6b-alum - CHM151LL: SYNTHESIS OF ALUM FROM ALUMINUM | 1...

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CHM151LL: SYNTHESIS OF ALUM FROM ALUMINUM | 1 Synthesis of Alum from Aluminum OBJECTIVES become more familiar with single-replacement redox reactions practice mass and volume measurement techniques calculate the theoretical yield and percent yield of a synthesis reaction DISCUSSION Alums are ionic compounds that crystallize from solutions containing sulfate ion, a trivalent cation such as Al 3+ , Cr 3+ , or Fe 3+ and a monovalent cation such as K + , Na + , or NH 4 + . Six of the water molecules bind tightly to the trivalent metal ion; the remaining six molecules bind more loosely to the monovalent cation and the sulfate anion. In this experiment you will prepare an alum–KAl(SO 4 ) 2 · 12H 2 O [potassium aluminum sulfate dodecahydrate]–from elemental aluminum. This alum compound is widely used in dyeing fabrics, making pickles, making paper and purifying water. Although aluminum sits well above hydrogen in the activity series, it reacts only slowly with dilute acids because a thin coating of aluminum oxide protects the metal surface. Aluminum reacts more rapidly with alkaline solutions to produce hydrogen because the excess hydroxide ion first attacks the tough Al 2 O 3 layer so the metal can react. Aluminum converts to the tetrahydroxoaluminate ion Al(OH) 4 . Slow addition of acid to a solution of this ion causes the precipitation of solid Al(OH) 3 followed by the dissolving of the precipitate to form the aluminum ion Al 3+ . The solid Al(OH) 3 will also dissolve in excess base due to formation of Al(OH) 4 . We call a hydroxide that can react with either acids or bases amphoteric . Al 3+ add OH ! " ! ! ! add H + # ! ! ! Al(OH) 3 add conc. OH ! " ! ! ! ! add H + # ! ! ! Al(OH) 4 Except for metals in Group 1, Ca 2+ , Sr 2+ , and Ba 2+ , all metal ions form insoluble hydroxides. These insoluble hydroxides dissolve in acid solution. Amphoteric hydroxides also dissolve in strong base solutions (concentrated solutions of hydroxide ion made by dissolving solid NaOH or KOH in water). The equations given below show the complete sequence of reactions involved in our experiment. 1. Reaction of aluminum with KOH (the dissolution step) 2 Al ( s ) + 2 KOH ( aq ) + 6 H 2 O ( l ) 2 KAl(OH) 4 ( aq ) + 3 H 2 ( g ) 2. Initial addition of sulfuric acid (precipitation of Al(OH) 3 ) 2 KAl(OH) 4 ( aq ) + H 2 SO 4 ( aq ) 2 Al(OH) 3 ( s ) + K 2 SO 4 ( aq ) + 2 H 2 O ( l ) 3. Further addition of sulfuric acid (dissolving of Al(OH) 3 ) 2 Al(OH)
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This note was uploaded on 02/28/2012 for the course CHEM 151 taught by Professor Wiley during the Spring '12 term at Mesa CC.

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6b-alum - CHM151LL: SYNTHESIS OF ALUM FROM ALUMINUM | 1...

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