The purpose of this lab experiment was to recover as much of the initial mass of copper wire as possible after it undergoes the copper cycle. The value of the final mass of copper metal can help determine the percent yield (recovery) in comparison to the initial mass of copper. The percent yield is calculated using the following formula:
(Final Mass of Copper) /(Initial Mass of Copper) * 100 = % Yield (Volland, 2005).
This value will illustrate whether more copper was produced (% yield greater than 100) or consumed (% yield less than 100). In order to get the closest possible value to a 100% recovery, certain precautions must be taken. Any loss of copper due to spilling or being left behind in beakers must be avoided in order to achieve the best results.
Procedure
The experimental procedure used for this experiment was outlined in the CHEM 120L lab manual, under Experiment #1. All steps were followed with no changes in the procedure.
Results and Observations
Table 1: The Reaction of Cu with Nitric acid (HNO3)
Table 2: The Formation of Cu (OH)2
Table 3: The Conversion of Cu (OH)2 to CuO
Table 4: The Formation of CuSO4 from CuO
Table 5: The Formation of Copper metal from CuSO4
Table 6: The Removal of Zinc with HCl
∙ Calculation of Percent Yield
Initial Mass of Copper = 1.0773 g
Final Mass of Copper = (Mass of Evaporating Dish + Copper) – (Mass of Evaporating Dish)
= (64.4557 g) – (63.0500 g)
= 1.4057 g
Percent Yield = ((Final Mass of Copper)/(Initial Mass of Copper)) * 100
= ((1.4057 g ) / (1.0773 g) * 100
= 130 %
Discussion
For this lab, the general objective was to recover as much of the initial amount of copper after it went through the copper cycle. In the first chemical reaction, the copper wire was dissolved in nitric acid and became copper nitrate. Since this reaction was performed under a fume hood, the NO2 gas was not detected (University of Maine, 2001). This was an example of a REDOX reaction, where an element gains electrons and another element loses electrons (University of North Carolina, 1998). In this specific reaction, copper metal lost electrons (became oxidized), while nitrogen gained electrons (became reduced). During the second reaction, sodium hydroxide was added to the newly created copper nitrate and the new product formed was calcium hydroxide. Unlike the first chemical reaction, this reaction was a double displacement reaction where the two compounds were interchanged to form new compounds (Westbroek, 2000). This reaction can also be classified as a precipitation reaction, where an insoluble product results from the reaction of two or more soluble reactants (Westbroek, 2000). The newly created calcium hydroxide, which contained chunks of black precipitate, was then heated and copper (II) oxide was created. This was done by adding heat, which means that this reaction can be classified as a dehydration reaction, where there was a loss of water in the reacting molecules. Following this reaction, copper (II) oxide was filtrated and, with the addition of sulphuric acid, the solution now had an aqua blue colour. This reaction was also a double displacement reaction. Finally, zinc metal was added to the copper (II) sulphate solution, which resulted in a copper precipitate. This final reaction was classified as a single displacement reaction, where zinc metal displaced the copper and became zinc sulphate (Westbroek, 2000). Before the copper was weighed, zinc was removed by the addition of hydrochloric acid and any excess water was removed by the addition of heat. Once again, a dehydration reaction occurs because of the loss of water. In the end, more copper metal was obtained after the copper cycle then the initial mass of copper wire.
As shown in the calculation of the percent recovery, the final amount of copper produced was greater than that of the initial amount of copper. As a result, the percent recovery was greater than 100% (130%). An error that accounted for this increase may have been that some of the excess water and/or zinc were not removed by heat and the hydrochloric acid, respectively. As a result, the final mass of copper recorded may have included a combination of water and/or zinc, along with the copper. Other incomplete reactions throughout the copper cycle may have also contributed to the increase in the final mass of copper metal.
Questions
During part 1 of this lab investigation, sodium hydroxide was added very slowly to the dissolved copper wire. The reason why the sodium hydroxide was added slowly to the solution was that if the sodium hydroxide was added to quickly, the solution would react very quickly, producing lots of heat, resulting in gas and possible spills. This is a very dangerous safety hazard, as the sodium hydroxide must be handled with lots of precaution. One of the acids used in this experiment that must have been handled with precaution was the sulphuric acid. Sulphuric acid is highly corrosive, which means that any liquid splashes could cause very severe burns (European Fertilizer Manufacturers Association, 1997). Therefore, eye and hand protection (i.e. goggles and gloves) must be worn at all times while this chemical is being handled. Proper ventilation must be given to this acid and any inhalation must be avoided (European Fertilizer Manufacturers Association, 1997).
There were several terms in this experiment that must be clarified. A supernatant is a clear liquid that lies above a precipitate (Saunder, 2004). Decant, or decanting, is the process of pouring out a liquid, with the intent of leaving some of the liquid behind (Saunder, 2004). The third term, which is dissolution, is the act of going into a solution. Usually, it involves a chemical change from one form to an aqueous form (Saunder, 2004). The final term, which is bumping, occurs because liquids do not boil in an even fashion (Saunder, 2004). As a result, the bubbles do not form right away, which could cause eruptions when the bubbles actually do form (Saunder, 2004). This is why constant stirring of the solution is required in order to prevent any of these eruptions.
The following is the theoretical yield of copper (II) hydroxide if the initial mass of pure copper was 1.000g:
1.000 g Cu(s) * 1 mol * 1 mol Cu(NO3)2 * 1 mol Cu(OH)2 * 97.562 g
63.546 g 1 mol Cu(s) 1 mol Cu(NO3)2 1 mol
= 1.535 g Cu(OH)2
The following is the theoretical yield of CuSO4 ∙ 5H2O if the initial mass of pure copper was 1.000g:
1.000 g Cu(s) * 1 mol * 1 mol Cu(NO3)2 * 1 mol Cu(OH)2 * 1 mol CuSO4 * 159.612 g
63.546 g 1 mol Cu (s) 1 mol Cu(NO3)2 1 mol Cu(OH)2 1 mol
= 2.512 g CuSO4 ∙ 5H2O
Conclusion
The purpose of this experiment was to retain as much of the initial amount of copper after it had gone through the five-step copper cycle. Using the values of the initial and final masses of copper, a percent yield was calculated, which came out to be 130%. As a result, it can be concluded that more copper was produced during the chemical reactions of the copper cycle. Therefore, the objective of this lab experiment was partially achieved because the final mass of 1.4075 grams of copper metal exceeded the expected value of 1.0773 grams of copper metal. Further studies in the area of chemical synthesis can include introducing new compound cycles and determining the amount of copper metal present in the opposite direction of the copper cycle.
References
Casiday, R. and Frey, R. (1998) “The Chemistry Behind Airbags” 14 October 2007.
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Department of Chemistry: 2007 First Year Chemistry Laboratory Manual. Synthesis of Copper Compounds. University of Waterloo, Waterloo. pp. 10-15
European Fertilizer Manufacturers Association (1997) “Guidance For The Compilation of Safety Data Sheets For Fertilizer Materials Sulphuric Acid” 15 October 2007.
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Saunder, WB (2004) “ Dorland’s Illustrated Medical Dictionary” 15 October 2007.
<http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd-a-b-000zPzhtm >
University of Maine (2001) “The Copper Cycle” 14 October 2007.
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University of North Carolina (1998) “Oxidation-Reduction Reactions” 14 October 2007.
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Volland, Dr. W. (2005) “Percent Yield” 14 October 2007.
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Westbroek, G. (2000) “Types of Chemical Reactions” 14 October 2007.
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