A Cycle of Chemical Reactions of Copper PRE-LABORATORY ASSIGNMENT Finish the pre-laboratory assignments before the laboratory experiment. Read the manual of this experiment and complete the following questions. 1. a. Identify oxidation-reduction reactions in the copper cycle experiment. For each oxidation-reduction reaction, write half reactions to show what is oxidized and what is reduced. Identify acid-base reactions and precipitation reactions in the copper cycle experi- ment. Write equations for the corresponding reactions. b. How many mL of 3.0 M NAOH are required to neutralize 4.0 mL of 16.0 M HNO2? 2. 1N Copper. AplL tions and interpreting them in terms of chemical equations. You will also use a simple classification scheme for grouping chemical reactions by type. Finally, you will practice quantitative lab techniques by trying to recover the copper with minimal loss. AN OVERVIEW OF THE COPPER CYCLE One of the most fascinating aspects of laboratory experiments to new chemistry students is the variety of colors, odors, and textures encountered. The present experiment demonstrates that aspect by a sequence of striking changes in color, appearance, and bulk. By going through this experiment, students will develop laboratory techniques by the challenges of shepherding copper through all chemical changes with minimal loss. A schematic outline of the chemical reactions involved in the experiment is shown in Figure 5-1 HNO Cu(NO)2 Cu (1) NaOH (2) Zn (5) Cu(OH), 9VOA CUSO je() A (heat) 1000 Ht H,SO Cuo Figure 5-1. A schematic outline of the series of chemical reactions in the copper cycle. The overall copper cycle is then given as Cu(NO,)2(aq) Cu(s) Cu(OH)2(s) CuO(s) CUSO,(aq) Cu(s) The balanced equations corresponding to these steps are: Nitric acid is not only a strong acid but also a strong oxidizing agent, making it a very effective solvent for most metals. 1. (1) 4HNO,(aq)+ Cu(s) Cu(NOs)2(aq) + 2H20(l) + 2NO2(g) Many heavy metals form insoluble hydroxides which can be precipitated from solutions by sol- uble hydroxides such as NaOH. 2. (2) Cu(OH)2(s) + 2NANO,(aq) Cu(NO,)2laq) + 2NaOH(aq) Many transition metal hydroxides lose water upon heating, thus changing to oxides. For copper, this reaction happens at a relatively lower temperature and is accompanied by a striking color change, from blue to black. 3. (3) Cu(OH)2(s)Cu0(s) + H20(e) 4. Metal oxides, being base an hydrides, form salts with acids just as readily as do the bases themselves. CuO(s) + H,SO(aq) CuSO(aq) + H20(e) (4) 5. Chemically active metals, such as zinc, readily displace less active metals from their salts. CuSO&(aq) + Zn(s) Cu(s) + ZnSO(aq) (5) 6. The reaction used to remove excess Zn(s) left over from eq(5): Zn(s) +2H*(aq) Zn2t(aq) + H(g) (6) The last reaction is not explicitly shown in the cycle, but actually occurs twice. The first reaction is with H from excess HSO, as a side reaction during the reduction of CuSO and the second one is with HCL, as an intentional step for the removal of excess Zn remaining in solution after reduction of Cu2+ to Cu. Each equation, as given above, describes one of the colorful transformations, but it is easy to lose sight of their physical meaning if you merely look upon them as a bunch of symbols written on paper. To become more familiar with the chemical equations it is essential to try the following: Find out what the compounds, represented by the formulas, actually look like and what their chemical properties are. a. What class of compounds they belong to, e.g. acid, salt, etc., and what the generic properties of this class are, ie. what types of reactions they undergo. b. Similarly, try to categorize the equations (actually, the reactions they represent): oxidation- reduction (redox) reaction, acid-base reaction, precipitation reaction, replacement reaction, C. etc CLASSIFICATION OF CHEMICAL REACTIONS Precipitation In a precipitation reaction, two soluble substances react to form an insoluble compound, e.g. (7) CaCl2+ Na COs CaCO,(s) + 2NaCl CACO3 is insoluble in water so this salt will form a precipitate and fall out of the solution. 45 LAB 5: A CYCLE OF CHEMICAL REACTIONS OF COPPER