Purpose
The purpose of this lab was to focus on the Law of Conservation of Mater and how the sources of error in a lab can alter the expected data.
Qualitative and Quantitative Data
Copper Quantitative Data
Initial Mass of Copper
Mass of Evaporating Dish
Mass of Evaporating Dish and Copper
Mass of Recovered Copper
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2.004 g
50.165 g
57.01 g
6.845 g
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Qualitative Data from Reactions
Reaction
Cu + HNO 3
Cu(NO3) 2 + NaOH
Cu(OH)2
CuO + H2SO4
CuSO4 + Zn
Zn + H2SO4
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Observations
Exothermic and produced nitrogen dioxide gas
Exothermic, precipitate formed, solution became thicker, and gas was produced.
Black Copper Oxide precipitate formed and water formed. Black precipitate is denser than water and sinks to the bottom of the beaker.
Copper Sulfate and water formed. Copper Sulfate is a blue precipitate, but since it is soluble in water, it dissolves and the mixture is turned a bright blue color.
Zinc sulfate is formed and is aqueous. Hydrogen gas is also formed and released. Solid copper is found on the bottom of the beaker.
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Net Ionic Equations
1.
Cu (s) + HNO3 (aq) NIE: Cu(s) + 4H+(aq) + 2NO3-(aq)---> Cu2+(aq) +2NO2(g) + 2H2O(l)
2.
Cu(NO3)2 (aq) + NaOH (aq)
NIE: Cu2+(s) + 2OH-(s) ---> Cu(OH)2O(s)
3.
Cu(OH)2 (s) decomposes NIE: Cu(OH)2(s)---> CuO(s) + H2O(l)
4.
CuO (s) + H2SO4 (aq) NIE: CuO(s) + 2H+(aq)---> Cu2+(aq) + H2O(l)
5.
CuSO4 (aq) + Zn(s) NIE: Cu2+(aq) + Zn(s)---> Cu(s) + Zn2+(aq)
Calculations:
Final Mass of Copper
Mass of evaporating dish and copper - mass of evaporating dish = mass of copper
57.01 g - 50.165 g = 6.845 g
57.01 g - 50.165 g = 6.845 g
Final Moles of Copper
Final mass of copper X (moles of copper)/(molar mass of copper) = Final moles of copper
6.845 g Cu X ( 1 mole Cu) / ( 63.564 g Cu) = 0.1077 moles Cu
6.845 g Cu X ( 1 mole Cu) / ( 63.564 g Cu) = 0.1077 moles Cu
Initial Moles of Copper
Initial mass of copper X (moles of copper) / (molar mass of copper) = Initial moles of copper
2.004g Cu X ( 1 mole Cu)/ (63.564g Cu)= 0.0315 moles Cu
2.004g Cu X ( 1 mole Cu)/ (63.564g Cu)= 0.0315 moles Cu
Percent Yield
Percent Yield = (moles of copper recovered) / (initial moles of copper) X 100
Percent Yield = 0.1077 moles Cu / 0.0315 moles Cu X 100 = 341.9 %
Percent Yield = 0.1077 moles Cu / 0.0315 moles Cu X 100 = 341.9 %
Conclusions
The initial mass of copper used in the experiment was 2.004 grams, and the initial moles of copper was 0.0315. Due to the Law of Conservation of Mass, one would predict that after any number of chemical reactions and phase changes, the final mass and number of moles of copper would remain the same as the initial amounts. After the lab was finished, the final mass of the copper was 6.845 g, and the final moles was 0.1077. Clearly these values are not the same as hypothesized, due to a variety of sources of error throughout the lab. The percent yield of copper calculated was 341.9%.
Discussion of Theory
The main chemical concept that this lab involves is the Law of Conservation of Mass. This states that in a chemical reaction, matter can neither be created or destroyed, or more simply, the mass of the products must equal the mass of the reactants. In this lab, we tested this by putting copper through five different reactions and many phase changes. We started our experiment with 2.004 g of solid copper, and based on the Law of Conservation of Mass, once this copper is returned to its original state, it will have the same mass. In our lab, this was not the case, and we ended up with approximately three and one half times the amount of copper we started with, According to the law previously stated, this is impossible. So how did this happen? After going back and analyzing the reactions in the lab, we concluded that he majority of the sources of error in the lab would cause a loss of copper and a decrease in the final mass. So then how could the final mass possibly be larger than the initial mass? After more thought, we came to the conclusion that the final amount of copper that was weighed to calculate the final mass contains impurities introduced by the reactions in the lab, and these impurities altered the mass of the final product. In the final reaction, solid zinc was added to aqueous copper sulfate. If there was any solid zinc that did not react with the copper sulfate and was left over with the copper precipitate, zinc particles may have significantly changed the mass of the final product. As a result, the Law of Conservation of Mass was still held true, and the same amount of copper that was used initially was also found in the final product of the reaction.
Sources of Error
According to the Law of Conservation of Mass, the mass of the products must equal the mass of the reactants, so logically one would expect that if 2 g of copper was used to start the lab, the lab would result in 2 g of copper. Unfortunately, that was not the case in this lab, and the final mass of copper exceeded the initial mass by 4.841g. There were many different sources of error throughout this lab, and I believe that this was the reason for such a significant difference between the initial and final masses and moles of copper that were calculated. The first step of the lab was to measure 2 g of copper and place it in a beaker. Error may have occurred at this step if the balance that was used to weight the copper was not calibrated correctly, or if the amount of copper weighed was not exactly the 2 g required. Even a small deviation, such as that of 0.004g in my group, can influence the final results. There were many possibly sources of error in the lab that could have been due to incorrect measurement from systemic or random error. Throughout the entirety of the lab, there were many steps that presented possible sources of error such as adding too much or too little of a compound to the copper solution, or the loss of copper during transport or by being left on the stirring rod. Another situation which was a source of error was during step 5 when the copper solution and water was heated so that the contents of the beaker would boil. The error in this step was caused by the variable heating of the solution to prevent bubbling of the solution. The length of heating was another source of error because the mixture may have needed to be heated less or more than the 5 minutes stated in the direction to get to complete the step. During step 5, stirring of the mixture was required, so another source of error was that copper from the beaker may have been left on the stirring rod. During the next step, error may have occurred if there was copper precipitate left on the side of the beaker instead of washed with water. Decanting also proposed a source of error because copper may have been accidentally lost, or not enough water may have been decanted from the beaker. When the final mass of copper was weighed, the balance may have not been calibrated correctly, and the copper in the evaporating dish may have contained impurities, such as un-reacted zinc, that changed the mass that was measured. All in all, there was an abundance of sources of error in this lab.
Analysis Questions
1. The reaction between copper and nitric acid is an exothermic reaction. The function of using the ice bath in this step is to control the temperature of the reaction, because the reaction may become too violent if nothing is used to cool it down. Ice water is used as an absorber for the heat released in this reaction.
2. A double replacement reaction took place in step 4 between the copper nitrate and sodium hydroxide. The reaction that occurred in step 7 was a dehydration reaction. The reaction that occurred in step 9 was a single replacement reaction.
3. The reaction between the excess zinc and sulfuric acid in step 9 is essential to ensure that there is so un-reacted solid zinc mixed in with the copper, and that any excess zinc is reacted to form zinc sulfate than is aqueous and can be poured out with water. If this was an incomplete reaction, there would be leftover solid zinc that would be weighed with the copper precipitate, and the zinc would increase the measured final mass of the copper and cause a significant difference between the initial and final masses of the copper. The reaction between zinc and sulfuric acid is as follows: Zn(s) + H2SO4(aq) ---> ZnSO4(aq) + H2(g)
4. When the CuO was washed, the excess hydroxide ions that could have remained from the previous reaction were removed.
5. After sulfuric acid was added to the beaker, copper was found as copper ions with a 2+ charge instead of the previous copper(ii) oxide form.
6. In the final step of the lab when the copper precipitate was washed, zinc ions were removed. The previous reaction that took place involved aqueous copper(ii) sulfate and solid zinc. The products of the reaction were solid copper and aqueous zinc sulfate. After finding the net ionic equation of the reaction, one determined that the products of the reaction were solid copper and aqueous zinc ions. After this knowledge was obtained, one can conclude that the zinc ions were washed out with the water during the final step.
2. A double replacement reaction took place in step 4 between the copper nitrate and sodium hydroxide. The reaction that occurred in step 7 was a dehydration reaction. The reaction that occurred in step 9 was a single replacement reaction.
3. The reaction between the excess zinc and sulfuric acid in step 9 is essential to ensure that there is so un-reacted solid zinc mixed in with the copper, and that any excess zinc is reacted to form zinc sulfate than is aqueous and can be poured out with water. If this was an incomplete reaction, there would be leftover solid zinc that would be weighed with the copper precipitate, and the zinc would increase the measured final mass of the copper and cause a significant difference between the initial and final masses of the copper. The reaction between zinc and sulfuric acid is as follows: Zn(s) + H2SO4(aq) ---> ZnSO4(aq) + H2(g)
4. When the CuO was washed, the excess hydroxide ions that could have remained from the previous reaction were removed.
5. After sulfuric acid was added to the beaker, copper was found as copper ions with a 2+ charge instead of the previous copper(ii) oxide form.
6. In the final step of the lab when the copper precipitate was washed, zinc ions were removed. The previous reaction that took place involved aqueous copper(ii) sulfate and solid zinc. The products of the reaction were solid copper and aqueous zinc sulfate. After finding the net ionic equation of the reaction, one determined that the products of the reaction were solid copper and aqueous zinc ions. After this knowledge was obtained, one can conclude that the zinc ions were washed out with the water during the final step.