The Copper Cycle Lab Report

THE COPPER CYCLE

OBJECTIVES

  • To observe the chemical properties of copper through a cycle of chemical reactions, including oxidationreduction, word image 2604 decomposition, and acid-base reactions.
  • To use a variety of separation and recovery techniques to isolate copper-containing compounds from solution.
  • To calculate percent recovery of copper through a cycle of chemical reactions.

INTRODUCTION & BACKGROUND

Copper is a valuable and versatile elcmcnt capable of forming a variety of compounds in both nature and the laboratory environment. In iß elemental form, copper is a soft metal with a characteristic shiny red-brown hue, and is valued for its use in jewelry and metal alloys such as bronze and brass. Although relatively unreactive toward air oxidation and simple inorganic acids, copper can be oxidized by stronger oxidizing agents, allowing for formation of a variety of colorful (frequently blue or green) inorganic compounds word image 2605

In the lab experiment, copper’s versatility can be observed through a cyclical series of redox, precipitation, acidbase, and decomposition reactions. Performing this “cycle” of reactions permits students to gain experience in a variety of laboratory techniques, while demonstrating the challenges to efficient recovery ofchemical products in small-scale reactions.

Originating with metallic copper, the sequence of compounds formed in this experiment is shown in the diagram

below:

word image 2606

Reaction

Reaction

C

Reaction

D

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In Reaction A, aqueous nitric acid (a strong oxidizing agent), is added to a small amount Of copper wire word image 2607 word image 2608 wire is readily oxidized to water-soluble copper(ll) nitrate (a blue solution), with fomation dioxide — a dcnsc, toxic, reddish-brown gas. fie balanced molecular equation for this reaction is: word image 2609 cu(s) + 4 HN03 (an -+

In Reaction B, the aqueous solution is treated with aqueous sodium hydroxide and undergoes an observable double displacement (metathesis) reaction to form the light blue precipitate, copper(ll) hydroxide:

+ 2 NaOH (an -+ (s) + 2 NaN03 (aq)

In Reaction C, heat is applied to the solid Cu(OH)2, resulting in formation of black, insoluble copper(ll) oxide, with accompanymg loss ofwater:

word image 2610 -+ Cuo (s) + H20 (0 word image 2611

word image 2612 word image 2613 In Reaction D, the insoluble Cuo reacts readily with aqueous sulfuric acid to form a sky-blue aqueous solution of copper(ll) sulfate (and water):

Cuo (s) + H2S04 (an -+ cuS04 (an + H20 (1)

Finally, in Reaction E, magnesium metal acts as a reducing agent to convert the aqueous copper(ll) sulfate to pure (reduced) copper metal, Cu:

cuS04 (an + Mg(s) -+ cu (s) + MgS04(aq)

In a competing reaction, magnesium also reduces sulfuric acid, with formation ofhydrogen gas and aqueous magnesium sulmte•

Mg(s) + H2S04(aq) -+ H2(g) + MgS04(aq)

Consequently, any excess magnesium metal that remains after the completion of Reaction E can be conveniently consumed by addition ofa small excess of aqueous sulfuric acid. The recycled copper metal can be washed, isolated, and dried, allowing for precise calculation ofthe percent recovery of copper.

Fall 18

The Copper Cycle

Experiment Procedure

Materials: Two (2) large disposable test tubes, 100-mL beaker, two pre-cut copper wire pieces, disposable transfer pipets, deionized water, 15 M HN03, 6 MNaOH, 6MH2S04, small waste beaker, 150-mL beaker with boiling stones, hot plate, pre-cut Mg strips, acetone

General Instructions:

l. Students must wear laboratory goggles, lab aprons, and disposable gloves at all times.

  1. The experiment requires use of concentrated acids and bases, a reactive metal (Mg), and acetone. Spills of any type should be reported immediately to the instructor and cleaned up carefully.
  2. Bottles of concentrated acids and bases must remain in the fume hoods at all times.
  3. For each Reaction A — E described below, record däailed observations, as well as a balanced molecular equation and a balanced net ionic equation, on the Report Sheet.
  4. Students will complete two trials of the procedure concurrently. Using permanent marker, label test tubes with “Trial l” and “Trial 2”, as well as initials. Complete identical procedural steps with each test tube.
  5. Check with lab instructor to determine if quantitative analysis ofthe ‘Tercent Recovery of Copper” is required for the lab report; if quantitaüve analysis is required, record all masses to the nearest 0.000 lg.

Reaction A: Copper Metal Copper(ll) Nitrate

l. If required, record the mass of labeled test tube on the Report Sheet.

  1. Obtain a pre-cut length of copper wire (approx. 1.5 cm); fold the wire into a circular shape, and place in test tube. If required, record the combined mass of the test tube and copper wire on the Report Sheet and word image 2614 determine the mass of the copper wire by subtraction (approximately 0.02 g).
  2. Place test tube in a clean 100-mL beaker. Warning —Perform this step in thefume hood and do not inhale the evolved nitrogen dioxide gas! Use a disposable I-mL transfer pipet to add 5 – 6 drops (no more than 0.25 mL) of concentrated (15 M) nitric acid. Do not add an excess of nitric acid. Allow the copper wire to react (disappear) completely before continuing to the next step. If the copper wire has not word image 2615 disappeared after 10 minutes of gentle mixing, consult your instructor.
  3. Add —l mL of deionized water (using plastic pipet) to the test tube and mix by gently agitating the test tube.

Do not remove fre test tube and beaker from ffe fume hood until the brown gas has dissipated word image 2616

[Looking ahead — Prepare a boiling water bath for use in Part C by adding 100 mL of tap water and a few boiling stones to a 150-mL beaker, using a hot plate to heat the water to boiling.]

Reaction B: Copper(ll) Nitrate Copper(ll) Hydroxide word image 2617

l. Slowly add 10 drops of6 M NaOH to the test tube with gentle agitation. Formation of the blue precipitate word image 2618 should be observed; the test tube may feel warm to the touch.

  1. Allow the test tube to cool briefly, then add an additional 10 drops of 6 M NaOH and gently agitate the solution to assure complete mixing. Centrifuge the test tube for 60 seconds at 1000 rpm.
  2. Remove fre test tube from the centrifuge and observe; a blue precipitate should be at the bottom of the test word image 2619 tube, and a colorless or very light blue liquid (the “supematant”) should be visible above the precipitate word image 2620 Consult your insü-uctor if no precipitate is visible, or if the supematant still has a significant blue tint.
  3. Test for complete precipitation of copper(ll) hydroxide by adding 3 more drops of 6 M NaOHto ffe supernatant without agitation. If a “ribbon” of additional precipitate forms, add 5 more drops ofNaOH, agitate gently to mix. Centrifuge the test tube for an additional 60 seconds at 1000 rpm.
  4. Remove the test tube from the centrifuge and confirm that the blue precipitate is at the bottom of the test word image 2621 tube. Carefully decant (pour off) the colorless supernatant from ffe test tube into a small liquid waste beaker; altematively, use a disposable transfer pipet to withdraw the supematant from the test tube.

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Reaction C: Copper(ll) Hydroxide Copper(ll) Oxide

l. Place the test tube in the boiling water bath; observe that the precipitate changes color from blue to black, which is evidence of formation of copper(ll) oxide. delete

2. Continue to heat the test tube until all the precipitate, including any precipitate on the walls ofthe test tube, tums completely black. (Note that the contents of the test tube do not have to be heated to dryness; the contenß will appear to be fivet”.)

word image 2622

Reaction

D:

Copper(ll)

Oxide

Copper(ll)

Sulfate

  1. Carefully add no more than 20 drops of 6 M sulfuric acid to the solid CuO, agitate gently, and observe the change in the test tube. Do not add excess sulfuric acid. The disappearance of the black precipitate and simultaneous appearance of a sky-blue solution is evidence ofthe formation of soluble copper(ll) sulfate word image 2623 Continue to agitate the tube gently and allow all black solid to react. Consult your instructor if any black word image 2624 solid remains after 2-3 minutes.
  2. Use a I-mL disposable transfer pipetto add I mL of deionized water to the test tube containing the ft copper(ll) sulfate solution, and agitate gently to mix.

Reaction E: Copper(ll) Sulfate Copper Metal (and Dissolution of Excess Magnesium Metal)

l. Obtain two (2) I-cm magnesium strips for each test tube. Secure each test tube in the 100-mL beaker; add word image 2625 the Mg sü-ips one at a time to the copper(ll) sulfate solution. A rapid reaction should be observed, with vigorous generation of gas bubbles, generation of heat, loss ofblue color from the solution, and formation of red-brown copper metal on the surface of the Mg strips.

  1. If necessary, use a clean glass stimng rod to guide the Mg strips into the solution, break up the Cu metal forming on die surface ofthe Mg strips, and expose fresh Mg to the solution. Do not agitate vigorously.
  2. Allow the reaction to continue for approx. 15 minutes. The solution should fade to light blue, evidence that most of the soluble copper(ll) sulfate has been removed from solution by reduction to insoluble Cu metal word image 2626 word image 2627 Consult instructor if the solution still has a significant blue color or if no copper metal has formed, especially if all the Mg has disappeared — more magnesium may be needed.

word image 2628

  1. Only if necessary, add I — 2 drops of 6 M sulfuric acid to consume any unreacted Mg. Continue this process until few bubbles are observed and no unreacted Mg is apparent in the test tube. Do not add excess word image 2629 sulfuric acid.
  2. Centrifuge the test tube containing the recovered Cu metal for 60 seconds at 1000 rpm and decant the supernatant into the liquid waste beaker. Wash the red-brown Cu metal with two l- mL portions of deionized water, followed by two I-mL portions of acetone. Decant the supematant after each washing, and immediately dispose of acetone waste in the “Organic Waste” bottle. Be careful to keep the recovered red-brown Cu metal in the test tube!

word image 2630 If quantitative analysis of “Percent Recovery of Copper” is required, continue with steps 6 — 7:

  1. Remove as much liquid as possible, then secure the test tubes in a dry 100-mL beaker and place in a 150 o c word image 2631 oven for 15 minutes to allow the Cu metal to dry word image 2632
  2. word image 2633 Remove test tubes from the oven and confirm visually that the recovered Cu metal is dry. Allow the test tubes to cool to room temperature, and confirm that the exterior of each tube is clean and dry — remove any visible white residue with a kimwipe. Record the combined mass of each test tube and recovered copper on the Report Sheet. Determine the mass of recovered copper by subtraction and calculate percent recovery.

Disposal and Cleanup

  • Dispose of aqueous waste in ‘Aqueous Hazardous Waste’ container, acetone in Organic Waste and recovered solid waste in Solid Waste container.
  • Wash all glassware and rinse with deionized water. Return all glassware and equipment to original storage locations. Dispose of used test tubes in “Sharps” container.

Fall 18

The Copper Cycle

Prelaboratory Assignment

Date: Name:

word image 2634 Partner(s)Name:

l. Two of the reactions in the “Copper Cycle” are classified as oxidation-reduction (“‘redox”) reactions.

Identify the two reactions, and identify the oxidizing agent and the reducing agent in each word image 2635

  1. Which one of the redox reactions above can be classified as a single displacement reaction? word image 2636
  2. One of the reactions in fis experiment is classified as a decomposition reaction. Identi$r it: word image 2637
  3. One of the reactions in fis experiment is classified as a precipitation reaction. Identifr it: word image 2638
  4. One of the reactions in fis experiment is classified as an acid-base reaction. Identifr it: word image 2639
  5. How many reaction trials are to be completed? Are the trials completed sequentially or concurrently?
  6. Which reaction requires a boiling water bath? Which reaction(s) use the centrifuge? word image 2640

word image 2641

  1. In Reaction A, students are instructed to add no more than 0.25 mL of 15 M nitric acid. What volume of 15 M nitric acid is required to react with 0.030 g of copper metal? If 1.0 mL of acid contains approximately 20 drops, how many drops of nitric acid are needed?
  2. A student uses a 0.0245 g sample of copper wire to perform the Copper Cycle experiment. If 0.0132 g of dry copper is recovered upon completion ofthe reaction cycle, calculate the percent recovery of Cu.

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The Copper Cycle

Report Sheet Name:

word image 2642

Date.

Partner(s) Name:

Complete the following table if quantitative analysis of “Percent Recovery of Copper” is required as part of report:

 
 

Trial 1

Trial 2

Mass of Empty Test Tube (g)

  

Mass of Test Tube + Copper Wire(g)

  

Mass of Copper Wire (g)

  

Mass of Test Tube and Recovered Cu (g)

  

Mass of Recovered Copper (g)

  

Percent Recovery of Copper

  

Average Percent Recovery

  

Reaction A: Copper Metal to Copper(ll) Nitrate

Observations:

Balanced Molecular Equation:

Balanced Net Ionic Equation:

Reaction B:Copper(II) Nitrate to Copper(ll) Hydroxide

Observations:

Balanced Molecular Equation:

Balanced Net Ionic Equation:

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Reaction C: Copper(ll) Hydroxide to Copper(ll) Oxide

Observations:

Balanced Molecular Equation:

Balanced Net Ionic Equation:

Reaction D: Copper(ll) Oxide to Copper(ll) Sulfate

Observations:

Balanced Molecular Equation:

Balanced Net Ionic Equation:

Reaction E: Copper(ll) Sulfate to Copper Metal (and Dissolution of excess Mg)

Observations:

Balanced Molecular Equation 1:

Balanced Net Ionic Equation 1:

Balanced Molecular Equation 2:

Balanced Net Ionic Equation 2:

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word image 2643The Copper Cycle

Post-lab Questions

Date: Name:

word image 2644 Partner(s) Name:

I. A variety of procedural errors can impact the effectiveness of the copper reaction cycle. Comment on the following documented errors:

  1. When NaOH (aq) is added to ffe solution in Reaction B, the Cu(OH)2 (s) occasionally does not

precipitate immediately, and excess NaOH must be added. What other substance present in the reaction mixture might preferentially react with the NaOH and prevent precipitation of the Cu(OH)2? Explain.

  1. Students sometimes use HN03 (aq) instead ofH2S04 (aq) in Reaction E, Step 4, assuming that both strong acids will accomplish the same purpose. Briefly describe the consequences of this error.
  2. Students occasionally observe the formation of a solid white side product, sometimes appearing as a white residue on the walls offre test tube, in the recovery and isolation ofthe copper metal in Reaction E. Speculate on the identity of this white solid. (Use of online resources may be helpful.)
  3. Errors in experimental technique can result in the percent recovery of copper being too low or too high. Give at least one specific example of each type of error in technique, and explain what could be done to ensure that those errors do not occur in the recovery.
  4. Commentonffe percent recovery of copper achieved by your group. Indicate whether it was less than 100% or greater than 100%. Do any ofthe explanations provided in Question 3 apply to the results you obtained?

The Fall 18

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