Results and Calculations
Table 2 Masses Recorded Before and After Reactions
% Yield/% Recovery = (mass of copper recovered / mass of initial copper) x100%
= (2.4729g/1.1680g) x100% = 211.72%
Discussion
In this lab, the synthesis of different copper compounds reacted as predicted in the theoretical expectations. The first chemical reaction with copper and nitric acid evolved a red-brown gas, which is discovered to be nitrogen dioxide. In this reaction copper reacts with nitric acid to produce copper (II) nitrate, nitric dioxide, and water.
Cu(s)+4HNO3 (aq) → Cu(NO3)2 (aq) + 2NO2 (g) + 2H2O (l)
Since copper as a reactant has an oxidation state of 0 and copper in Cu(NO3)2 has an oxidation state of +2, it loses 2e-. In the reactant side, N has an oxidation state of +5 and in N in NO, has an oxidation of +2, thus N gains 3e-. Therefore, this reaction is a redox reaction because copper is oxidized, and nitrogen is reduced (Petrucci, 2009).
The second reaction is the synthesis of copper (II) hydroxide. This is done by adding 20% NaOH to the solution of copper (II) nitrate. In this reaction, a double displacement occurs and thus copper (II) hydroxide is formed, in the form of a blue/green precipitate.
Cu(NO3)2 (aq) + 2NaOH(aq) → 2NaNO3 (aq) + Cu(OH)2 (s)
The third reaction requires the synthesis of copper (II) oxide from copper (II) hydroxide. This reaction is done by heating up the solution of copper (II) hydroxide to a boil. During the heating of this mixture, the precipitate in the solution begins to turn from its original blue/green colour to a darker shade, eventually turning all of the precipitate black.
Cu(OH)2 (s) →CuO(s) + H2O(g)
During this reaction, water is expelled from Cu(OH)2 to form CuO. Thus, this reaction is a dehydration reaction because of the lost water molecule (Cudennec & Lecerf, 2003).
The fourth reaction synthesizes copper (II) sulphate from copper (II) oxide. This is done by adding dilute sulphuric acid to the mixture of copper (II) oxide. In this reaction the acid, H2SO4 reacts with the basic copper (II) oxide in a neutralization reaction to create CuSO4, copper (II) sulphate salt, and water. The balanced chemical equation for this reaction is: CuO(s) + H2SO4 (aq) →CuSO4 (aq) + H2O(l) `The fifth reaction that takes place forms Cu metal from CuSO4. To do this, zinc was added to the aqueous solution of CuSO4. During this reaction, it finally recovers some of the copper that was started with in the form of a brown precipitate. The solution also turns clear eventually from blue as the copper is being precipitated and being replaced with zinc, forming zinc sulphate.
Zn(s) + CuSO4(aq) → Cu(s) + ZnSO4 (aq)
The above reaction is another redox reaction. This is because zinc, started off with an oxidation state of 0 before the reaction, and after the reaction zinc ended up with an oxidation state of +4, thus zinc loses 4e- and is in turn oxidized. In the case of copper, it has an oxidation state of 10 in CuSO4 and after the reaction it ended up with an oxidation state of 0, therefore copper is reduced as it gains 10e-.
In the sixth reaction, hydrochloric acid, HCl, is added to the mixture of copper and zinc precipitate. As the HCl was being added, the mixture of copper and zinc became cloudy with bubbles (which indicates a release of a gas) until it reacts with all of the leftover zinc, thus isolating copper to progress in the experiment. The reaction is as follows:
Zn(s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g)
This reaction is a single displacement reaction but also a redox reaction. This is because Zn in the reactant side has an oxidation state of 0, and in ZnCl2 it has a +2 oxidation state, Zn loses 2e- thus it is oxidized. Hydrogen has an oxidation state of +1 in HCl, and ends up with a 0 oxidation state after the reaction, gaining 1e-. Therefore, oxygen is reduced.
Although the results of the reactions were predicted through the theoretical information provided. The results that were obtained were however, erroneous; this is because the percent recovery was above 100%, due to the recorded mass of copper retrieved (2.4729g) being above the initial mass of copper (1.1680g. There cannot be more copper created. One of the observed sources errors that could have caused this could be time itself, seeing as there was only a limited amount of time provided and many of the procedures were time based, such as the evaporating of the water in the dish. Another observed source of error is that some of the zinc was still remaining in the copper precipitate that did react with the hydrochloric acid. The last source of error observed was that residue of the copper compounds from the previous reactions could have been trapped within the recovered copper, thus making it impure.
Questions
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Identify one factor that could cause the percent recover of copper to be higher, and on factor that could cause the percent recovery of the copper to be lower, than 100%.
One factor that could have caused the percent recovery of copper to be higher is if small traces of water remained within the isolated copper.
A factor that could have caused the percent recovery to be lower could be due to the very thick and clingy nature of the precipitates being worked with, because of this, losing some of the precipitate throughout the process is inevitable.
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In Part 1, after the copper wire was dissolved, 25 ml of 20% NaOH was slowly added to the solution. Why is it important to make this addition slowly?
It was important to make this addition slowly so that none of the NaOH would spill, allowing the reaction to precipitate most if not all of the copper through copper (II) hydroxide. Also, NaOH is very basic, and very corrosive, therefore it does pose a risk. Minimizing spillage is important for chemical safety.
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Suggest a reason for allowing the last traces of water to evaporate slowly rather than rapidly expelling the water by intense heating.
One reason for allowing the last traces of water to evaporate rather than rapidly expelling the water by intense heating so that the energy from boiling the water does not expel some of the copper precipitate out of the dish.
Conclusion
In conclusion, the methods are being experimented with to synthesize and isolate are effective when using them for large quantities of substances. Mass production of these compounds would be best for employing these methods, as many of the processes synthesis do take time and thus, using as much material to synthesize or isolate, as possible would minimize down time. For small quantities, other methods should be used.
In terms of how useful these methods are for the chemical industry, these methods are relatively simple and will be able to do the job for mass production of synthesized compounds or even isolating an element. Although the results from this experiment were erroneous, there is no doubt that the simplicity of the processes is invaluable to the chemical industries.
References
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Cuddenec & Lecerf. 2003. The transformation of Cu(OH)2 into CuO, revisted. Elsevier SAS. pp. 1471-1472.
< http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W6W-49Y3VNS-4&_user=1067412&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1048115892&_rerunOrigin=google&_acct=C000051246&_version=1&_urlVersion=0&_userid=1067412&md5=b34325a660ea01689338535ab0c0d2b3>
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Petrucci et al. 2007. General Chemistry: Principles & Modern Applications, 9th edition. Pearson Education, Inc. pp. 110.
- Department of Chemistry 2009 Chemistry 120 Lab Manual. University of Waterloo, Waterloo. pp. 13-16.
