Take several strands of copper filament and roll it into a small ball (half a pea size). Place it in a small test tube under the fume hood. Add 5 to 6 drops of concentrated nitric acid (HNO3). The brown gas produced in this conversion is NO2 . It is poisonous, so be careful not to inhale it. Using the bottle of distilled water under the hood, add enough distilled water to your test tube to fill it to approximately 1/3 full.
Observations from Conversion I: (For each conversion write a brief paragraph summarizing what you observed happening. Use complete sentences. Pay attention to grammar and spelling.)
The chemical reaction for conversion I is:
3 Cu + 8 HNO3 --> 3 Cu(NO3)2 + 2 NO + 4 H2O
(NO is colorless. It goes on to react in air with oxygen to form brown NO2.)
Rewrite this equation giving the names of the compounds.
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The blue color of the solution is characteristic of the Cu(II) ion in a water solution.
Procedure (forming the hydroxide):
Use a glass stirring rod to place a drop of the solution from your 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.
What is the pH of the copper (II) nitrate solution? _____
Place a drop of sodium hydroxide solution on the other end of your pH test paper.
What is the pH of the NaOH solution?_____
Half fill a 250-mL beaker with ice water. Place your test tube into the ice water. Slowly and cautiously add NaOH, with constant stirring, to your solution. Continue adding NaOH until the solution is basic (has a pH greater than 7 on your pH paper).
The sodium hydroxide, a base, neutralizes the leftover nitric acid and thereby destroys its acidic properties. Neutralization reactions produce a large amount of heat. The pale blue precipitate is copper(II) hydroxide.
Observations from Conversion II:
The chemical reaction for conversion II is:
Cu(NO3)2 + 2 NaOH --> Cu(OH)2 + 2 NaNO3
Rewrite this equation using names instead of symbols.
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Procedure (forming the oxide):
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. This is copper(II) oxide. Remove the test tube from the heat, label, stopper it loosely, and place it in the rack until tomorrow.
Observations from conversion III:
The chemical reaction for conversion III is:
Cu(OH)2 --> CuO + H2O
Rewrite this equation using names instead of symbols.
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Procedure (dissolving the oxide):
Carefully pour the liquid out of the test tube (or use a Beral pipet to remove the liquid) being careful not to lose any of the precipitate. 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.
Observations from conversion IV:
The chemical reaction for conversion IV is:
CuO + H2SO4 --> CuSO4 + H2O
Rewrite this reaction using names instead of symbols.
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Procedure (regeneration of the copper):
Place your test tube in a cold tap water bath in a 250-mL beaker. Add one small spatula full of granulated zinc to the solution. Stir it gently with a glass stirring rod until the blue color disappears and all the excess zinc is used up by the acid. Since this second reaction produces hydrogen gas, the absence of bubbles as well as the disappearance of the blue color shows that the reaction is finished. This reaction also will produce a lot of heat so it can convert the free copper back to copper(II) oxide if it gets too warm.
This last conversion is a two step process. First, the zinc reacts with the copper(II) sulfate to give back our original copper (Back at last!).
CuSO4 + Zn --> Cu + ZnSO4
Then the excess zinc reacts with the excess sulfuric acid in the solution.
Zn + H2SO4 --> ZnSO4 + H2
Remove the test tube from the water bath and observe the metallic copper in the bottom of the test tube. Compare it to the copper you started with. How is it similar and/or different?
Label your test tube, stopper it, and return it to the rack so that your product may be inspected.
| Compound | Formula | Color |
| nitric acid | ||
| sodium hydroxide | ||
| sulfuric acid | ||
| copper(II) oxide | ||
| copper(II) sulfate | ||
| copper(II) hydroxide | ||
| copper(II) nitrate | ||
| zinc sulfate |
You will be working with a variety of hazardous chemicals in this experiment. Concentrated nitric acid, dilute sodium hydroxide, and dilute sulfuric acid cause blindness. Wear your safety glasses at all times. If you spill any chemicals on your hands, wash them thoroughly. 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.
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 very dangerous if used improperly.
| Compound | Formula | Color |
| nitric acid | HNO3 | colorless |
| (old soln. yellow) | ||
| sodium hydroxide | NaOH | colorless |
| sulfuric acid | H2SO4 | colorless |
| copper(II) oxide | CuO | black |
| copper(II) sulfatehydrate | CuSO4•5H2O | blue |
| copper(II) hydroxide | Cu(OH)2 | pale blue |
| copper(II) nitrate hydrate | Cu(NO3)2•4H2O | blue |
| zinc sulfate | ZnSO4 | colorless |