This laboratory report describes the synthesis of alum crystals from aluminum can strips through a series of sequential chemical reactions, in which the products of each reaction serve as reactants in the next. Using potassium hydroxide and sulfuric acid under controlled heating, the experiment demonstrates the energy requirements and percent yields associated with aluminum recycling. The report documents materials, methods, observations, and results, concluding that aluminum recycling can be accomplished through relatively straightforward chemical processes that are far less energy-intensive than primary aluminum production from ore. The experiment provides a hands-on illustration of reaction types commonly encountered in introductory chemistry.
This experiment demonstrates the synthesizing of alum from aluminum cans through sequential reactions, in which the products of one chemical reaction become the reactants in the next. The sequential reactions involved illustrate, to a meaningful degree, the energy requirements and percent yields associated with recycling aluminum. In this way, the experiment provides a firsthand demonstration of the importance of aluminum recycling and certain of the key processes involved. It also offers a clear physical example of the fairly typical reaction types that are commonly identified and theoretically mapped by chemistry students.
The reactants used in this experiment included strips of an aluminum can cut into small pieces (1.0085 g), a 1.4 M solution of potassium hydroxide (KOH), a 6 M sulfuric acid (Hâ‚‚SOâ‚„) solution, a 50% ethanol solution, ice, and distilled water. Equipment used included two graduated cylinders (10 mL and 50 mL), an aspirator, rubber tubing, a standard hotplate, a BĂĽchner funnel, a stirring rod, filter paper, weigh paper, a clamp, a ring stand, a beaker, and a scoopula. Standard classroom laboratory equipment was utilized, and none of the reactants were especially exotic.
The mass of the aluminum strips was obtained by first weighing the beaker (104.3116 g), then adding the aluminum and recording the new weight (105.3201 g), and taking the difference (1.0085 g) as the mass of aluminum used.
After weighing the aluminum, the beaker was placed on a hotplate set to low heat and 50 mL of the KOH solution was added while venting under a laboratory hood. As the reaction proceeded, distilled water was added at intervals to maintain a volume of approximately 25 mL throughout the reaction period of roughly 30 minutes. An aspirator system was set up and used to filter the resulting reaction mixture, with the beaker rinsed and poured through the filter system multiple times to collect the solid substance formed from the aluminum during the reaction.
Next, 20 mL of the 6.0 M sulfuric acid was added to the reaction mixture, again over low heat on the hotplate and with venting, until no new solids formed and all existing solids had dissolved. The beaker was then placed in an ice bath. Observations were recorded throughout the entire procedure.
During the first reaction, after adding the KOH solution to the beaker containing the clean aluminum strips, the solution turned a dark purple color, appearing almost black at times. The aluminum visibly began to dissolve, and bubbles formed on the surface of the solution. As the solution cooled following the reaction, the bubbling and the vapor emitted by the mixture — which necessitated venting — became far less apparent. After filtration was carried out multiple times, the solids were removed from the reaction mixture, leaving a colorless filtrate in the reaction beaker.
Adding 20 mL of 6 M sulfuric acid to the colorless filtrate caused the mixture to become cloudy and white, and solid precipitates began to form. These solids were broken down and dissolved by the heat of the hotplate and the ongoing reaction, and by the end of the reaction the solution was again clear and colorless. An ice bath was then prepared, and without the need for any nucleation sites to be introduced, solid crystals began to form and precipitate out of the solution, settling to the bottom of the beaker.
The filter apparatus was again assembled and used to remove the crystals from the mixture. The beaker was rinsed twice with 10 mL of the 50% ethanol solution, which also served to dry the alum crystals and allow for more accurate weighing. The cleaned and dried beaker was weighed (104.3153 g), the alum crystals were added to the beaker, and the new weight was recorded (109.1553 g). The difference (4.84 g) represents the mass of alum crystals formed through the sequential reactions. Using the starting mass of the aluminum strips and the final mass of the alum crystals, it is possible to determine the quantities of the reaction elements and calculate the percent yield.
Recycling aluminum is not as energy-intensive as mining aluminum and can be achieved with fairly simple chemical reactions. This experiment demonstrated the ease of certain aluminum-based reactions using standard laboratory equipment and common chemicals. Alum crystals were successfully created from aluminum cans, providing a practical illustration of the chemistry underlying aluminum recycling.
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