Methods
The method was followed as per the practical manual (page 15-20). As this is a practical, perfect results can’t always be obtained, but we can get a good indication of what will occur. There were slight deviations. These include, more H2SO4 was added to dissolve the Al(OH)3 to give a clear solution.
Results and observations
The results and observations obtained in this experiment are summarised below in table 1 and in calculations. Details of the observations and results recorded in the laboratory can be found on the attached sheet.
Table 1: Observations recorded at each step
Calculations
Theoretical yield of alum from the equation below:
2 Al(s) + 2 KOH (aq) + 4 H2SO4 (aq) + 22 H2O (l) → 2 KAl (SO4)2.12H2O (s) + 3 H2 (g)
Percentage yield of alum:
Discussion
The experiment was performed according to the procedure outlined in the practical manual. The only deviation occurred during the dissolution of Al(OH)3 when more H2SO4 had to be added to give a clear, red see through colour. Care was taken to ensure that the product was quantitatively transferred at each step so that a highest possible yield could be achieved, which correlates with the accuracy of the experiment. In this way a final yield of 11.8g (76%) of alum was obtained after using the Buchner filter system.
Aluminium was cut into pieces so that it can react faster with the aqueous KOH and then heated.
It can be suggested that the yield, of 76 %, of the reaction (alum) may improve if the reaction could run longer i.e. the flasks placed longer on the steam baths, at warmer temperatures, in the fume hoods and that the filter runs longer so that the product is essentially dry. It is also noted that more H2SO4 had to be added excessively to dissolve Al(OH)3, so maybe in future experiments a super acid could be used for a more effective yield.
The reason why this procedure is not used in the industry is because H2 gas burns explosively1 which means it is highly pollutant and on a large scale i.e. a big industry, the pollution will be catastrophic. KOH is also very corrosive so it can attack the machines in the factory, industry, which means it will rust and cost the company a lot of money. Sulfuric acid is also very hazardous.
Conclusion
The experiment illustrated an interesting example of the reduction of an environmental waste product. In doing so, reducing aluminium to potassium aluminium sulphate dodecahydrate, a final percantage yield of 76% was obtained. It was also found that a theoretical yield of 11.8g should have been yielded if all the procedures were done 100% efficiently, but because this is a practical a final yield of 9.00g was obtained.
References
- Inorganic chemistry 244 practical manual, University of Stellenbosch, 2010, p. 8-12
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John A. S. Green (2007). Aluminum recycling and processing for energy conservation and sustainability. Materials Park, Ohio: ASM International. p. 198.
- Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Ed.), Oxford:Butterworth-Heinemann.
Pre-laboratory questions
Amphoterism: An amphoteric substance is one that can react as either an acid or base. Many metals and metalloids have amphoteric oxides or hydroxides e.g.
Formulas for ions that make up potash alum:
Discussion
The experiment was performed according to the procedure outlined in the practical manual. The only deviation occurred during the dissolution of Al(OH)3 when more H2SO4 had to be added to give a clear, red see through colour. Care was taken to ensure that the product was quantitatively transferred at each step so that a highest possible yield could be achieved, which correlates with the accuracy of the experiment. In this way a final yield of 11.8g (76%) of alum was obtained after using the Buchner filter system.
Aluminium was cut into pieces so that it can react faster with the aqueous KOH and then heated.
It can be suggested that the yield, of 76 %, of the reaction (alum) may improve if the reaction could run longer i.e. the flasks placed longer on the steam baths, at warmer temperatures, in the fume hoods and that the filter runs longer so that the product is essentially dry. It is also noted that more H2SO4 had to be added excessively to dissolve Al(OH)3, so maybe in future experiments a super acid could be used for a more effective yield.
The reason why this procedure is not used in the industry is because H2 gas burns explosively1 which means it is highly pollutant and on a large scale i.e. a big industry, the pollution will be catastrophic. KOH is also very corrosive so it can attack the machines in the factory, industry, which means it will rust and cost the company a lot of money. Sulfuric acid is also very hazardous.
