The Synthesis of Copper Metal

Transcription

1 CHEM 109 Introduction to Chemistry Revision 1.0 The Synthesis of Copper Metal To learn about Oxidation-Reduction reactions. To learn about Half-Reactions and Half-Cells. To learn about the Activity of Metals. In this exercise we will convert Cupric Ion (Cu 2+ ) into Copper metal (Cu) via an oxidationreduction reaction with Zinc (Zn). Cu 2+ (aq) + Zn(s) Cu(s) + Zn 2+ (aq) (Eq. 1) We will also observe a reaction between Copper (Cu) and Silver Ion (Ag + ), another oxidationreduction reaction. 2 Ag + (aq) + Cu(s) 2 Ag(s) + Cu 2+ (aq) (Eq. 2) Recall, oxidation involves the transfer of electrons from one species, that being oxidized, to another, that being reduced. Here we note Zinc is being Oxidized in the above reaction while the Cupric Ion is being Reduced. Electrons are being transferred from the Zinc metal to the Cupric Ion and the system behaves like a shorted battery.

2 The primary difference between the elemental metals, such as Zn, Cu and Ag, is the ease with which they undergo chemical reactions; i.e. are oxidized. The elements toward the bottom-left corner of the Period Table, which have the most metallic character, are chemically very reactive, and, hence, are the most Active metals. These metals very readily shed their electrons to become oxidized and form cations. The Activity of metals can be evaluated by means of their ability to undergo a chemical reaction in a Single Replacement reaction. In single replacement reactions, one metal displaces another metal, or hydrogen, from a compound or aqueous solution. In general, this can be represented as: M ' + M " Z M ' Z + M " (Eq. 3) where M ' is the metal which will displace M ". The metal M ' that displaces M " does so because it has a greater tendency to undergo a reaction. The metal M ' is said to be more "Active" than M ". Thus, the Activity of a metal is a measure of its ability to compete in a single replacement reactions. An Activity Series is a sequence of metals arranged according to their Activity. The activity of a metal can be loosely quantified by its oxidation half cell potential ox o as measured electrochemically. For our Cupric Ion-Zinc metal reaction, a corresponding voltaic cell can be constructed as in the diagram below. Recall, half-cell potentials are tabulated as reduction potentials; red o. So, the half-reaction with the most negative half-cell potential red o is the one most easily carried out as an oxidation. Ease of oxidation, hence activity, is quantified as a low half-cell reduction potential. For example, Liithium, with red o = -3.05V, is more easily oxidized than Aluminum, with red o = -1.66V. So, we say Lithium is a stronger reducing agent than Aluminum. And, Lithium is more active than Aluminum.

3 So, we will observe our reactions involving Copper, Zinc and Silver and arrange them according to their relative activity. Then, we will compare this arrangement with that of each metal's reduction half-cell potential.

4 Pre-Lab Safety Questions 1. Consult an SDS for 3M Sulfuric Acid. What first aid measures should be taken if this substance gets on your skin or in your eyes? 2. Consult an SDS for 1M Silver Nitrate. What are some of the hazards associated with this solution? 3. What are the incompatible materials for Silver Nitrate?

5 Procedure 1. Weigh out 2.0g of Cupric Sulfate (CuSO 4 ) Pentahydrate and add it to a 125 ml Erlenmeyer flask. Add 50 ml of Water to the flask. 2. Heat with occasional stirring on a hot plate to hasten the dissolution. Once the salt has dissolved, remove the solution from the hot plate and allow it to cool briefly. 3. Add 6 ml of 3M Sulfuric Acid (H 2 SO 4 ). Then add two crushed pieces of mossy Zinc, which have been previously weighed. This must be done in this order. Some gas will be evolved at this point. 4. Place the flask back on the hot plate and heat with occasional stirring until the solution is nearly colorless. 5. Carefully decant the liquid from the solids. 6. Add 25 ml of Water and swirl. Decant the liquid again. Repeat this washing procedure. 7. Remove any unreacted Zinc with a spatula. Wash any adhering bits of Copper back into the flask with a stream of Water from a wash bottle. 8. Add just enough Water to the flask so as to be able to wash the Copper into a filter funnel. Do this, adding additional Water to the flask to wash the remaining Copper into the funnel as well. 9. Allow the Copper to air dry on the filter paper for a few minutes. 10. Weigh the Copper. 11. Observe the reaction between Copper metal and Silver Nitrate, as demonstrated by your Teaching Assistant. (This reaction must be carried out in a limited fashion due to the high price of Silver.)

6 Data Analysis 1. What is the Limiting Reagent in this reaction? (You must support your answer quantitatively.) What happens to the excess reagent for the reagent that is not limiting? 2. Determine the theoretical yield, actual yield and percentage yield of the Copper in your reaction? 3. Prepare an Activity Series for the three metals used in this lab. 4. Using the table of reduction half-cell potentials in your text, look up the appropriate reduction for the three metals used in this lab. Compare these with your Activity Series. 5. Which metal is the strongest reducing agent? Which is the weakest? 6. Which metal is the strongest oxidizing agent?

7 Post Lab Questions None