 drop 0. 15 M KOH Gel • Al 3+ + 3 KOH(0. 15 M) Al(OH)3(gel) + 3 K+ Insol(s) • Add 1 drop 1. 5 M KOH to gel Dissolves. • Al(OH)3(gel) +KOH(1. 5 M) KAl(OH)4(aq) • Simple hydroxides ADD COMPLEX

#26 Test for SO 42 - ion in Alum KAl(SO 4)2. 12 H 2 O Alum(aq) 1 drop Alum(aq) + 1 drop Ba. Cl 2(aq) white ppt, Ba. SO 4(s) Sulfate present. SO 42 -(aq) + Ba. Cl 2(aq) Ba. SO 4(s) + 2 Cl-(aq)

#27 Flame test sample prep. • Place some Alum on a watch glass. • Heat nail. • Touch Alum with Red Hot Nail

#28 Evolution of gases and Lilac flame, K+ ion. • Slowly bring close to the flame. • Watch it swell as gases H 2 O(g) and SO 3(g) evolve. • Bring it into the flame. • Lilac color, “K+” ion.

#29 Heating Alum (Chemistry) 2 SO 3 (g) 24 H 2 O (g) Alum 2 KAl(SO 4)212 H 2 O Al 2 O 3 (s) insoluble HEAT K 2 O (s) soluble + H 2 O Base 2 KOH

#30 Tests in heated Alum sample • 2 KAl(SO 4)2. 12 H 2 O 24 H 2 O(g) + 4 SO 3(g) + Al 2 O 3(s) + K 2 O(s) Remove Alum RESIDUE Crush it in a ~2 cm pool of distilled water.

#31 Tests with HEATED Alum sample • Check p. H, Basic. WHY? • K 2 O(s) + H 2 O(g) 2 KOH(aq) Filter over cotton. Test with Ba. Cl 2(aq) No SO 42 -, No ppt. CAUTION: Any Undecomposed alum may give positive test with Ba. Cl 2.

Acknowledgements Experiments: Ms. Virginia Roll Power Point Assistance: Mr. Bill Hager Christopher Byers