Introduction
In this lab a piece of copper was weighed and put through a series of cycles, beginning with it being dissolved in a solution and in the end being returned to its original solid state. This lab was given with the intention of showing how the Law of Conservation and Mass and percent yield are correlated. The initial amount of copper used must be the same as the final amount of copper used, and if not the percent yield must either be too high or too low because of some discrepancies in the lab.
Materials and Methods
The lab set up began by placing a hot plate on a ring stand. Once set, a fifty-milliliter beaker was placed on the hot plate and an iron ring was attached to the ring stand, in order to hold beaker in place. Next, the fume hood was created by attaching a connecting a funnel to the aspirator by using a rubber hose. In order to use the fume hood the water was turned on in the aspirator to make a vacuum and capture any harmful gasses given off by the chemical reactions.
Our cycle began by measuring out 0.2550 grams of copper turnings and transferring them over to the beaker. Next, 5 mL of a 6 molarity solution of HNO3, nitric acid, was measured in a 10 mL graduated cylinder. After turning on the aspirator, the solution was slowly added to the beaker. Following the first reaction, the aspirator funnel was removed and 10 mL of distilled water was added. The stir bar was then activated and while being stirred 6 M NaOH, sodium hydroxide was then added dropwise. As the NaOH solution was being added, samples of the copper solution were taken and drop onto red litmus paper until the paper was stained blue. A total of 161 drops of NaOH solution was added. Once the reaction had clearly taken place, the hot plate and stir bar were then turned on and left until the solution was a dark black, a total of approximately 10 min, and left for another 5 min to cool down. Once cool, the liquid was decanted into its appropriate waste container and 10 milliliters of distilled water was added to the copper solution. 3.9 milliliters of the 3 M H2SO4 was then added dropwise into the solution, and another reaction took place, making the solution a light blue. Another 5 mL of distilled water was added to the solution, and the beaker was then transferred to a magnetic stir plate. The stir bar was then activated and .3125 g of 20 mesh zinc was added slowly to the solution until it was colorless, after about 10 min. The stir bar was then removed and about 5 mL of 6 M HCl was added to the solution to react with any excess zinc. The copper was now ready to be recovered. After taking the weights of a dry filter paper and a dry watch glass, the vacuum filtration apparatus was constructed by clamping the filter flask to the ring stand and connected the hose from the fume hood to the flask. A Büchner funnel was attached to the flask and the filter paper was put in the filter. The aspirator was turned on and wetted with distilled water in order for the vacuum to set the filter at the bottom of the funnel. The solution was run through the filtration device and recovered on the filter paper and weighed on the watch glass.
Results and discussion
For the first reaction, the .2550 g of copper turnings was reacted with 5 mL of 6 M HNO3. After adding an initial 4 mL of nitric acid solution the stir bar was activated in order to begin the reaction When the copper began to dissolve, the solution began to turn into a clear blue and NO2 gas began to be released. In order to accelerate the reaction, another mL of nitric acid was added and the hot plate was turned on. The equation to the first reaction was:
Cu(s)+ 4 HNO4(aq) → Cu(NO3)2(aq) + 2 NO2(g) + 2H2O(l)
The solid copper reacted with the nitric acid to create an aqueous solution of copper (II) nitrate, NO2 gas, and 2 molecules of water. This is an example of a gas forming metathesis reaction as well as a redox reaction because both nitrogen atoms lose at least one oxygen atom.
The second reaction took place when the 6 M NaOH solution was added dropwise. At 91 drops the solution showed the beginning of the reaction by turning a bluish-green color and as more drops were added, the solution began to turn mostly blue. At 161 drops the solution stained red litmus paper blue. The blue staining of the red litmus paper indicated that the solution was basic. The equation to the second reaction was:
Cu(NO3)2(aq) + 2 NaOH(aq) → Cu(OH)2(s) + 2 NaNO3(aq)
The copper (II) nitrate reacts with the sodium hydroxide in order to create a solid copper hydroxide and sodium nitrate. This reaction is another example of a metathesis reaction but instead is a precipitation reaction because a solid is formed.
In the third reaction, no chemical was added to the solution. Instead, heat and water were used to convert the copper (II) hydroxide into copper (II) oxide and water. This reaction was shown when the solution changed from blue to black. The equation to the third reaction was:
Cu(OH)2(s) → CuO (s) + H2O (l)
This equation is an example of a decomposition reaction and a precipitation reaction. The solution was then decanted and 10 milliliters of distilled water was added. Leaving only the copper (II) oxide and water in the solution.
The fourth reaction was started by the addition of 3.9 mL of a 3 M H2SO4 solution. The H2SO4 solution dissociates leaving the 2 H+ ions to react with the oxygen of copper oxide, creating 1 water molecule and 1 copper (II) sulfate molecule. The reaction was indicated by the discoloration of the previous black solution. The equation to the fourth reaction was:
CuO(s) + H2SO4 → CuSO4(aq) + H2O(l)
This reaction is an example acid-base metathesis reaction.
The final reaction was the addition of .3125 g of zinc mesh to the copper (II) sulfate solution in order to create solid copper and zinc (II) sulfate. The addition of zinc caused the solution to turn a light blue color and also caused the copper to clump and solidify. The equation to the fifth reaction was:
CuSO4(aq) + Zn(s) → Cu(s) + ZnSO4(aq)
This is an example of a redox reaction, where electrons are transferred between reactants and a solid is formed from a compound.
The final step of the lab was to calculate the actual yield and find the percent yield of the copper product. The weights of the filter paper and watch glass were .3433 g and 53.9715 g, respectively. The final weight of the watch glass and filter paper with the copper was 55.3807 g. Once the weights were subtracted the actual yield of copper was .9659 g and the percent yield was calculated to be 378.8%. The actual yield was considerably higher than the original .2550 g of copper because of discrepancies in the lab, one major one being that the filter paper was not completely dry. The extra weight of water played a big role in the calculation of actual yield in the solution. Another reason for the greater actual yield could have been from an excessive addition of the HCl solution causing the solution retained by the filter paper to be heavier.
Conclusion
Throughout this lab the copper was transferred from various states of matter, keeping the original amount of copper each time. The “actual yield” of copper was calculated to be higher than the original amount, but the overall amount of copper used in the cycle was retained according to the Law of Conservation of Mass. Although that Law couldn’t be proven an understanding of writing different types of ionic equations, calculating percent yield, handling concentrated acid and base solutions, and perform a vacuum filtration, making the lab an overall success.