Chemical Reactions of Copper and Percent Recovery

Transcription

1 and Percent Recovery EXPERIMENT 9 Prepared by Edward L. Brown, Lee University To take copper metal through series of chemical reactions that regenerates elemental copper. Students will classify the various reactions, write their net ionic equations and identify the driving force responsible for the reaction. beaker (250 ml) graduated cylinder wire gauze ring clamp ring stand A P P A R A T U S Bunsen burner boiling stones glass stirring rod evaporating dish beaker tongs OBJECTIVE APPARATUS AND CHEMICALS C H E M I C A L S Copper wire in numbered test tube Aluminum foil concentrated Nitric Acid (HNO 3 ) Methanol 3.0 M NaOH Acetone 3.0 M HCl In this lab you will perform three of the five basic types of chemical reactions: 1. Synthesis two elements come together to form a compound. Cl + 2Na (s) 2NaCl Equation 9.1 (s) 2. Decomposition a compound is broken down into smaller molecules and/or elements. In lab today, you will decompose water into its elements according to the following chemical equation: 2H2 O (l) 2H + O Equation 9.2 Copyright 2005 Chem21 LLC. No part of this work may be reproduced, transcribed, or used in any form by any means graphic, electronic, or mechanical, including, but not limited to, photocopying, recording, taping, Web distribution, or information storage or retrieval systems without the prior written permission of the publisher. For permission to use material from this work, contact us at info@chem21labs.com. Printed in United States of America.

2 3. Combustion An element reacts with oxygen to form an oxide (this is also a Synthesis) 2Mg (s) + O 2MgO Equation 9.3 (s) A compound consisting of carbon, hydrogen and/or oxygen (C 8 H 18 is called octane) reacts with oxygen to form carbon dioxide and water. 2C8H 18 (l) + 25O 16CO + 18H2 O Equation 9.4 (l) 4. Metathesis two ionic species (C 1 A 1 and C 2 A 2 ) containing a cation (C) bonded to an anion (A) are mixed together. The cations (C 1 and C 2 ) exchange anionic partners to form the products shown (C 1 A 2 and C 2 A 1 ) when one of the four criteria (also called driving force) below is met. C A + C A C A + C A Equation a. A precipitate is formed (use solubility rules to determine when a precipitate will form). AgNO 3 + HCl AgCl (s) + HNO3 Equation 9.6 b. A weak electrolyte is formed either C 1 A 2 and C 2 A 1 is a weak electrolyte. An example of a weak electrolyte is a weak acid. Weak acids are all species where a hydrogen atom is attached to fluorine, oxygen, or to a positive nitrogen, excluding the seven strong acids (HCl, HBr, HI, H 2 SO 4, HNO 3, HClO 3, HClO 4 ). The Lewis structure of the product, HC 2 H 3 O 2 (acetic acid), shows that a hydrogen is attached to an oxygen making it a weak acid. HCl + NaC2H3O 2 HC2H3O 2 + NaCl Equation 9.7 The structure above is a common class of weak acids called organic acids (or carboxylic acids). The distinguishing feature of this category of acids is the COOH group. H H C H O C O H c. A non electrolyte (water) is formed. This reaction is also called a neutralization reaction where an acid and a base react to form a salt and water. HCl + NaOH H2 O (l) + NaCl Equation 9.7 Experiment 9 9-2

3 d. A gas is formed upon mixing two compounds and/or solutions. The gases (and their smells) commonly formed in chemical reactions are H 2 S (rotten egg), SO 2 (burnt match), CO 2 (odorless, doesn t support combustion), O 2 (odorless, supports combustion), H 2 (odorless, explosive), and NH 3 (ammonia). Equation 9.8 shows a general reaction of carbonates and acids. 2HCl + Na 2CO 3 CO + H2 O (l) + 2NaCl Equation Oxidation / Reduction a more active metal gives its electrons to the cation of a less active metal. The more active metal becomes a cation and the cation of the less active metal becomes a metal. This process of transferring electrons is called a REDOX reaction and requires that REDuction occurs only in the presence of Oxidation (i.e. they are coupled). Oxidation is defined as the loss of electrons and reduction is the gain of electrons. In total, you will perform five reactions involving copper, starting with the oxidation of copper metal to copper ion and ending with the reduction of copper ion to regenerate elemental copper as the final product. Unlike experiments where we analyzed the reactions qualitatively (we were concerned with observing what happened and not with the question of how much reacted), this lab will focus on analyzing the various reactions quantitatively - analysis based on amount rather than observation. This laboratory exercise will require familiarization with several separation techniques to isolate a single pure product. The separation of the components in many mixtures routinely encountered by chemists is based on the use of one or more of the following techniques: 1. Decantation. The process of separating a liquid from a denser solid. The solid component settles to the bottom of the container (sedimentation) leaving the less dense liquid at the top where it is removed by pouring. 2. Extraction. The process of separating substances by exploiting their differing solubilities in a particular solvent. Ideally, only one component of a mixture will dissolve in the solvent of choice leaving the other component as a solid or an immiscible liquid. 3. Filtration. The process of separating a liquid from a solid by moving the solution through a material that is porous to the liquid but not the solid. Typically, filter paper and gravity are used to accomplish this separation. 4. Evaporation. The process of providing conditions in which a volatile component (usually a liquid) is removed from a less volatile component (usually a solid) by Experiment 9 9-3

4 prolonged exposure to an atmosphere with which the volatile component attempts to establish an equilibrium. 5. Crystallization. The process of providing conditions in which only one substance in a solution will form crystals that can be collected by filtration. H2O H2O Place receiving flask in an ice/water bath. 6. Distillation. A process in which components of mixtures are separated based on differing boiling points the lowest boiling component is collected first followed by the more volatile components of the mixture. 7. Sublimation. A process in which a solid passes directly into the gas state without becoming a liquid. All substances can accomplish this feat at some particular temperature and pressure, however, at atmospheric pressure relatively few substances sublime. 8. Chromatography. A process in which components of a mixture are exposed to two very different physical environments a stationary phase and a mobile phase. Usually, no two components in a mixture will have the same affinity for both phases. The extent of separation depends on each components time in A B A&B the mobile phase the longer a component is in Step 1 Step 2 Step 3 this phase, the farther it will travel along the stationary phase. In a later lab, you will use paper as the stationary phase and water/acetone as the mobile phase to separate cations from one another. Each person will be given a mass of copper wire which will be converted into Cu(NO 3 ) 2 Cu(OH) 2 CuO CuCl 2 Cu(s). The unbalanced reactions involved are as follows: Cu (s) + HNO 3 Cu(NO 3 ) 2 + H 2 O + NO 2 Equation 9.9 Cu(NO 3 ) 2 + NaOH Cu(OH) 2 + NaNO 3 Equation 9.10 Cu(OH) 2 (s) CuO + H 2 O Equation 9.11 CuO (s) + HCl CuCl 2 + H 2 O Equation 9.12 Experiment 9 9-4

5 Al (s) + CuCl 2 AlCl 3 + Cu Equation 9.13 excess Al (s) + HCl AlCl 3 + H 2 Equation 9.14 Several items should be noted about Equations : Some of the products are gases (i.e. NO 2 and H 2 ) which are the easiest products to remove from a reaction. Water is formed in three steps and its removal is accomplished by decantation or evaporation. Sedimentation and decantation techniques are only employed when the copper compound is insoluble (s) in water the (s) stands for solid. When the copper compound is insoluble, all other products are water soluble (NaNO 3 and AlCl 3 ) and are removed by allowing the copper compound to sediment and decanting the water soluble by-products away from the copper compound. Any water present at the end of the reaction is removed by combining it with methanol, then acetone, followed by evaporation. Following this final evaporation, you will obtain elemental copper the same material used in the very first reaction. If you obtain the same amount that you started with, then you will have recovered 100 percent. This percent recovery is one of the most important laboratory calculations. This value gives chemists a quick understanding of just how well a single reaction (or in this case, multiple reactions) has occurred. This value will be dependent on two main factors: the degree of completion of each reaction and the ability to isolate a single product at each step. The percent recovery is calculated using Equation actual mass % Recovery = 100 theoretical mass Equation 9.15 The actual mass is the mass of the pure, final product as determined by a balance in the laboratory. The theoretical mass is the mass of the product that should be present if every reactant molecule participated in the written reaction(s) AND if the technique(s) used to obtain the pure, final product were successful in isolating only product molecules without losing any in the process. In today s lab, a rare use of Equation 9.15 occurs. Since we are starting and ending with the same pure substance (elemental copper), the theoretical mass is the starting mass. In most reactions, the theoretical mass is calculated by converting the mass (in grams) of Substance A to the mass (in grams) of Substance B. A: Equation 9.9: Addition of Concentrated HNO 3 PROCEDURE 1. Obtain a clean, 250 ml, graduated beaker from your lab drawer. 2. Place your name or initials on the beaker. 3. Obtain a piece of copper wire from your instructor and determine the mass of the copper [Data Sheet Q1]. Experiment 9 9-5

6 4. Twist the copper wire into a flat, tight coil (the size of a penny) and place it in the bottom of your beaker. 5. Place your beaker in the hood. 6. Either you or your instructor will place ~ 5 ml of concentrated nitric acid (conc HNO 3 is very CAUSTIC Avoid Contact!! Use gloves!!) in the beaker. The copper metal will be oxidized to copper ions (~5 minutes). The noxious gas, NO 2, is produced in this reaction so the beaker must be kept inside the hood. 7. Record your observations [Data Sheet Q2]. Note the state (s, l, g, aq) of each product you will need to know this to balance the equations online. Equations give the states of the reactants but not the products. 8. After the copper wire has disappeared, fill your beaker to the 100 ml mark with distilled water (use the graduations on the beaker to measure the water). 9. Write a balanced net ionic equation [On-Line Report Sheet Q3] and classify the type of reaction this represents [On-Line Report Sheet Q4]. B: Equation 9.10: Addition of NaOH 10. Use a graduated cylinder to add 30 ± 2 ml 3.0 M NaOH to the copper nitrate solution. 11. Record your observations [Data Sheet Q5]. 12. Write a balanced net ionic equation [On-Line Report Sheet Q6] and classify the type of reaction this represents [On-Line Report Sheet Q7]. 13. What is driving force (reason) for the reaction [On-Line Report Sheet Q8]. C: Equation 9.11: Addition of Heat 14. Place the beaker on a wire gauze seated on a ring clamp [Figure 9.1]. Place a few boiling stones in the beaker and stir the solution (constant stirring) with a glass stirring rod as you bring the solution almost to boiling. Leave the stirring rod in the solution to avoid losing material on your lab bench. 15. Near the boiling point, the solution will form a brown / black precipitate and all the blue color will disappear. Remove the heat at this point and allow the solution to cool (5-10 minutes) without stirring. 16. Record your observations [Data Sheet Q9]. 17. Write a balanced net ionic equation [On-Line Report Sheet Q10] and classify the type of reaction this represents [On- Line Report Sheet Q11]. 18. If some of the solid has not settled to the bottom of the beaker during the cooling period, add an additional 2 ml 3.0 M NaOH dropwise on top of the floating solid. Figure Decant (pour off) the clear solution with minimal loss of the precipitate. The trick is not to pour a little and then stop; then pour a little more, then stop, etc this will just stir up the solid in the bottom of the beaker. The trick to decantation is to pour slowly and constantly until the solid in the bottom of the beaker is just about to be poured out then, stop decanting! 20. Add 100 ml boiling water (use beaker tongs) to the precipitate in the beaker any non-copper impurities will dissolve in the hot water. Allow the solid to sediment for Experiment 9 9-6

7 5 minutes. Decant the water taking care to minimize loss of the copper-containing solid. What are you removing in this step [On-Line Report Sheet Q12]? D: Equation 9.12: Addition of HCl 21. Use a graduated cylinder to add ml 3 M HCl to the beaker. 22. Record your observations [Data Sheet Q13]. 23. Write a balanced net ionic equation [On-Line Report Sheet Q14] and classify the type of reaction this represents [On-Line Report Sheet Q15]. 24. What is driving force (reason) for the reaction [On-Line Report Sheet Q16]? E: Equation 9.13: Addition of Aluminum 25. Place the beaker containing the CuCl 2 into a larger beaker filled ¾ full with ice and water. 26. After 5 minutes, remove this beaker from the ice bath and add ~ 0.25 g aluminum foil to the beaker. At this point two reactions are occurring in the beaker one between the copper ion and aluminum metal [Equation 9.13] and one between the hydrochloric acid and the aluminum metal [Equation 9.14]. 27. This reaction will take 5-10 minutes. You are ready to move on to the next step when the red precipitate of copper no longer forms on the surface of freshly added aluminum foil and there are no shiny pieces of metal (Al) in the beaker and the solution is no longer blue. If aluminum is still present at the end of the reaction, add more 3.0 M HCl; if solid copper is still forming on fresh aluminum foil, add more aluminum foil. Record your observations [Data Sheet Q17]. 28. Write a balanced net ionic equation for the reaction between Al and CuCl 2 [On-Line Report Sheet Q18] and classify the type of reaction this represents [On-Line Report Sheet Q19]. What is driving force (reason) for the reaction between Al and HCl [On- Line Report Sheet Q20]? 29. Write a balanced net ionic equation for the reaction between Al and HCl [On-Line Report Sheet Q21] and classify the type of reaction this represents [On-Line Report Sheet Q22]. What is driving force (reason) for the reaction between Al and HCl [On- Line Report Sheet Q23]? F: Isolation of Copper: 30. Decant most of the liquid above the copper metal into another beaker (leave ~ 20 ml). 31. Transfer this remaining 20 ml and the solid copper to a clean, dry evaporating dish. Decant the liquid from the evaporating dish back into the beaker to aid in transferring all the copper. Repeat as necessary so that all the copper has been transferred to the evaporating dish. Finally, decant the water in your evaporating dish away from the copper metal that has settled. 32. Rinse the copper in the evaporating dish with 5 ml water. Use a stirring rod to break up the clumps of copper. Decant the water. 33. Repeat Step 32 with another 5 ml of water (decant the water). 34. Repeat Step 32 a third time with another 5 ml of water (decant the water). 35. Take your evaporating dish and a small beaker to the lab station where the methanol is located (DO NOT TAKE THE METHANOL OR ACETONE TO YOUR LAB STATION!!). Rinse the copper in the evaporating dish with 5 ml methanol. Experiment 9 9-7

8 Use a stirring rod to break up the clumps of copper. Decant the methanol into your beaker and then place it in the methanol waste container before proceeding to Step Acetone is located at the same lab station. Rinse the copper in the evaporating dish with 5 ml acetone. Use a stirring rod to break up the clumps of copper. Decant the acetone into your beaker and place it in the acetone waste container before proceeding to Step At your lab station, place the evaporating dish on the top of a beaker containing ½ inch of boiling water [Figure 9.2]. Allow the heat of the boiling water to remove the remaining traces of acetone and dry the copper (Take Care Not To Allow The Flame To Come Near The Evaporating Acetone!!! Acetone Is Extremely Figure 9.2 Flammable!!! If the Acetone does ignite, Do Not Panic the fire will not hurt your product and will go out when all the acetone has evaporated.). You may use a glass stirring rod to break up any clumps of copper, but make sure you don t lose any of your product by setting your stirring rod on the counter. Scrape your stirring rod on the side of the evaporating dish to remove any adhering copper. 38. Remove the heat when the copper appears dry and no longer clumps on your stirring rod. To test for complete evaporation of the acetone, briefly pass the flame from your Bunsen burner over the surface of the copper in the evaporating dish if any residual acetone is present, it will burn briefly and then die out. 39. Use non-rubberized tongs to place your evaporating dish on the lab bench to cool. 40. When the evaporating dish is cool to the touch, WIPE THE OUTSIDE DRY WITH A PAPER TOWEL and take it to the balance area. 41. Tare a piece of weighing paper (0.000 g) and quantitatively transfer the solid copper onto the weighing paper. Record its mass [Data Sheet Q24]. 42. Crease the weighing paper and transfer the product to a small vial. Cap the vial and affix a label containing your name and the mass of the copper 10 POINTS NO VIAL, NO POINTS. 43. Determine the % Recovery [On-Line Report Sheet Q25]. 1/2 inch H 2 O Waste Disposal Any aqueous solutions generated in this lab can be flushed down the sink with plenty of water. Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will be entered and lab attendees will be permitted to access the online data / calculation submission part of the lab report (click on Lab 9 Chemical Reactions of Copper). Enter your data accurately to avoid penalty. The lab program will take you in order to each calculation. If there is an error, you will be given additional submissions (the number and penalty to be determined by your instructor) to correct your calculation. Experiment 9 9-8

9 Post-Lab Questions: The questions for this lab can be found at Do Not Wait Until The Last Minute!!!! Computer Problems and Internet Unavailability Happen, But Deadlines Will Not Be Extended!! On the Internet, complete any Post Lab Questions for Laboratory 9. The computer program will check your answer to see if it is correct. If there is an error, you will be given additional submissions (the number and penalty to be determined by your instructor) to correct your answer. Late Submission: Late submission of the lab data / calculations is permitted with the following penalties: - 10 points for submissions up to 1 day late, - 20 points for submissions up to 2 days late. Experiment 9 9-9

10 Laboratory 9 Student Data Sheet Lab 9 Chemical Reactions of Copper Name: Mass: A: Equation 9.9: Addition of HNO 3 1. Mass of Copper Wire g 2. Observations: Cu (s) + HNO 3 B. Equation 9.10: Addition of NaOH 5. Observations: Cu(NO 3 ) 2 + NaOH C. Equation 9.11: Addition of Heat 9. Observations: Cu(OH) 2 (s) + heat D. Equation 9.12: Addition of HCl 13. Observations: CuO (s) + HCl E. Equation 9.13: Addition of Aluminum 17. Observations: CuCl 2 + Al (s) AND Al (s) + HCl F. Isolation of Copper 24. Mass of Copper g Experiment

11 Laboratory 9 Instructor Data Sheet Name: A: Equation 9.9: Addition of HNO 3 1. Mass of Copper Wire g 2. Observations: Cu (s) + HNO 3 B. Equation 9.10: Addition of NaOH 5. Observations: Cu(NO 3 ) 2 + NaOH C. Equation 9.11: Addition of Heat 9. Observations: Cu(OH) 2 (s) + heat D. Equation 9.12: Addition of HCl 13. Observations: CuO (s) + HCl E. Equation 9.13: Addition of Aluminum 17. Observations: CuCl 2 + Al (s) AND Al (s) + HCl F. Isolation of Copper 24. Mass of Copper g Experiment