Chemical Reactions of Copper and Percent Yield
Names: Andrew Yaksic
Purpose: To gain some familiarity with basic laboratory procedures, some chemistry of a typical transition element, and the concept of percent yield.
Equipment: Analytical balance, 2 250-mL beakers, evaporating dish, stirring rod, towel, wire gauze, Bunsen burner, rubber hose, 100-mL graduated cylinder, weighing paper, boiling chips, ring stand, iron ring
Materials: 0.5 g piece of copper wire, 6 M H2SO4, methanol, concentrated HNO3, 3.0 M NaOH, granular zinc, acetone, concentrated HCl
Introduction: Most chemical syntheses involve separation and purification of the desired product from unwanted side products. Common methods of separation include filtration, sedimentation, extraction, decantation, and sublimation. This experiment will use chemical reactions to separate copper from copper compounds. Oxidation-reduction (redox) reactions and metathesis (single-replacement) reactions will be used. The objective of the experiment is to recover as much of the original copper as possible. The quantitative measure of success in this experiment is percent yield, which is the ratio of recovered mass to initial mass.
A list of the reactions occurring in this experiment is shown below:
Cu(s) + 4HNO3(aq) → Cu(NO3)2(aq) + 2NO2(g) + 2H2O (l)
Cu(NO3)2(aq) + 2NaOH (aq) → Cu(OH)2(s) + 2NaNO3(aq)
Cu(OH)2(s) ∆ → CuO(s) + H2O(g)
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s)
This experiment utilizes two very flammable drying agents, methanol and acetone. Open flames must not be used in the presence of either of these substances.
Procedure:
1. Approximately one-half gram of copper wire was weighed. Its mass was recorded. It was placed in a 250-mL beaker.
2. Approximately 5 mL of concentrated HNO3 was added to the beaker. The reaction was allowed to occur in the fume hood.
3. Approximately 100 mL of distilled water was added to the beaker after the reaction was complete.
4. 30 mL of 3.0 M NaOH was added to the solution in the beaker. Three boiling chips were added to the beaker. The solution was carefully heated while stirring with a stirring rod.
5. The precipitate in the beaker was allowed to settle. The supernatant liquid was decanted.
6. 200 mL of distilled water was heated. The hot water was added to the beaker with the precipitate. The contents of the beaker were stirred. The precipitate was allowed to settle. The supernatant liquid was decanted once more.
7. 15 mL of 6.0 M H2SO4 was added to the beaker.
8. In the fume hood, 2.0 g of zinc metal was added to the beaker. The contents of the beaker were stirred until the supernatant liquid was colorless. More zinc was added as necessary.
9. When gas evolution had ceased and the supernatant liquid was colorless, the solution was decanted. The precipitate was transferred to a weighed porcelain evaporating dish. The copper was washed with 5 mL of distilled water. The water was decanted.
10. The copper was washed with 5 mL of methanol. The methanol was decanted.
11. The copper was washed with 5 mL of acetone. The acetone was decanted.
12. A steam bath was prepared according to Figure 6.1 shown below. The copper was dried on the steam bath for 5 minutes. The bottom of the evaporating dish was dried with a towel. The boiling chips were removed from the copper. The evaporating dish and copper were weighed and the mass was recorded.
<img src="crcpy6_01.jpg" alt="Figure 6.1: Steam bath">
Observations:
Initial mass of copper: .5651 ± .0001 g
Mass of copper and evaporating dish: 43.3403 ± .0001 g
Mass of evaporating dish: 42.7942 ± .0001 g
When HNO3 was added to the copper wire, a blue solution was formed and a gas evolved. The reaction was exothermic. When water was added, the solution turned light blue. When NaOH was added, the solution turned blue again. After heating the new solution, a black precipitate formed. By adding distilled water to this new precipitate and then decanting, all stray ions and other precipitates except the desired CuO are removed from the beaker.
When H2SO4 was added, a clear green-blue solution was formed. When zinc was added to the solution, a pungent gas evolved quickly. Zinc(II) sulfate is present in solution. The gas that evolved was a sulfate gas. The choking feeling that accompanied breathing it confirms that. After the reaction, a copper-brown-colored substance remained. Washing that solution removed all other ions or precipitates from the beaker. The copper precipitate was uniform in appearance.
Results:
Mass of recovered copper. (Mass of copper and evaporating dish minus mass of evaporating dish.) =
43.3403 ± .0001 g – 42.7942 ± .0001 g =
.5461 ± .0002 g
Percent yield. (Mass of recovered copper divided by initial mass of copper multiplied by 100.) =
.5461 ± .0002 g / .5651 ± .0001 g x 100=
.5461 ± .04% g / .5651 ± .02% g x 100 =
96.64 ± .06% percent yield =
96.64 ± .06 percent yield
Discussion: There are many sources of error in this experiment. This experiment includes decantation processes, which always produce error. Some precipitate is always lost when supernatant liquid is separated from the precipitate. Decantation errors could be avoided by employing filtration techniques instead of decantation techniques. This experiment also included the transfer of precipitate from a beaker to an evaporating dish. It is difficult to remove all of the precipitate from the beaker using simply liquid. Some precipitate sticks to the walls of the beaker and is extremely difficult to remove. It is possible for the percent yield to be greater than 100%. This occurs when reactions are not allowed to finish. This means that some of the previous unwanted precipitate remains with the desired product. The unwanted precipitate will not participate in subsequent reactions, causing a constant unwanted mass in addition to the desired mass of reaction products. If a significant amount of unwanted precipitates remained in the final copper precipitate, then the final mass would have been higher than the initial copper mass, causing a percent yield greater than 100%.
The theory associated with this experiment is the atomic theory. The reason that the same mass of copper will be produced after all of the reactions that occur is because a constant number of copper molecules were present throughout the experiment. The original copper sample contained a specific number of moles of copper, and that amount of copper was present in every reaction, precipitate, and solution. Thus, the same number of moles of copper was produced in the final reaction because the amount of copper remained constant throughout the experiment. The atomic theory predicts this behavior.
There are several ramifications of this experiment. Personal ramifications include increased laboratory experience and increased understanding of redox and metathesis reactions. This experiment has far-reaching real-world applications. For example, if an ore containing an element were found, the element could be separated from the ore using reactions similar to those used in this experiment. If an element existed as a solid, but it needed to be transported as a liquid for some reason, then a solution of that solid and some other substance could be created with the assurance that the same amount of the element that went into solution could later be extracted from that solution.
Questions:
1. If the percent yield of copper was greater than 100%, several errors are plausible. They are listed in the Discussion section above.
2. a. If 2.5 moles of methane reacted with 3 moles of oxygen, the limiting reagent would be oxygen.
b. 1.5 moles of CO2 can be made from the mixture above. 1.5 moles of CO2 equates with 66.0 grams of CO2.
3. If 8.00 g of CH4 burned in the presence of 6.00 g of oxygen, the following amounts of materials would remain after the reaction was complete:
CH4: .5 moles were present when the experiment began. Only 3/16 moles were used in the reaction. This means that 5/16 moles of CH4 remain after the reaction. This is equivalent with 5.0 g of CH4.
O2: 3/8 moles were present when the experiment began. All of the O2 was consumed during the reaction. Thus, 0 g of O2 remains.
CO2: 3/16 moles were produced by the reaction. This is equivalent to 8.25 g of CO2. H2O: 3/8 moles were produced by the reaction. This is equivalent to 6.75 g of H2O.
4. If .80 g of CuO are present, then .01 moles of CuO are present. Thus, .01 moles of H2SO4 are necessary to react with CuO. If 3.0 M H2SO4 is available, then .01/3.0 L or 3.3 mL of 3.0 M H2SO4 are necessary to react.
5. If 3.00 g of Zn is allowed to react with 1.75 g of CuSO4, then .046 moles of Zn are reacting with .01 moles of CuSO4. This means that .036 moles of Zn will remain after the reaction. This is equivalent to 2.35 g of Zn.
6. A limiting reagent is whichever reagent in a reaction that is used up completely, preventing the nonlimiting reagent(s) from being consumed completely.
Conclusion: This experiment was successful. The percent yield was reasonable. Familiarity with basic laboratory procedures was gained, and the concept of percent yield was explored.