Experiment 5
Chemistry of Copper
INTRODUCTION
In previous laboratory experiments we were able to take advantage of the physical and
chemical properties of a substance for identification or for separation. In this experiment
we take that understanding one step further.
Copper is an element that is chemically combined into a variety of compounds in nature,
but most commonly in the form of a sulfide, as in chalcocite, Cu2S, and chalcopyrite,
FeCuS2. Copper metal is an excellent conductor of heat and electricity and is an
alloying element in bronze and brass. Copper is a soft metal with a characteristic
bright orange-brown color, which we often call "copper color". Copper is relatively inert
chemically; it does not readily air oxidize (react with oxygen in air) and is not attacked
by simple inorganic acids such as sulfuric and hydrochloric acids. Copper that oxidizes
in air is called patina.
Copper(ll) ion forms a number of very colorful compounds; most often, these
compounds are blue or blue-green, although other colors are found depending on the
copper(II) compound.
We will observe several chemical and physical properties of copper through a sequence
of reactions that produce a number of colorful compounds. Starting with metallic copper
at the top, the sequence of products formed is shown on the diagram:
Mg
Cu
HNO3
CuSO4
Cu(NO3)2
H2SO4
NaOH
CuO
∆
Cu(OH)2
Dissolution of Copper Metal
Copper reacts readily with substances called oxidizing agents (substances that remove
electrons from other substances). In this experiment aqueous nitric acid, HNO3, is the
substance that oxidizes copper metal to the copper(II) ion.
Cu(s) + 4 HNO3(aq) → Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)
(5.1)
As products of this reaction, copper(II) nitrate, Cu(NO3)2, is a water-soluble salt that
produces a blue solution, and nitrogen dioxide, NO2, is a dense, red-brown gas. The
solution remains acidic because an excess of nitric acid is used for the reaction.
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Precipitation of Copper(II) Hydroxide from Solution
For Part B of the Experimental Procedure, the solution containing the soluble Cu(NO 3)2
is treated with sodium hydroxide, NaOH, a base. Copper(II) hydroxide, Cu(OH)2, a light
blue solid, precipitates from the solution.
Cu(NO3)2(aq) + 2 NaOH(aq) → Cu(OH)2(s) + 2 NaNO3(aq)
(5.2)
Sodium nitrate, NaNO3, is a colorless salt that remains dissolved in solution.
Conversion of Copper(II) Hydroxide to a Second Insoluble Salt
Heat applied to solid copper(II) hydroxide causes black, insoluble copper(II) oxide, CuO,
to form.
Cu(OH)2(s) → CuO(s) + H2O(g)
(5.3)
Dissolution of Copper(II) Oxide
Copper(II) oxide dissolves readily with the addition of aqueous sulfuric acid, H 2SO4,
forming a sky blue solution as a result of the formation of the water-soluble salt,
copper(II) sulfate, CuSO4.
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
(5.4)
Re-formation of Copper Metal
Finally, the addition of magnesium metal, Mg, to the copper(II) sulfate solution
completes the copper cycle. In this reaction, magnesium serves as a reducing agent (a
substance that provides electrons to another substance). Magnesium reduces
copper(II) ion from the copper(II) sulfate solution to copper metal and water-soluble
magnesium sulfate, MgSO4.
CuSO4(aq) + Mg(s) → Cu(s) + MgSO4(aq)
(5.5)
Magnesium metal also reacts with sulfuric acid. Therefore when magnesium metal is
added to the copper(II) sulfate solution, a second reaction occurs that produces
hydrogen gas, H2, and additional magnesium sulfate.
Mg(s) + H2SO4(aq) → H2(g) + MgSO4(aq)
(5.6)
Therefore hydrogen gas bubbles are observed during this reaction step of the cycle.
This reaction is also used to remove any excess magnesium metal that remains after
the copper metal has been removed.
Reaction Types That Occur in This Experiment
As you proceed in this course, you will often observe similar types of reactions. An
organization of chemical reactions helps to simplify our overall understanding,
interpretation, and predictability of chemical behavior. Some of the reaction types that
occur in this experiment are listed.
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Equation 5.1.
Equation 5.2.
Equation 5.3.
Equation 5.4.
reaction
Equation 5.5.
Equation 5.6.
A single displacement reaction and an oxidation-reduction reaction
A double displacement (or metathesis) reaction
A decomposition reaction
A double displacement (or metathesis) reaction and an acid-base
A single displacement reaction and an oxidation-reduction reaction
A single displacement reaction and an oxidation-reduction reaction
EXPERIMENTAL PROCEDURE
Procedure Overview: Copper metal is successively treated with nitric acid, sodium
hydroxide, heat, sulfuric acid, and magnesium in a cycle of chemical reactions to
regenerate the copper metal. The chemical properties of copper are observed in the
cycle.
A. Copper Metal to Copper(II) Nitrate
Perform the series of reactions in a test tube that is compatible with your laboratory
centrifuge. Consult with your laboratory instructor.
1. Preparation of the Copper Metal Sample. Obtain a less than 0.02-g sample of
Cu wool. Determine the mass (±0.001 g) of the selected test tube. Roll and
place the Cu wool into the test tube (must fit into a centrifuge) and measure their
combined mass. Record the mass of the copper sample.
2. Reaction of the Copper Metal. Perform this step in the fume hood. Using a
dropper bottle or a dropping pipet, add drops (≤10 drops) of concentrated HNO3
to the copper sample until no further evidence of a chemical reaction is observed.
(Caution: Concentrated HNO3 is very corrosive. Do not allow it to touch the skin.
If it does, wash immediately with excess water. Nitric acid will turn your skin
yellow, a way to check your laboratory technique!) At this point the Cu metal has
completely reacted. (Caution: Do not inhale the evolved nitrogen dioxide gas.)
What is the color of the gas? Add 10 drops of deionized water. Show the
resulting solution to your laboratory instructor for approval and save the solution
for Part B.
B. Copper(II) Nitrate to Copper(II) Hydroxide
1. Preparation of Copper(II) Hydroxide. Agitate or continuously stir with a stirring
rod the solution from Part A.2 while slowly adding 10 drops of 6 M NaOH.
(Caution: Wash with water immediately if the NaOH comes into contact with the
skin.) This forms the Cu(OH)2 precipitate. After the first 10 drops are added, add
10 more drops of 6 M NaOH. Using a wash bottle and deionized water, rinse the
stirring rod, allowing the rinse water to go into the test tube. Centrifuge the
solution for 30 seconds (refer to Technique 11 F on pages 8 & 9 of your
Laboratory Technique Guide for a review of how to use the centrifuge).
2. A Complete Precipitation. Test for a complete precipitation of Cu(OH)2 by
adding 2-3 more drops of 6 M NaOH to the supernatant (the clear solution in
the test tube). If additional precipitate forms, add 4-5 more drops and again
centrifuge. Repeat the test until no further formation of the Cu(OH)2 occurs. The
solution should appear colorless and the precipitate should be light blue. Obtain
your laboratory instructor's approval and save for Part C.
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C. Copper(II) Hydroxide to Copper(II) Oxide
Heating to Dryness. Decant (pour off) and discard the supernatant from the test tube.
Carefully (very carefully!) and slowly heat the test tube with a cool flame (adjust the
flame to have a low supply of fuel) until the Cu(OH)2 precipitate changes color. Avoid
ejection (and projection) of your copper compound by not holding the test tube over the
direct flame for a prolonged period of time. Obtain your instructor's approval and save
for Part D.
D. Copper(II) Oxide to Copper(II) Sulfate
Dissolution of Copper(II) Oxide. To the solid CuO in the test tube from Part C, add
drops (<20 drops, 1 mL) of 6 M H2SO4 with agitation until the CuO dissolves. (Caution:
Do not let sulfuric acid touch the skin!) (Slight heating may be necessary, but be careful
not to eject the contents!) The solution's sky blue appearance is evidence of the
presence of soluble CuSO4. Obtain your instructor's approval and save for Part E.
E. Copper(lI) Sulfate to Copper Metal
1. Formation of Copper Metal. Using fine steel wool, polish about 5-7 cm of Mg ribbon.
Cut or tear the ribbon into 1-cm lengths. Dilute the solution from Part D with deionized
water until the test tube is half-full. Add the Mg strips, one at a time to oh- serve the
chemical change, to the solution until the solution is colorless. Describe what is
happening. What is the coating on the magnesium ribbon? What is the gas?
If a white, milky precipitate forms [from the formation of magnesium hydroxide,
Mg(OH)], add several drops of 6 M H2SO4. (Caution: Avoid skin contact.) Break up the
red-brown Cu coating on the Mg ribbon with a stirring rod. After breaking up the Cu
metal and after adding several pieces of Mg ribbon, centrifuge the mixture.
2. Washing. Add drops of 6 M H2SO4 to dissolve any excess Mg ribbon. (Caution:
Avoid skin contact.) Do this by breaking up the Cu metal with a stirring rod to expose
the Mg ribbon, coated with Cu metal, to the H2SO4 solution. Centrifuge for 30 seconds,
decant, and discard the supernatant. Wash the red-brown Cu metal with three 1-mL
portions of deionized water. Wash the Cu metal by adding water, stirring the mixture
with a stirring rod, and allowing the mixture to settle. Rinse the stirring rod in the test
tube. Centrifuge, decant, and discard each washing.
3. Determination of the Mass of Recovered Copper. Very gently dry the Cu in the
test tube over a cool flame (If you can feel the heat of the flame with the hand holding
the test tube clamp, the flame is too hot!). Allow the tube and contents to cool and
determine the mass (± 0.001 g). Repeat the heating procedure until a reproducibility in
mass of + or - 1% is obtained. Record the mass of Cu recovered in the experiment.
Disposal. All solutions used in the procedure can be disposed of in the sink, followed by
a flush with tap water. Dispose of the copper metal in the Waste Solids container.
CLEANUP: Rinse all glassware twice with tap water and twice with deionized water.
Discard all rinses in the sink.
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Name _______________________________
Experiment 5 Prelaboratory Assignment
The Copper Cycle
1. What is the purpose of this lab?
2. The element copper is cycled through a number of compounds in this experiment,
starting with elemental copper, what is the final chemical form of copper in this
experiment?
3. Name the compounds of copper formed in this experiment that have the following
formulas:
a. Cu(NO3)2
b. Cu(OH)2
c. CuO
d. CuSO4
4. A number of “Caution” chemicals and solutions are used in this experiment. Refer to
the Experimental Procedure and the corresponding sections to identify the specific
chemical or solution that must be handled with care.
Experimental Procedure
Precautionary Chemical/Solution
Part A.2
Part A.2
Part B.1
Part D.1
Part E.1
5. Refer to Technique 11 F on pages 8 & 9:
a. What function does a centrifuge perform?
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b. Describe the technique for "balancing a centrifuge" when centrifuging a sample.
6. A 0.0194-g sample of copper metal is recycled through the series of reactions in this
experiment. In Part E.3, a 0.0169-g sample of copper is isolated. What is the percent
recovery of the copper metal?
7. What volume, in drops, of 16 M HNO3 (or concentrated HNO3) is required to react
with 0.020 g of Cu metal? See Equation 5.1. Assume 20 drops per milliliter. The
expression "16 M HNO3" indicates that 16 moles HNO3 is present in each liter of the
HNO3 solution.
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Name _______________________________
Experiment 5 The Copper Cycle
DATA & CALCULATIONS TABLE (Show work & formulas if applicable.)
1. Mass of test tube (g)
2. Mass of test tube + Cu wool (g)
3. Mass of Cu sample (g)
Synthesis of
Observation
Instructor
approval
BCE
a. Cu(NO3)2
b. Cu(OH)2
c. CuO
d. CuSO4
e. Cu
4. Mass of test tube + Cu recovered
1st mass (g)
2nd mass (g)
3rd mass (g)
5. Mass of Cu recovered
6. Percent Yield
7. Discuss the variance of your percent yield from the theoretical recovery.
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CONCLUSION QUESTIONS
1. Part A.2. What is the color of NO2 gas?
2. Part B. What observation would indicate the loss of copper from the cycle?
3. Part E. Sulfuric acid has a dual role in the copper cycle of reactions. Identify each.
4. Part E.2. If sulfuric acid were unavailable to remove the unreacted magnesium from
the mixture and nitric acid were used instead, what observations (if any) would occur in
the experiment?
5. In Part E.3.
a. Explain what factors may contribute to a greater than 100% recovery of copper in the
cycle of chemical reactions.
b. If the percent recovery is less than 100%, identify specific steps in the cycle where
the error (the loss of copper) may have occurred.
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