Alum - The Preparation of Common Alum from Aluminum Foil...

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The Preparation of Common Alum from Aluminum Foil Objective: The purpose of this experiment is to prepare common alum, KAl(SO 4 ) 2 12H 2 O, from a piece of aluminum foil and to practice quantitative laboratory techniques by determining the percentage yield of a chemical reaction. Safety Tips: Wear safety goggles at all times in the laboratory. KOH solution is very corrosive and will damage clothing and skin. Wash skin immediately with lots of water if you come in contact with KOH. H 2 gas burns explosively in air. Keep flames away from beaker in which the H 2 gas is being produced and perform this step in the hood. H 2 SO 4 solutions are very corrosive. Wash skin immediately with lots of water if you come in contact with H 2 SO 4 . Ethyl alcohol is flammable. Keep away from open flames. Background: Chemical synthesis is the process of producing new substances from other, usually simpler, substances. This laboratory exercise demonstrates a chemical synthesis in which aluminum can be converted chemically into another chemical compound. Aluminum foil will be converted chemically into hydrated potassium aluminum sulfate, KAl(SO 4 ) 2 12H 2 O, or common alum. Alum is a versatile substance used in dyeing fabrics, to harden photographic film, preparing pickles, tanning leather, and clarifying water in municipal water treatment plants. Medicinally, it has applications as an astringent and a styptic (coagulates blood to stop bleeding). Derivatives of alum are also widely used in making antiperspirants because the Al 3+ ions bind water strongly. The reactions discussed in the following paragraphs will be used to produce common alum. The first part of the experiment involves dissolving the aluminum foil with potassium hydroxide (KOH) solution to produce hydrogen gas and a soluble salt, KAl(OH) 4 . ) ( 3 ) ( ) ( 2 ) ( 6 ) ( 2 ) ( 2 2 4 2 g H aq OH KAl l O H aq KOH s Al + + + Equation (1) This reaction explains why alkaline products like liquid detergents and bleaches are never stored in aluminum containers. The aluminum would slowly dissolve, producing hydrogen gas. A high pressure of hydrogen gas could build up in a sealed container, possibly causing the container to rupture or explode if the hydrogen gas was ignited. Sulfuric acid is added after the aluminum has dissolved. Sulfuric acid will convert the soluble salt, KAl(OH) 4 , to insoluble aluminum hydroxide, Al(OH) 3 .
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) ( 2 ) ( ) ( ) ( 2 ) ( ) ( ) ( 2 2 4 2 3 4 2 4 l O H aq SO K s OH Al aq SO H aq OH KAl + + + Equation (2) The insoluble aluminum hydroxide, Al(OH) 3 , is neutralized by the excess sulfuric acid to form aluminum sulfate, Al 2 (SO 4 ) 3 , which is a water soluble ionic compound. ) ( 6
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Alum - The Preparation of Common Alum from Aluminum Foil...

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