CHEM 231 Experiment 3 A Cycle of Copper Reactions, Summaries of Quantitative Techniques

Download CHEM 231 Experiment 3 A Cycle of Copper Reactions and more Summaries Quantitative Techniques in PDF only on Docsity! CHEM 231 Experiment 3 A Cycle of Copper Reactions In this experiment, you will begin with the element copper, and carry out a series of chemical transformations in which you will see copper in other forms. The last reaction returns the copper to its original metallic form which you will recover and weigh. The purposes of the experiment are  to observe various types of chemical reactions and relate observations to these reactions.  to reinforce the idea of atoms being preserved in chemical reactions.  to practice quantitative techniques by attempting to recover the original copper and computing the percent recovered. Please note that in their pure forms, all of the compounds observed are solids. In this experiment, we may not see them as solids because they may be dissolved in water. Water is present from the first step until the recovered copper is dried in the last step. In making observations, it is important to distinguish between a solution and a solid suspended in water. Procedure Step 1: The oxidation of copper metal by nitric acid Cut a length of pure copper wire that weighs about 0.5 g (about a 10-cm length). Polish it with a piece of sandpaper to remove any oxides or other coating. (Some copper wire may have a very thin plastic coating.) Weigh the wire to at least to the nearest mg and record this mass. Coil the wire and place it in the bottom of a 250-mL beaker. The next step must be done in a fume hood because of the possibility of noxious gas being produced. Add 4.0 mL of concentrated nitric acid (a solution of HNO3). Swirl the solution around in the beaker until the copper has completely dissolved. Add water until the beaker is about half full. The remainder of the experiment can be carried out at your lab bench. Record in your notebook a description of everything you see in this step. Background information The reaction that takes place can be written two different ways. Here is the first: 3Cu(s) + 8HNO3(aq)  3Cu(NO3)2(aq) + 2NO(g) + 4H2O Another way of writing the reaction recognizes the fact that ionic compounds in solution exist as separate ions when they are in solution and the same is true of strong acids such as nitric acid. 3Cu(s) + 8H+(aq) + 8NO3 -(aq)  3Cu2+(aq) + 6NO3 -(aq) + 2NO(g) + 4H2O The nitric oxide gas (NO) that is produced is not observed in this experiment because it almost instantly reacts with atmospheric oxygen to produce nitrogen dioxide: 2NO(g) + O2(g)  2NO2(g) The reaction involving copper is an example of an oxidation reaction. When an element increases in charge it is said to be oxidized. In solution, most ions are colorless. Copper(II) ion, however is an exception. Questions Based on the background information and your observations, answer the following questions before continuing: What element is oxidized? What observations can you make about nitrogen dioxide gas? Where is the original copper after the reaction? Is copper(II) nitrate soluble or insoluble? The above reactions should be written in your notebook and all observations should be related to the compounds which appear in those reactions. Step 2: Formation of copper(II) hydroxide While stirring the solution from the last step with a glass rod, add 30 mL of 3.0 M NaOH. Write down any observations you make directly in your notebook. Background information Sodium hydroxide is a strong base which can neutralize acids. The solution you have from the first step was made with excess nitric acid and thus is strongly acidic. After all the excess acid is gone, additional sodium hydroxide can react with the copper compound in the following manner: Cu(NO3)2(aq) + 2NaOH(aq)  Cu(OH)2(?) + 2NaNO3(aq) Questions Answer the following questions before continuing: in the presence of excess acid, there is a significant amount of H+ ion present in the solution. Besides being an acid, H+ ion can also act as an oxidizing agent, accepting electrons from another element, and producing hydrogen gas (H2). In this secondary reaction, zinc is oxidized by the hydrogen ion: Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g). Notice that this reaction competes with the first one for the zinc metal and, thereby works against the overall goal of reducing copper to its metallic state. However, with the directions given in this procedure, we should still see most of the copper ion reduced even in the presence of this competing reaction. Questions Answer the following questions before continuing: What do you observe that indicates that copper(II) ion is being consumed in this step? What do you observe that indicates that copper metal is being formed? What do you observe that indicates that the second reaction is taking place (in which the zinc is oxidized by hydrogen ion and making hydrogen gas)? Step 6: Purification and recovery of copper metal Decant the liquid from the last step. Add 10 mL of 6 M HCl and warm, but do not boil, the solution. The purpose of this step is to oxidize any remaining zinc metal in order to remove it. You may see the production of bubbles of hydrogen gas as this is taking place. When this hydrogen evolution is apparently complete, decant the supernatant liquid, and transfer the copper to a porcelain dish. Use a spatula or rubber policeman. Wash the product with a small amount of distilled water, allow it to settle, and decant the wash water. Repeat the washing and decanting two more times. Move to the hood, away from all flames, and wash and decant with methanol. Place the porcelain dish in the place provided to dry. If this is at the conclusion of a lab period, allow the methanol to evaporate until the following week. If this is at the beginning of a lab period, allow it to evaporate until the end of the period. Weigh an empty beaker, transfer the purified copper metal into the beaker and weigh again. Compare the final mass to the initial mass of copper used. Determine the percent recovery. Background information In step 5, although the oxidation of zinc by the acid competes with the desired reaction, it also helps accomplish another important task. Specifically, if we wish to recover purified copper, we must remove any excess zinc metal from the product. Oxidizing it to soluble zinc ion which can then be rinsed away accomplishes this. In this last step, we add more strong acid in the form of HCl to be certain that this process is complete. All that remains is solid copper and a number of ionic impurities all of which are dissolved in the water which remains. The rinsing and decanting steps help remove these impurities. The purpose of the final rinse with methanol is to help remove the water. Unless the solution is heated to dry it, the recovered copper metal is likely to be wet. Methanol will remove the water and is much more likely to evaporate completely because it is so much more volatile. Questions Answer the following questions concerning the final step: Assume that, at the end of Step 3, you had only solid copper oxide and water. Considering the reactions which took place in Steps 4, 5, and 6, which ions are present in the water before the rinsing and decanting in Step 6? What percent of the original copper did you recover in the end? If this amount is not equal to 100%, to what do you attribute the difference? Summary and conclusions In this experiment, you saw different forms of the element copper. Summarize the various states of copper that you observed in this reaction by filling out a table such as that shown. Substance Formula Appearance (color) Water soluble? (Yes/No) Copper(II) nitrate Copper(II) hydroxide Copper(II) oxide Copper(II) sulfate Copper metal Your conclusion should summarize in words the results displayed in the table and the reactions that produced these compounds. You should also include balanced chemical reactions for each step. Your observations should be related to the reactions.