Series of Copper Reactions (W.G.)
In this experiment, you will perform a series of reactions. Metallic copper, Cu atoms, will be
dissolved in nitric acid to give a blue solution that contains copper(II) nitrate. The soluble
copper(II) nitrate will then be converted to (a) a series of insoluble copper(II) compounds,
(b) a soluble complex copper(II) compound, and (c) finally back to metallic copper. Since
mass is neither created nor destroyed in ordinary chemical reactions, the weight of metallic
copper recovered at the end of the experiment should be the same as the amount used at
the beginning of the experiment. At the end of the experiment you will be asked to
calculate the mass of the copper you were issued. You will also write balanced equations for
the reactions observed. (Note that water is formed as a product in several of the reactions.
You will find that the production of water will help balance many of the equations!)
Purpose:
In this investigation, you will take copper through various types of chemical
reactions and recover the copper metal product. You will write balanced
equations for each reaction.
Materials:
Copper wire, 6.0M nitric acid (Caution: corrosive), 4.0M sodium hydroxide
solution (Caution: corrosive), 6.0M hydrochloric acid (Caution: corrosive),
6.0M aqueous ammonia, 6.0M sulfuric acid solution (Caution: corrosive), 30mesh zinc, methanol, acetone, pH paper
Procedures:
1.
Obtain an unknown envelope that contains a piece of copper wire. Record your
unknown number.
Preparation of blue copper(II) nitrate from copper and nitric acid (Caution: deadly
nitrogen dioxide gas is also produced).
Place the copper in a clean 250 mL beaker . Working under the fume hood, add 8.0
mL of 7.9M nitric acid. (The use of the fume hood is extremely important. The
brown nitrogen dioxide gas produced is deadly!) Do not remove from the fume hood
until all of the brown gas is dissipated and the solution has become very blue (not
green). If some solid copper remains at this time, add small amounts of nitric acid
and stir until all the copper is dissolved. Record your observations and write a
balanced equation for the reaction.
2.
Preparation of light blue copper(II) hydroxide from copper(II) nitrate and sodium
hydroxide.
After all the copper is dissolved and the reaction has ceased, add 20.0 mL of 4.0M
sodium hydroxide solution. Allow the light blue precipitate to settle slightly and
observe the supernatant (top) liquid. The solution should be colorless and basic
(turns red litmus blue). If necessary, add a few more drops of sodium hydroxide
with stirring until the solution tests basic and precipitation is complete. Show your
light blue precipitate to your instructor, record your observations, and write a
balanced equation for the reaction. (If your solution was hot, some black copper(II)
oxide may have also formed.)
3.
Preparation of copper(II) oxide from heating copper(II) hydroxide.
Add enough water to the beaker to bring the total volume to 100 mL. Cover with a
watch glass and gently heat the solution while stirring, just to the boiling point. Heat
gently for 5 minutes. The light blue copper(II) hydroxide should decompose
completely to give black copper(II) oxide and water. Show the black precipitate to
your instructor and write a balanced equation for the reaction. Allow the solid
copper(II) oxide to settle slightly and observe the solution at the top (it should be
colorless). If color remains in the solution, heat the solution and let the precipitate
settle again. [Consult your instructor before proceeding. If time is short, place the
labeled beaker in your lab drawer to settle until the next lab period.]
Carefully decant the supernatant liquid into a beaker using a stirring rod. Do not
decant directly into the sink, to avoid the loss of any solid product. Add about 150
mL of very hot distilled water, stir, and allow to settle again for several minutes.
Decant the supernatant once again. This decanting and stirring removes the sodium
nitrate from solution.
4.
Preparation of copper(II) chloride from copper(II) oxide and hydrochloric acid
To the beaker containing the black precipitate, add 10 mL of 6.0M hydrochloric acid
and stir. If any black precipitate remains, fresh 6.0M hydrochloric acid should be
added in 1.0 mL portions and stirred. Add 100 mL of distilled water, show the green
copper(II) chloride solution to your instructor, and write a balanced equation for the
reaction.
5.
Preparation of a deep blue complex compound [Cu(NH3)4](OH)2, from copper(II)
chloride
(Note: Copper(II) chloride reacts with a limited amount of aqueous ammonia
to precipitate the light blue copper(II) hydroxide solution. As more aqueous
ammonia is added, the insoluble copper(II) hydroxide dissolves to form a
soluble compound [Cu(NH3)4](OH)2, containing a complex ion, [Cu(NH3)4]2+,
which is deep blue. As you add 6.0M aqueous ammonia observe carefully and
you will see the light blue precipitate dissolve as excess ammonia is added.)
To the green copper(II) chloride solution, add 25 mL of 6.0M aqueous ammonia slowly
with constant stirring until all the copper(II) hydroxide dissolves. If necessary, add
additional ammonia in 5 mL portions until deep color change occurs. Show the deep
blue solution to your instructor and use the following to write the two balanced
equations: Copper(II) chloride reacts with ammonia and water to form copper(II)
hydroxide and ammonium chloride. The copper(II) hydroxide then reacts with
ammonia to form [Cu(NH3)4](OH)2(aq).
6.
Preparation of copper(II) sulfate from [Cu(NH3)4](OH)2
(Note: [Cu(NH3)4](OH)2 reacts with sulfuric acid and water to produce
[Cu(OH2)4]SO4 and ammonium sulfate. In the aqueous solutions of copper(II)
sulfate produced, copper(II) ions exist as [Cu(OH2)4]2+ rather than as simple
Cu2+ ions. Use [Cu(OH2)4]2+ for the Cu produced in the balanced equation!)
Very cautiously, add 20 mL of 6.0M sulfuric acid to the deep blue solution. A great
deal of heat is generated in this reaction, so the 6.0M sulfuric acid should be added
a few drops at a time with vigorous stirring. After all the acid has been added, the
solution should be sky blue in color. Handle the hot solution very carefully. Show the
sky blue solution to your instructor and write a balanced equation for the reaction.
7.
Preparation of metallic copper from copper(II) sulfate solution
(In this reaction, zinc will reduce copper(II) ions to metallic copper as the
metallic zinc is oxidized to zinc ions. In the balanced equation, the metal ions
are associated with sulfate. In a simplified form of this reaction, the
[Cu(OH2)4]2+ ions from the preceding reaction can be represented as Cu2+
ions.)
Slowly add about 2 grams of 30-mesh zinc to the blue copper(II) sulfate solution.
Stir the solution. If the solution is still blue after the reaction stops producing gas,
add very small amounts of zinc and allow it to react until no further gas production is
observed. Zinc sulfate solution is colorless and thus the disappearance of the blue
color from the solution shows that all the copper(II) ions have been displaced from
the reaction. When you believe all the zinc has been reacted (no more bubbles),
decant the supernatant liquid and discard. Write a balanced equation for the
reaction.
In order to recover the solid copper, the remaining zinc must be removed. Add 20
mL of 6.0M hydrochloric acid, stir, and allow it to react with the remaining zinc.
Once bubbles slow, cover with a watch glass and heat gently on a hot plate (do not
boil) until gas evolution stops.
Weigh a porcelain evaporating dish to the nearest 0.0001 gram. Add 50 mL distilled
water to the beaker of copper and stir. Decant the supernatant again and transfer
all the solids to the weighed evaporating dish. You may wash with small amounts of
distilled water to aid in the transfer. Add 10.0 mL water to the solid, stir, and
decant the supernatant liquid. Repeat this washing two more times.
Wash the solid with 5.0 mL methanol (keep away from flames!). Decant the
supernatant and discard in a labeled organic waste container.
Wash the solid with 5.0 mL of acetone (keep away from flames!). Decant the
supernatant and discard in a labeled organic waste container. Evaporate the
remaining acetone by placing the evaporating dish in a drying oven. Stir the solid
occasionally while drying. Spread the copper out in a thin layer in the dish to speed
the process. Cool the evaporating dish and weigh.
Report Form:
Name _____________________
Unknown number: #____________ (Very important!)
After each step, obtain your instructor’s approval (initials).
Observations:
Instructor’s initials
_____________________________________1. Copper(II) nitrate solution
____
_____________________________________2. Copper(II) hydroxide (solid)
____
_____________________________________3. Copper(II) oxide (solid)
____
_____________________________________4. Copper(II) chloride solution
____
_____________________________________5. Soluble [Cu(NH3)4](OH)2
____
_____________________________________6. Copper(II) sulfate solution
____
_____________________________________7. Metallic copper
____
Balanced Equations for Reactions:
1.
2.
3.
4.
5a.
5b.
6.
7.
Calculation of mass of copper used:
Mass of evaporating dish
____________
Mass of copper and evaporating dish
____________
Mass of copper recovered
____________