EXPERIMENT 3 SYNTHESIS OF AN ALUM KAL SO4 2 12H2O INTRODUCTION Potassium aluminum sulfate dodecahydrate KAl SO4 2 12H2O belongs to a class of inorganic compounds called alums which is accented on the first syllable The composition of alum is represented by the general formula M M3 SO4 2 12H2O In this formula M is a monoatomic ion such as Na K Tl or Ag or is NH4 Although the name alum sounds as if the compound must contain aluminum the M3 might be from any of several main group metals p block elements and transition metals d block elements that form triply charged ions Al Ti V Cr Mn Fe Co Ga In and Ir Some examples are NaAl SO4 2 12H2O which is a component of some baking powders NH4Al SO4 2 12H2O which is used in dyeing textiles and KCr SO4 2 12H2O which is used in tanning leather The term double salt also describes KAl SO4 2 12H2O A double salt is an ionic compound that contains two different cations or two different anions In aqueous solution double salts give positive tests for all three of the ions they contain KAl SO4 2 12H2O would give positive tests for K Al3 and SO4 2 Hydrate is another term that describes KAl SO4 2 12H2O A hydrate is a substance that contains a specific number of water molecules per formula unit in its solid form Hydration is common with ionic compounds for example Na2SO4 10H2O and CoCl2 6H2O The water of hydration is included when calculating the compound s formula mass The water molecules are part of the crystal of a compound along with cations and anions but they can break out of the crystal when it is heated This dehydration might occur at a specific temperature characteristic of the particular compound If the compound is heated in a relatively closed container the escaping water might dissolve the remaining salt forming a liquid solution Consequently this combination of dehydration and dissolving looks like melting the temperature range at which it occurs the solid to liquid transition can be measured in the same way that a melting point is measured Reactions Involved in the Synthesis Metals dissolve in water or aqueous solutions only by reacting with water or some other component of the solution The reaction converts the metal to a positive ion Aluminum dissolves in acids by reacting with H3O and dissolves in bases by reacting with OH The reaction you will use to dissolve aluminum involves OH from KOH 2 Al s 2 OH aq 6 H2O 2 Al OH aq 3 H2 g 3 1 Next you will add sulfuric acid to the solution containing Al OH 4 As H3O reacts with Al OH 4 insoluble Al OH 3 forms and precipitates As more acid is added the precipitate Al OH 3 dissolves Experiment 3 3 1 4 Al OH aq H3O aq Al OH 3 s 2 H2O l Al OH 3 s 3 H3O aq Al3 aq 6 H2O l 3 2 3 3 The product forms and precipitates when the solution containing Al3 K from KOH and SO4 2 from K aq Al3 aq 2 SO aq 12 H2O l KAl SO4 2 12 H2O s 3 4 H2SO4 is cooled EQUIPMENT NEEDED B chner funnel balance beakers hot plate clamp conical funnel Erlenmeyer flasks filter flask isopropyl alcohol Al foil ice CHEMICALS NEEDED graduated cylinder melting point capillary tubes ring stand rubber O ring slit stopper temperature probe spatula watch glass 6 M H2SO4 sulfuric acid 3M KOH potassium hydroxide thymol blue solution filter paper 12 5cm and 7 0cm glass stirring rod PROCEDURE Dissolving Al by Reaction with KOH 1 Weigh a piece of aluminum foil on the laboratory balance Remove some Al from it or add more Al to it until you have 0 5 0 6 grams record the mass Tear the Al into pieces about the size of large postage stamps and put these pieces into an Erlenmeyer flask 2 Take a graduated cylinder to the hood and measure out 20mL of 3M KOH solution Caution caustic material Take the graduated cylinder back to your bench area and pour the 3M KOH solution into the flask containing the aluminum foil Within a few minutes the Al and KOH will begin to react producing hydrogen this reaction will become progressively more vigorous Be careful when handling the flask the reaction is so exothermic that the entire flask including the neck will become extremely hot After the obvious evolution of hydrogen ceases let the flask stand for a few minutes more to ensure complete reaction of the Al 3 If the solution in the flask is absolutely clear proceed to the next step in the synthesis If the solution is not absolutely clear filter it by gravity filtration Collect the filtrate in another Erlenmeyer flask Discard the filter paper in the regular trash container Experiment 3 3 2 4 24 Converting Al OH 4 To Al3 by Reaction with H2SO4 4 Take a graduated cylinder to the hood and measure out 16 0mL of 6M H2SO4 solution Caution caustic material Take the graduated cylinder back to your bench area To the clear filtered reaction solution add the 6M H2SO4 solution a few milliliters at a time After each addition swirl the flask to mix the contents Initially Al OH 3 will precipitate then some of it will dissolve as more H2SO4 is added If solid sticks to the inside wall of the flask wash it down with a few drops of H2SO4 After you have added all the H2SO4 gently heat the flask for several minutes have the liquid barely bubbling If the amount of H2SO4 added to the reaction mixture was sufficient for the amounts of Al and KOH you used all of the solid in the flask should dissolve 5 While the reaction mixture is heating fill an 800 mL beaker with ice and add just enough tap water to cover the ice Wash out the unused Erlenmeyer flask fill it with distilled water and place it in the ice water bath This will be your wash water Precipitating KAl SO4 2 12H2O by Cooling 6 Remove the flask of distilled water from the ice water bath and replace it with the flask containing the reaction mixture Occasionally stir the solution with a glass rod After crystals of the product begin to form cool the mixture for at least another ten minutes continue to stir it occasionally While you wait for the product to precipitate set up the equipment for filtration Collecting Washing and Drying KAl SO4 2 12H2O 7 Set up a B chner funnel apparatus Clamp the filter flask to a ring stand and connect the sidearm of the flask to the vacuum line with a long piece of pressure tubing Place the funnel on top of the flask and insert a piece of filter paper Open the vacuum line so that it is pulling air through the funnel Wet the filter paper in the funnel with some water from a beaker until it forms a seal with the base of the funnel 8 Remove …