Joseph Chang Chemistry, Period 4 Copper and Silver Nitrate Displacement Reaction Lab Purpose: The purpose of conducting this experiment is to use an experiment to determine the mole ratio between the copper and silver in the reaction. By finding the mole ratio, we can empirically prove the precepts of the relationships of moles in "displacement reactions" mainly that the reactants exchange electrons, or in our equation: Cu (s) -> Cu2+ (aq) + 2e- producing solid silver molecules from dissolved silver ions while the copper becomes copper ion and dissolve in the water, while the copper gives the silver ions to make the silver's charge neutral. Materials: Copper strips, beakers, silver nitrate solution, filter paper, tweezers, and electronic balance (accurate to the hundredths place) Procedure: 1) Take a piece of copper, which you must brush the dust from and sand until the copper has a shiny luster (to remove impurities). 2) Place the copper in the prepared beaker, then carefully add the silver nitrate necessary to completely cover and react with the copper strip. (Do not touch the silver photosensitive silver nitrate) 3) Store the copper and wait for a day for the copper and silver nitrate to react. 4) Swirl the strip in the solution to get rid of any silver. 5) Use tweezers to remove the copper strip from the solution. 5) Next, weigh and record the filter paper. 6) Use the filter paper when you pour the silver solution from the beaker through a funnel into another beaker. 7) Use rubbing alcohol to rinse the beaker once more and pour the remaining fluid through the filter paper. 8) Store the filter paper with the strip of copper to allow it to dry for another day. 9) Determine and record the mass of the copper and the filter paper containing silver. 10) Return the silver to Mrs. Shah to be remade into silver nitrate. 11) Find the mass of copper dissolved and the mass of the silver formed. 12) Calculate the moles of copper dissolved and of silver formed-the mole ratio. Data: Mass of Copper (Cu) Before the Reaction 5.960 g After the Reaction 5.600 g Mass of Filter Paper Before the Filtration 0.862 g After the Filtration 2.130 g Volume of Silver Nitrate (AgNO3) used: 40 mL. Calculations: 1. The mass of copper dissolved: 5.960 g - 5.600 g = 0.3600 g Cu 2. The mass of silver formed (accrued on the filter paper): 2.130 - 0.862g = 1.27 g Ag 3.The number of moles of copper dissolved (given copper = 63.55 g/mol): 0.3600g/ 63.55g/mol = 0.005664 mol Cu dissolved The number of moles of silver formed (given silver = 107.9 g/mol) 1.27g/ 107.9g/mol = 0.0118 mol Ag dissolved 4. The ratio of moles of silver to moles of copper: 0.0118 mol Ag / 0.005664 mol Cu = 2.08 mol Ag/ mol Cu Cu (s) + 2Ag+ (aq) à Cu2+ (aq) + 2Ag (s) Conclusion: The oxidation/reduction reaction in which copper and silver exchange ions can be written: Cu (s) + 2Ag+ (aq) -> Cu2+ (aq) + 2Ag (s) or Oxidation ½: Cu (s) -> Cu2+ (aq) + 2e- Reduction ½: 2 Ag+(aq) 2e--> 2 Ag (s) Questions: 1. The mole ratio my lab group formed when reacting the copper with the silver nitrate was 2 moles of silver to every 1 mole of copper. Thus, considering the charge of cupric is 2+ and the charge of Ag is 1+, it would make sense for two moles of silver ions (1+) to interact and exchange electrons with one mole of copper to form one mole of cupric (2+). Therefore, one mole of copper of no charge would give electrons and gain negative charge to two moles of silver of positive charge 1, giving the copper a charge of plus 2 and the silver a neutral charge. 2. In the case of my experiment, instead of obtaining 2.00 mol Ag/ mol Cu, I obtained 2.08 mol most likely out of experimental error. The integrity of the experiment relied on the relative purity of the copper, silver nitrate, cupric, and silver. If any of the substances involved in the reaction were contaminated, then they would result in a small, but discernable error. For example, if the filter paper filtered and accrued substance other than the pure silver, then the recording of the mass of silver would be inaccurate, which would have made the calculation of the silver mass and thus the number of silver moles incorrect. This could have given me a mass of a few hundredths or tenths of a gram because silver itself weighed 1.27 grams, allowing for an error to be relatively large compared to the measured mass. Another example of a possible error would be if the electronic balance was inaccurate, such as if it was not properly calibrated and set to zero before we conducted the measurement of the mass. This could result in an error from several thousands to a few hundredths. This would have affected my mole ratio likewise by several ten thousandths or hundred thousandths. Also, my group experienced some problems with our scale, probably contributing to some error on our measurements. Another error could have occurred if my group did not completely scrape off the dust and residue, which would give the initial copper a mass larger than it actually was. However, dust and residue, if they were not scraped off are probably do not have a very large mass, perhaps affecting the mass by a few hundredths, the number of moles less, and the mole ratio even less. Finally, if my group did not completely scrape all the silver that was precipitated from the reaction, then my numbers for the mass of the silver would be incorrect. This could be very possible since the method of scraping was not completely comprehensive. However, since most of the silver naturally lumped together, it was somewhat easier to collect the silver. Nonetheless, the possibility of inaccuracy when collecting a silver could have been greater than the error of washing the residue, since my group could have left behind and not recorded much silver, around several hundredths of a gram. 3. The mole ratio of silver to cupric ions was 2.08 or approximately 2 to 1q given experimental error. Yes it would be possible to find the balanced chemical equations. By observing the empirically obtained mole ratio, then I can also deduce that the equation would involve one copper molecule reacting with and giving two silver ions an electron. Considering that I know Cu2+ and 2Ag+ ions exist as single ions and not diatomic, I do not need to worry about complications in the equation, and I can simplify it to one copper and two silver exchanging electrons. 4. If the silver still contained water, then my previous assumption, that the silver is pure would be invalidated, and thus the number of moles I calculated would be incorrect, and therefore the mole ratio of silver to copper would be incorrect. Since it is the ratio of silver to copper, then the presence of water would increase the weight and create a ratio larger than the actual ratio. 5. If some of the copper crumbled and mixed with the silver, it would remain in the filter paper with the silver (since it can not go through the filter paper). That would decrease the mass of the copper beyond the actual mass, as well as increase the mass of silver beyond the actual mass. Since the mole ratio determined was that of the moles of silver to the moles of copper, then it would thus be increased by both the increase of mass in the numerator as well as the decrease of the mass in the denominator. 6. If magnesium (Mg2+) were to act, atom-for-atom, exactly like the copper in this experiment, then, since it has the same charge as cupric (Cu2+), the exact same number of moles of silver would also interact and exchange, taking the exact same number of electrons from the exact same number of moles of magnesium as it would from copper. The equation would be the same as the one with copper (II): Mg (s) + 2Ag+ (aq) -> Mg2+ (aq) + 2Ag (s) If there is 1 g Ag, then given that two silvers react with one magnesium and silver's molar mass of 107.9 g/mol, then there would be 1 g Ag/ 215.8 g Ag/mol = 0.004634 mol Ag So, since 1 gram of silver contains two ions of 0.004634 mol Ag+ that react with one Mg, it would require 0.004634 mol Mg2+ to react with it, which would have a mass of: 0.004634 mol ( 24.31 g Mg/mol) = 0.1127 g Mg