Expt 016 -- Copper Reactions

Copper metal is dissolved in nitric acid. Copper (II) hydroxide is formed by reaction with sodium hydroxide. Copper (II) oxide is formed by heating. Copper (II) oxide is dissolved in acid. Copper (II) ions are reduced with zinc metal to produce copper metal.

Safety

• Do not attempt this experiment without a hood for the nitric acid step.
• Nitric acid, sodium hydroxide, and sulfuric acid are corrosive. Several of the chemicals are toxic. Nitric acid stains skin. Wear goggles and apron. Wash spilled chemicals immediately wit large amounts of water. Do not ingest the chemicals. Wash hands frequently. The gas produced when copper reacts with nitric acid is toxic. Perform this reaction under a hood or with ventilation such that none of the gas (at all) is inhaled.

Procedure

Prepare an ice bath in a 250 mL beaker and a hot water bath in a second 250 mL beaker.

Dissolving copper:

1. Measure a 12 inch piece of light copper wire or light strands.

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2. Roll the copper wire into a small ball (half a pea size). Place it in a small test tube.

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3. Do not attempt the next step without a good working hood.
4. Under the fume hood, add 10 drops of 6 M HNO₃. Do not inhale the brown gas produced in this conversion; NO₂ is toxic and noxious. If reaction ceases with unreacted copper strands present, add 2 more drops of 6 M HNO₃. Repeat this step until all of the copper has been reacted.

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5. Use a plastic squeeze bottle of distilled water stored under the hood to add enough water to the test tube to fill it to approximately 1/3rd full.

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6. Wash hands.
7. Write a brief paragraph summarizing observations. Use complete sentences; pay attention to grammar and spelling.

Forming the hydroxide:

1. Collect pH paper, pH standards (usually on the container), stirring rod, NaOH, and Cu solution to measure the pH.

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2. Use a glass stirring rod to place a drop of the solution from the test tube on a piece of pH test paper. If the color of the paper indicates a pH of less than 7, the solution is acidic. A pH of 7 is neutral and a pH above 7 indicates a basic solution. Find and record the pH of the copper (II) nitrate solution.

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3. Place a drop of sodium hydroxide solution on the other end of the pH test paper. Find and record the pH of the NaOH solution.

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4. Half fill a 250-mL beaker with ice water. Place the test tube into the ice water.

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5. Slowly and cautiously add NaOH, with constant stirring, to the solution. Continue adding NaOH until the solution is basic (has a pH greater than 7 on the pH paper).

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6. Wash hands. Write a brief paragraph summarizing observations. Use complete sentences; pay attention to grammar and spelling.

Forming the oxide:

1. Place the test tube in a hot water bath on a hot plate and heat it, with occasional stirring, until all the solid material is converted to a black-brown substance.

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2. Remove the test tube from the heat, label, stopper it loosely, and store it to allow the CuO to settle. Or centrifuge if you have one available and enough time to go on.

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3. Write a brief paragraph summarizing observations. Use complete sentences; pay attention to grammar and spelling.

Dissolving the oxide:

1. Carefully, use a plastic transfer pipet to remove the liquid, or pour the liquid out of the test tube into disposal jar 1 provided by the instructor. Be careful not to lose any of the precipitate. A small amount of liquid may be left behind.

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2. Place the tube in an ice water bath. Add 3 mL of 3M H₂SO₄ to the precipitate and stir gently until the oxide dissolves with the formation of a blue copper(II) sulfate solution.

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3. Wash hands. Write a brief paragraph summarizing observations. Use complete sentences; pay attention to grammar and spelling.

Producing copper metal:

1. Place the test tube in a cold tap water bath in a 250-mL beaker.

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2. Add one small spatula full of granulated zinc to the solution.

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3. Stir occasionally with a glass stirring rod until the blue color disappears and all the excess zinc is used up by the acid.

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4. Zinc produces hydrogen gas in acid; the absence of bubbles as well as the disappearance of the blue color indicates that reaction is complete. Observe the final product.

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5. Carefully pour the liquid out of the test tube into disposal jar 2 provided by the instructor (or use a plastic transfer pipet to remove the liquid) being careful not to lose any of the solid.
6. Write a brief paragraph summarizing all observations. Use complete sentences; pay attention to grammar and spelling. Be sure to compare the properties of the metallic copper used at the outset with the copper metal produced in the last step.

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7. Label the test tube with your name, stopper it, and place it in the storage device (test tube rack) provided by the instructor so that the product may be inspected.
8. Wash hands.

Questions

1. Suppose the initial weight of the copper sample were 0.54 grams, and the recovered weight of copper were 0.50 grams. Find the yield of copper in this series of reactions.
2. The disappearance of the blue color in the reduction of the copper ion with zinc metal signals that the reaction is complete. Explain.
3. Fill in the following chart with the appropriate formulas and colors for the compounds used in this experiment:

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 016 Copper Reactions

Use the movies and pictures to answer the questions.

Curriculum-

• This experiment focuses on chemical transformations. It centers on the notion that atoms may be found in elements or compounds, but that chemical processes do not cause those atoms to disappear or change in a fundamental way. This experiment, therefore, is usually performed early in an introductory course.
• This experiment takes several days to complete. Usually students can get from copper metal to copper oxide in one 45-minute period. They can dissolve the copper oxide and treat it with zinc during part of a second period. Finally, isolation of the copper metal requires a third period.
• Teachers often use the chemicals without indication of their concentrations. Postponing the notion of concentration may help to focus learners on the key issue of conservation of atoms that is taught early in the course.

Safety-

• Do not attempt this experiment without a hood for the nitric acid step.
• Nitric acid, sodium hydroxide, and sulfuric acid are corrosive. Several of the chemicals are toxic. Nitric acid stains skin. Wear goggles and apron. Wash spilled chemicals immediately wit large amounts of water. Do not ingest the chemicals. Wash hands frequently. The gas produced when copper reacts with nitric acid is toxic. Perform this reaction under a hood or with ventilation such that none of the gas (at all) is inhaled.
• The copper dissolving reaction is accomplished faster with concentrated nitric acid, but 6 M HNO₃ is safer and gets the job done. Skin is immediately transformed to a tough yellow mixture of compounds with any splash of concentrated HNO₃. Use care when preparing solutions.

Time-

Teacher Preparation: 15 minutes

Class Time: 80 minutes Store CuO between periods to settle out. Zn reaction takes a while in second class period. Students may leave Zn in tubes between periods if the class period is long enough and centrifuges are available.

Materials-

• ice
• distilled water
• 0.2 g copper turnings
• 1 mL of 6 M HNO₃ -- (Wear apron, goggles, and rubber gloves. Work under a hood. Add 38 mL of concentrated HNO₃ to enough water to make 100 mL of solution.)
• 2 mL of 3 M NaOH -- (Use stock solution as provided.)
• 3 mL of 3 M H₂SO₄ -- (Use stock solution as provided.)
• zinc metal (20 mesh, granulated)
• pH paper (1-14)
• fume hood
• test tube, 13 x 100-mm
• plastic transfer pipet
• hot plate or other heat source
• glass stirring rod
• (disposal jars, vinegar, NaHCO₃, test tube rack)
• stopper
• 2 250-mL beakers

Disposal-

Disposal jar 1 contains base and disposal jar 2 contains acid. Dilute each jar with an equal volume of water. Wear goggles and apron. Slowly and carefully while stirring add the contents of the jar with the smaller total volume to the other jar. Test the resulting solution with pH paper. Neutralize the solution, as appropriate, with vinegar or sodium bicarbonate, and discard the resulting solution at the sink.

Lab Hints-

• This reaction is often done at the beginning of the term before students have any background with the concept of concentration. For this reason, the solutions are often provided to students without an indication of their concentration.
• Be certain to review safety precautions with students. These reagents are dangerous and must be handled properly.

Background-

• The chemical reaction for dissolving copper is:

  3 Cu + 8 HNO₃ --> 3 Cu(NO₃)₂ + 2 NO + 4 H₂O (conc)

  or Cu + 4 HNO₃ --> Cu(NO₃)₂ + 2 NO₂ + 2 H₂O (dilute)

  (NO is colorless. It goes on to react in air with oxygen to form brown NO₂; 2 NO + O₂ --> 2 NO₂.)

• The copper metal dissolves forming a greenish solution and giving off a gas which, in air, is brown. Heat is evolved. When water is added, the reaction mixture turns blue.
• The blue color of the solution is characteristic of the Cu(II) ion in a water solution.
• The sodium hydroxide, a base, neutralizes the unreacted nitric acid and destroys its acidic properties. Neutralization reactions produce a large amount of heat.

  HNO₃ + NaOH --> NaNO₃ + H₂O

  H⁺ + OH^- --> H₂O

• More NaOH leads to a reaction. The pale blue, gelatinous precipitate is copper(II) hydroxide.

  Cu(NO₃)₂ + 2 NaOH --> Cu(OH)₂ + 2 NaNO₃

• Heating copper(II) hydroxide cause water to be lost.

  Cu(OH)₂ --> CuO + H₂O

• CuO is black. Copper(II) oxide dissolves in acid to form an aqueous copper(II) solution:

  CuO + H₂SO₄ --> CuSO₄ + H₂O

• Zinc reacts with the copper(II) sulfate to produce copper metal. Looking quite a bit worse for the wear, the original copper atoms are returned to the elemental metallic state.

  CuSO₄ + Zn --> Cu + ZnSO₄

• Excess zinc reacts with sulfuric acid in the solution producing bubbles of hydrogen gas.

  H₂SO₄ + Zn --> ZnSO₄ + H₂

• The overall result is the reddish solid copper metal beneath a clear, colorless aqueous solution.

Answers-

Q1. Suppose the initial weight of the copper sample were 0.54 grams, and the recovered weight of copper were 0.50 grams. Find the yield of copper in this series of reactions.

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A1. Yield = ( wt. recovered/ wt. reacted ) x 100% =

= (0.50/ 0.54) x 100

= 93%

Q2. The disappearance of the blue color in the reduction of the copper ion with zinc metal signals that the reaction is complete. Explain.

A2. Copper(II) ion is blue. The disappearance of the blue color indicates that all of the copper(II) has been reacted.

Q3. Fill in the following chart with the appropriate formulas and colors for the compounds used in this experiment:

A3.

Compound	Formula	Color
nitric acid	HNO3	colorless
sodium hydroxide	NaOH	colorless
sulfuric acid	H2SO4	colorless
copper(II) oxide	CuO	black
copper(II) sulfate	CuSO4	blue
copper(II) hydroxide	Cu(OH)2	pale blue
copper(II) nitrate	Cu(NO3)2	blue
zinc sulfate	ZnSO4	colorless
nitrogen dioxide	NO2	brown gas

Key Words 1-

reaction, observation, oxidation, reduction, redox

Elements-

Cu H N Zn S Na O