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Lab Experiment A Cycle of Copper Reactions, Lab Reports of Chemistry

Lane CollegeChemistry

Pre lab questions, safety precautions and procedure of this lab experiment are explained.

Typology: Lab Reports

2020/2021

Uploaded on 05/11/2021

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Experiment 11 – A Cycle of Copper Reactions
In this experiment, you will perform and observe several reactions of copper. This
is a cycle of reactions, because you start and end with the same substance, copper metal.
In the first reaction, copper metal is oxidized by nitric acid to form copper (II) nitrate,
Cu(NO3)2. It is then converted to copper (II) hydroxide, Cu(OH)2, by reaction with base.
When this compound is heated, it is transformed to copper (II) oxide, CuO. Copper (II)
oxide is then reacted with acid to form copper (II) sulfate, CuSO4. Finally, the copper
ions in the copper sulfate are reduced to copper metal by magnesium. In theory, you
should be able to recover all of the copper that you started with. (The number of atoms of
copper is the same throughout all of the reactions.) However, small amounts of copper
can be lost during transfers of copper compounds from one container to another.
Similarly, small amounts of copper usually get washed away during purification steps.
Your goal will be to recover as much copper as possible at the end of the experiment by
being very careful in all of the transfers and purification steps that you do.
The unbalanced molecular equations for each of the steps are given below:
1. Cu (s) + HNO3 (aq) → Cu(NO3)2 (aq) + NO2 (g) + H2O (l)
2. Cu(NO3)2 (aq) + NaOH (aq) → Cu(OH)2 (s) + NaNO3 (aq)
3. Cu(OH)2 (s) → CuO (s) + H2O (l)
4. CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
5. CuSO4 (aq) + Mg (s) → Cu (s) + MgSO4 (aq)
Safety Precautions:
• Wear your safety goggles.
• Concentrated HNO3 is very corrosive. If it comes into contact with your skin or
clothing, wash it off immediately with plenty of water.
• The first reaction must be done under the fume hood. It produces a brown gas, NO2,
which is toxic.
• If any NaOH or H2SO4 comes into contact with your skin or clothing, wash it off
immediately with plenty of water.
• When using the centrifuge, make sure to place another tube with the same volume of
liquid across from your tube. Forgetting to balance the centrifuge can ruin the
centrifuge and your experiment.
Waste Disposal:
• All solutions used in this experiment should be disposed of in the inorganic waste
containers (which have a blue label) in one of the fume hoods.
• Copper metal may be disposed of in the “Waste Copper” container in the hood.
Prelab Questions:
pf3
pf4

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Experiment 11 – A Cycle of Copper Reactions

In this experiment, you will perform and observe several reactions of copper. This is a cycle of reactions, because you start and end with the same substance, copper metal. In the first reaction, copper metal is oxidized by nitric acid to form copper (II) nitrate, Cu(NO 3 ) 2. It is then converted to copper (II) hydroxide, Cu(OH) 2 , by reaction with base. When this compound is heated, it is transformed to copper (II) oxide, CuO. Copper (II) oxide is then reacted with acid to form copper (II) sulfate, CuSO 4. Finally, the copper ions in the copper sulfate are reduced to copper metal by magnesium. In theory, you should be able to recover all of the copper that you started with. (The number of atoms of copper is the same throughout all of the reactions.) However, small amounts of copper can be lost during transfers of copper compounds from one container to another. Similarly, small amounts of copper usually get washed away during purification steps. Your goal will be to recover as much copper as possible at the end of the experiment by being very careful in all of the transfers and purification steps that you do. The unbalanced molecular equations for each of the steps are given below:

1. Cu (s) + HNO3 (aq) → Cu(NO 3 )2 (aq) + NO2 (g) + H 2 O (l)

2. Cu(NO 3 )2 (aq) + NaOH (aq) → Cu(OH)2 (s) + NaNO3 (aq)

3. Cu(OH)2 (s) → CuO (s) + H 2 O (l)

4. CuO (s) + H 2 SO4 (aq) → CuSO4 (aq) + H 2 O (l)

5. CuSO4 (aq) + Mg (s) → Cu (s) + MgSO4 (aq)

Safety Precautions:

  • Wear your safety goggles.
  • Concentrated HNO 3 is very corrosive. If it comes into contact with your skin or clothing, wash it off immediately with plenty of water.
  • The first reaction must be done under the fume hood. It produces a brown gas, NO 2 , which is toxic.
  • If any NaOH or H 2 SO 4 comes into contact with your skin or clothing, wash it off immediately with plenty of water.
  • When using the centrifuge, make sure to place another tube with the same volume of liquid across from your tube. Forgetting to balance the centrifuge can ruin the centrifuge and your experiment. Waste Disposal:
  • All solutions used in this experiment should be disposed of in the inorganic waste containers (which have a blue label) in one of the fume hoods.
  • Copper metal may be disposed of in the “Waste Copper” container in the hood.

Prelab Questions:

  1. Balance each of the equations (if necessary). For each, classify the type of reaction (i.e. gas-forming, decomposition, precipitation, redox, acid-base, etc.). For the redox reactions, state which species is oxidized and which is reduced.
  2. Why are you unlikely to get 100% of the metallic copper back?
  3. If you were to get more than 100% of the copper back, explain what could have happened (hint: according to the Law of Conservation of Mass, does this make sense?).
  4. List the safety rules important for each step.

Procedure

Reaction 1

  1. Weigh an empty pointed centrifuge tube on a digital balance, place approximately 0.1 g of copper metal in it, and weigh it again. Record your data.
  2. In the hood , add 2 mL of 6 M nitric acid. Warm the contents in a hot water bath, if necessary. Heat until the copper has dissolved completely and no more brown fumes (toxic nitrogen dioxide, NO 2 ) are evolving.
  3. Allow the resulting solution to cool to room temperature. (You can put the tube into a beaker of ice water to help it cool off.) Add 2 mL of deionized water to the solution.
  4. Record your observations of the reaction, and write the balanced net ionic equation. Reaction 2
  5. Add 6 M NaOH drop-by-drop until the solution is basic. Stir well with a stirring rod after the addition of each drop. To tell whether or not it is basic, touch the stirring rod to a piece of red litmus paper. If it turns blue, the solution is basic. (Be careful - don't confuse the blue precipitate with the litmus paper turning blue. You will have to look around the edges of the precipitate at the litmus paper itself.)
  6. Record your observations of the reaction, and write the balanced net ionic equation. Reaction 3
  7. Gently heat the tube in a beaker of boiling water until a chemical transformation occurs. (The evidence will be a color change.) Continue heating until the reaction is complete. Allow the solution to cool to room temperature on its own (do not use an ice-water bath to cool it).
  8. When the solution is cool, centrifuge the tube to collect the solid. (Don't forget to balance the centrifug e with a similar tube opposite yours that contains approximately the same volume of liquid.) Decant (pour off) the solution from the tube into a small beaker, leaving the solid behind. (This solution will go into the inorganic waste when you are done.) It is better to leave a little liquid mixed with the solid than to accidentally pour off some solid into the waste. Remember that you want to recover as much copper as possible at the end of the experiment.
  9. Wash the solid twice: each time, add 2 mL of deionized water, swirl the centrifuge tube vigorously, centrifuge the tube, and decant the wash water into your waste beaker, leaving the solid behind.
  10. Record your observations of the reaction, and write the balanced net ionic equation.

In your results and discussion section, you should thoroughly discuss reasons for loss of copper at each step of the reaction cycle. Any major errors you make should always be discussed in this section.

Questions:

(Remember - always explain your reasoning.)

  1. What is the purpose of adding the strips of magnesium?
  2. Why don't we need to weigh the magnesium?
  3. For each of the five steps in this procedure, discuss likely reasons for losing or not recovering copper.