PURPOSE

To observe a sequence of reactions of copper that form acycle, along with the color and physical property changesthat indicate those reactions. To gain skill in recordingobservations and interpreting them in terms of chemicalequations. To use a simple classification scheme for grouping chemical reactions by reaction type. Topractice quantitative laboratory techniques by determin-ing the percentage of the initial sample of copper that is recovered.

PRE-LAB PREPARATION

To a beginning student of chemistry, one of the most fas-cinating aspects of the laboratory is the dazzling array ofsights, sounds, odors, and textures that are encounteredthere. Among other things, we believe that this experi-ment will provide an interesting aesthetic experience.You will be asked to carry out a series of reactions involving the element copper and to carefully observe and record your observations. The sequence begins and ends with copper metal, so it is called a cycle ofcopper reactions. Because no copper is added or removed between the initial and final steps, and because each reaction goes to completion, you should be able to quantitatively recover all of the copper you started with if you are careful and skillful. This diagram shows in an abbreviated form the reactions of the cycle of copper:

Cu Cu(NO3)2 Cu(OH)2

CuOCuSO4

HNO3∫∫∫B(1)

Zn, HCl (5)

NaOH∫∫∫B

(2)

H2SO4b∫∫∫(4)

heat

(3)

∫∫

∫B

Like any good chemist, you will probably be curious toknow the identity of each reaction product and the stoi-chiometry of the chemical reactions for each step of thecycle. Here they are, numbered to correspond to the stepsshown in the chemical equation.1

8HNO3(aq) � 3Cu(s) � O2(g) B3Cu(NO3)2(aq) � 4H2O(l) � 2NO2(g) (1)

Cu(NO3)2(aq) � 2NaOH(aq) BCu(OH)2(s) � 2NaNO3(aq) (2)

Cu(OH)2(s) B CuO(s) � H2O(l) (3)

CuO(s) � H2SO4(aq) B CuSO4(aq) � H2O(l) (4)

CuSO4(aq) � Zn(s) B ZnSO4(aq) � Cu(s) (5)

These equations summarize the results of a large num-ber of experiments, but it’s easy to lose sight of this if youjust look at equations written on paper. You can easily beoverwhelmed by the vast amount of information foundin this lab manual and in chemistry textbooks. It is in facta formidable task to attempt to learn or memorize iso-lated bits of information that are not reinforced by yourpersonal experience. This is one reason why it is impor-tant to have a laboratory experience. Chemistry is preem-inently an experimental science. As you perform this—and any other—experiment, watch closely and recordwhat you see. Each observation should be a little hook inyour mind on which you can hang a more abstract bit ofinformation, such as the chemical formula for the com-pound you are observing.

71

A Cycle of Copper Reactions

5EXPERIMENT

1Labels specify the states of the reactants and products: (s) means a solid, (l)means a liquid, (g) means a gas, and (aq) means an aqueous (water) solution.

It is also easier to remember information that is orga-nized by some conceptual framework. Observations andfacts that have not been assimilated into some coherentscheme of interpretation are relatively useless. It would belike memorizing the daily weather reports when you haveno knowledge of or interest in meteorology.

Chemists look for relationships, trends, or patterns ofregularity when they organize their observations ofchemical reactions. The periodic table, which groups theelements into chemical families, is a product of this kindof thinking. Each element bears a strong resemblance toother members of the same chemical family but also hasits own unique identity and chemistry.

In a similar fashion, it is useful to classify reactionsinto different types. Because no one scheme is able toaccommodate all known reactions, several different kinds of classification schemes exist. A simple classifica-tion scheme we will use at the beginning is one based on ideas of precipitation (ion combination), acid-base(proton transfer), and redox (electron transfer). Here wepresent an outline and some examples of this kind ofclassification:

A SIMPLE SCHEME FOR CLASSIFYINGCHEMICAL REACTIONS

1. PRECIPITATION REACTIONS (THE COMBINA-TION OF POSITIVELY CHARGED IONS WITHNEGATIVELY CHARGED IONS TO FORM ANINSOLUBLE NEUTRAL COMPOUND THAT PRE-CIPITATES FROM SOLUTION). If we add a solutionof sodium chloride (NaCl) to a solution containing silvernitrate (AgNO3), an insoluble white solid forms. Thesolid, called a precipitate, is silver chloride (AgCl), and wemay write the following chemical equation to describethe reaction, using symbols to represent the substances insolution.

Na�(aq) � Cl�(aq) � Ag�(aq) � NO3�(aq) B

AgCl(s) � Na�(aq) � NO3�(aq)

First we eliminate the ions called spectator ions, whichappear on both sides of the equation but do not partici-pate in the precipitation reaction. What is left is the netionic equation:

Ag�(aq) � Cl�(aq) B AgCl(s)

The net ionic equation concisely summarizes the netresult of mixing the two solutions: the formation of aninsoluble precipitate when a positively charged silver ionis combined with a negatively charged chloride ion. The

solid AgCl precipitate is easily separated from the solu-tion containing the soluble sodium nitrate salt. If wedesired, we could also recover the sodium nitrate by evap-orating the water from the solution.

2. ACID-BASE (PROTON TRANSFER REACTIONS).An acid is a substance that reacts with water to formhydronium ions (H3O

�) by transfering a proton to awater molecule.

(a strong acid) HCl(g) � H2O B H3O�(aq) � Cl�(aq)

(a weak acid) CH3COOH(aq) � H2O j H3O�(aq)

� CH3COO�(aq)

HCl is a strong acid, completely dissociated in aqueoussolution, while acetic acid (CH3COOH) is a weak acidthat only partially dissociates into hydronium ion andacetate ion.

A base is a substance that forms hydroxide ions whendissolved in water:

(a strong base) NaOH(s) B Na�(aq) � OH�(aq)

(a weak base) NH3(g) � H2O j NH4�(aq) � OH�(aq)

NaOH is a strong base, completely dissociated in aqueoussolution, while NH3 is a weak base, only partially dissoci-ating into ammonium ion and hydroxide ion.

Acids react with bases to form salts and (usually) water.Both are neutral compounds, being neither strongly acidicnor strongly basic. So acid-base reactions are also calledneutralization reactions. Two examples follow:

(a strong acid � a strong base)

H3O�(aq) � Cl�(aq) � Na�(aq) � OH�(aq) B

2 H2O � Na�(aq) � Cl�(aq)

(the net ionic equation)

H3O�(aq) � OH�(aq) B 2 H2O

(a weak acid � a weak base)

CH3COOH(aq) � NH3(aq) B NH4�(aq) � CH3COO�(aq)

Note that in these two examples of acid-base reactions aproton is transferred from the acid to the base.

3. REDOX REACTIONS (ELECTRON TRANSFERREACTIONS). Oxidation-reduction reactions, calledredox reactions, are reactions that involve the shift ortransfer of electrons from one kind of atom to another. In

72 A CYCLE OF COPPER REACTIONS

some reactions, the transfer is obvious, as in this reaction:

Mg(s) � 2H3O�(aq) � 2Cl�(aq) B

H2(g) � Mg2�(aq) � 2Cl�(aq)

Here, each magnesium atom is giving up two electrons totwo hydrogen ions, forming magnesium ion and hydro-gen gas.

Sometimes, the transfer of electrons between atoms isless obvious, as in the following reaction:

2SO2(g) � O2(g) B 2SO3(g)

Here, the reactants and products are all gases, and no ionsare formed. In classifying this reaction as an oxidation-reduction reaction, we use the concept of assigning oxi-dation numbers (also called oxidation states) to eachatom in the compound. A simple set of rules defines theprocedure for assigning the oxidation number. For a sim-ple binary compound (a compound composed of twodifferent elements), we imagine that all of the electrons inthe chemical bonds are assigned to the atoms with thegreatest affinity for electrons. The ability of an atom toattract electrons to itself is called electronegativity, and theatoms having the greatest electronegativity are found inthe upper right hand corner of the periodic table, withfluorine having the most.

In sulfur dioxide, SO2, and sulfur trioxide, SO3, weimagine that all of the electrons in the S—O bonds areassigned to the O atoms, giving each oxygen atom a fullvalence shell; this formally gives each oxygen atom a netcharge of �2. So if oxygen is assigned oxidation number�2, the sulfur in SO2 must have oxidation number �4and the oxidation number of S in SO3 would be �6, sincethe sum of the oxidation numbers on all the atoms mustadd up to the net charge on the molecule (zero, in thiscase). The oxidation number of atoms in their elementalform is always assigned zero. This seems reasonable forO2 because the oxygen atoms are equivalent so that therewould be no tendency for one oxygen atom to transferelectrons to its partner in the O2 molecule.

Once we have assigned oxidation numbers to each ele-ment in the chemical reaction, we will see that in this par-ticular reaction the oxidation number of the sulfur atomsincreases from �4 to �6 while the oxidation number ofthe oxygen atoms decreases from zero (in O2) to �2 (inSO3). From this viewpoint, the change in oxidation num-ber is formally equivalent to transferring electrons fromsulfur to oxygen. We say that the sulfur atoms have beenoxidized (because their oxidation number increases),while the oxygen atoms in O2 have been reduced (becausethe oxidation number of oxygen atoms decreases fromzero in O2 to �2 in SO3).

We must be careful to note, however, that the oxidationnumbers we assign do not necessarily represent the real dis-tribution of electronic charge in the molecule. By assigningthe oxidation number according to fixed rules, we haveartificially assigned integer changes in oxidation number toparticular atoms (sulfur and oxygen in this case), but thechanges in the electron density on the sulfur and oxygenatoms may not be as large as implied by the assigned oxida-tion numbers. Nevertheless, it is reasonable to suppose thatthe sulfur atom in SO3 is more positive than the sulfuratom in SO2 because the added oxygen atom would tend todraw electrons away from the sulfur atom.

4. DECOMPOSITION REACTIONS (A SUBSTANCEBREAKING DOWN INTO SIMPLER SUBSTANCESUNDER THE INFLUENCE OF HEAT). Althoughmany chemical reactions can be classified into one of thethree reaction types we described earlier, it is possible tofind examples of chemical reactions that do not neatly fitinto this scheme. For example, when calcium carbonate isheated, it breaks down into simpler substances:

CaCO3(s) B CaO(s) � CO2(g)

This reaction might be called a decomposition or dissoci-ation reaction. Any substance heated to a sufficiently hightemperature will begin to decompose into simpler sub-stances, so this kind of reaction is common.

DOES A COMPREHENSIVE CLASSIFICATIONSCHEME EXIST? If we searched we would find exam-ples of other reactions that do not fit into these four cate-gories. Indeed, it is probably fair to say that there is nocompletely comprehensive classification scheme thatwould accommodate all known chemical reactions. How-ever, many, if not most, of the chemical reactions describedin your general chemistry text will fit into this simplescheme. As you carry out each step of the cycle of copperreactions, think about what is happening in each reaction,and try to fit it into one of the four categories we described.

EXPERIMENTAL PROCEDURE

Special Supplies: Infrared lamps or steam baths, porcelain evaporatingdish.

Chemicals: 18- to 20-gauge copper wire, concentrated (16 M) HNO3, 3 MNaOH, 6 M H2SO4, 30-mesh zinc metal, 6 M HCl, methanol.

! SAFETY PRECAUT IONS : Concentrated nitricacid, HNO3, is hazardous. It produces severe burns on the skin,and the vapor is a lung irritant. When you handle it, youshould use a fume hood while wearing safety glasses (asalways) and rubber or polyvinyl chloride gloves. A polyethylene

A CYCLE OF COPPER REACTIONS 73

squeeze pipet can be useful for transferring the HNO3 from asmall beaker to your 10-mL graduated cylinder. Rinse yourhands with tap water after handling HNO3. The dissolution ofthe copper wire with concentrated HNO3 should be carried outin a fume hood. If no hood is available, construct the apparatusshown in Figure 5-1 to substitute for the fume hood. Thebrown NO2 gas that is evolved is toxic and must be avoided.

NaOH solutions are corrosive to the skin and especially dan-gerous if splashed into the eyes—wear your safety glasses.

Methanol and acetone are flammable and their vaporsare toxic. Use them in the hood to avoid breathing the vapor,and keep them away from all open flames.

WASTE COLLECT ION : The supernatant solutionthat is decanted in Step 5 contains zinc sulfate and zinc chlo-ride and should be collected.

A waste container should also be provided for themethanol used to dry the product in Step 5.

1. Cu TO Cu(NO3)2. Cut a length of pure copper wirethat weighs about 0.5 g (about a 10-cm length of 20-gauge copper wire). If it is not bright and shiny, clean itwith steel wool, rinse it with water, and dry it with a tis-sue. Weigh it to the nearest milligram, recording theweight in your laboratory book. Coil the wire into a flatspiral, place it in the bottom of a 250-mL beaker, and—inthe fume hood—add 4.0 mL of concentrated (16 M)nitric acid, HNO3. (If a fume hood is not available, usethe apparatus shown in Figure 5-1.)

Record in your notebook a description of what you see.Swirl the solution around in the beaker until the copperhas completely dissolved. What is in the solution when thereaction is complete? After the copper has dissolved, adddeionized water until the beaker is about half full. Steps 2through 4 can be conducted at your lab bench.

2. Cu(NO3)2 TO Cu(OH)2. While stirring the solu-tion with a glass rod, add 30 mL of 3.0 M NaOH to pre-cipitate Cu(OH)2. What is formed in the solution besidesCu(OH)2? Record your observations in your lab book.

3. Cu(OH)2 TO CuO. Stirring gently with a glass rodto prevent “bumping” (a phenomenon caused by the for-mation of a large steam bubble in a locally overheatedarea), heat the solution just barely to the boiling pointover a burner, using the apparatus shown in Figure 5-2. Ifthe solution bumps you may lose some CuO, so don’tneglect the stirring. Record your observations. When thetransformation is complete, remove the burner, continue

74 A CYCLE OF COPPER REACTIONS

FIGURE 5-1If a fume hood is not available, substitute this apparatus.

FIGURE 5-2Setup for heating Cu(OH)2 to convert it to CuO.

CuOsuspension

Warmgently,only asnecessary

stirring for a minute or so, then allow the CuO to settle.Then decant (pour off) the supernatant liquid, beingcareful not to lose any CuO. Add about 200 mL of hotdeionized water, allow to settle again, and decant oncemore. What is removed by this washing and decantationprocess?

4. CuO TO CuSO4. Add 15 mL of 6.0 M H2SO4, whilestirring. Record your observations. What is in solutionnow? Now transfer operations back to the fume hood.

5. CuSO4 TO Cu. In the fume hood, add all at once 2.0 g of 30-mesh zinc metal, stirring until the supernatantliquid is colorless. What happens? What is the gas pro-duced? When the evolution of gas has become very slow,decant the supernatant liquid, and pour it into the wastecontainer provided. If you can see any silvery grains ofunreacted zinc, add 10 mL of 6 M HCl and warm, but donot boil, the solution. When no hydrogen evolution canbe detected by eye, decant the supernatant liquid, andtransfer the copper to a porcelain dish. A spatula or rub-ber policeman is helpful for making the transfer. Washthe product with about 5 mL of deionized water, allow itto settle, and decant the wash water. Repeat the washingand decantation at least two more times. Move to thehood, away from all flames. Wash with about 5 mL ofmethanol, allow to settle, and decant. Dispose of themethanol in the proper receptacle. Place the porcelain dishunder an infrared lamp or on a steam bath or hot plate,and dry the copper metal. What color is it? Using a spat-ula, transfer the dried copper metal to a preweighed 100-mL beaker and weigh to the nearest milligram. Cal-culate the mass of copper you recovered by subtractingthe weight of the empty beaker from the weight of thebeaker plus the copper metal.

CALCULATION OF PERCENTAGE RECOVERY.Express the percentage of copper recovered as

If you are careful at every step, you will recover nearly 100percent of the copper you started with.

☛Consider ThisIf you used a penny as the source of your original copperin this experiment, would it matter if you used a pre-1982 penny (essentially pure copper) or a post-1982penny (copper cladding over a zinc core)? Describe whatwould happen in each step if you used a post-1982penny. Test your predictions.

Why would it be hard to perform a cycle of oxygen or acycle of hydrogen experiment similar to this cycle of cop-per exercise? Can you design an experimental apparatusfor a cycle of oxygen lab?

BIBLIOGRAPHYBailar, Jr., J.C. “A Further Improvement on the Copper

Cycle Experiment,” J. Chem. Educ., 1983, 60: 583.Condike, G. F. “Near 100% Yields With the ‘Cycle of Cop-

per Reactions’ Experiment,” J. Chem. Educ., 1975, 52:615.

Todd, D. and Hobey, W.D. “An Improvement in the Clas-sical Copper Cycle Experiment,” J. Chem. Educ., 1985,62: 177.

Umans, T. and de Vos, W. “An Improved Copper CycleExperiment,” J. Chem. Educ., 1982, 59: 52.

� mass of recovered copperinitial mass of copper wire

� 100%percentagerecovery

A CYCLE OF COPPER REACTIONS 75

DATA AND CALCULATIONS

77

A Cycle of Copper Reactions NAME

SECTION LOCKER

INSTRUCTOR DATE

REPORT

5

1. Initial mass of copper wire g

2. Mass of beaker plus dry copper g

3. Mass of empty beaker g

4. Mass of recovered copper g

5. Percentage recovery

%

Show your calculation.

EQUATIONS AND OBSERVATIONS

For each step of the cycle, write the products of the reaction and balance the chemical equation(s). Using theclassification scheme presented in the Pre-Lab Preparation, write the reaction type (combination, decomposition, orsingle/double replacement). Also record your observations of what happens at each step, and answer the questionsposed earlier in the Experimental Procedure.

1. Cu � HNO3 � O2 B

Reaction type:

What is in the solution after reaction is complete?

Observations (Be sure to include color and texture changes that occur in each step.)

percentage recovery �mass of recovered copperinitial mass of copper wire

� 100% �

2. Cu(NO3)2 � NaOH B

Reaction type:

HNO3 � NaOH B

Reaction type:

What is formed in the solution besides Cu(OH)2?

Observations:

3. Cu(OH)2

Reaction type:

What is removed by the washing and decantation process at the end of Step 3? (Consider the products of thereaction as well as reagents from previous steps.)

Observations:

4. CuO � H2SO4 B

Reaction type:

What is in the solution at the end of Step 4?

Observations:

heatßßßßB

78

5. Zn � CuSO4 B

Reaction type:

Zn � H2SO4 B

Reaction type:

Zn � HCl B

Reaction type:

What happens when the zinc is added?

What is the gas produced in the reaction?

What is removed by the washing and decantation near the end of Step 5?

What color is the recovered copper?

Observations:

Sheet 2

NAMEREPORT

5

79

QUESTIONS

1. Describe whether the error introduced by each of the following problems would result in a high or a low value forthe Cu recovery, or would not affect the results.

(a) Some of the copper nitrate solution is splashed out of the beaker in Step 1.

(b) Insufficient NaOH is added in Step 2.

(c) The solution bumps in Step 3, splashing out some CuO.

(d) Some solid is lost in the decanting process.

(e) An excess of H2SO4 is added in Step 4.

(f) Some unreacted zinc remains with the product at the end of the experiment.

80

(g) The washings in Step 5 are insufficient to remove all of the solution residues from the copper.

(h) The copper is not completely dried.

2. How many milliliters of 3.0 M NaOH are required to react with 4.0 mL of 16 M HNO3?

3. How many milliliters of 3.0 M NaOH are required to react with 0.5 g of Cu2� to form Cu(OH)2?

4. (a) Add together the results you calculated for Questions 2 and 3 and compare the sum with the milliliters of 3.0 MNaOH added in Step 2.

(b) Is there an excess of NaOH above that required to react with the HNO3 and Cu2�?

(c) What would be the effect on the percentage recovery if not enough NaOH were added in Step 2 to react withboth the unreacted HNO3 and the Cu2� present at the end of Step 1?

Sheet 3

NAMEREPORT

5

81

☛Consider ThisIf you used a penny as the source of your original copper in this experiment, would it matter if you used a pre-1982penny (essentially pure copper) or a post-1982 penny (copper cladding over a zinc core)? Describe what would hap-pen in each step if you used a post-1982 penny. Test your predictions.

Why would it be hard to perform a cycle of oxygen or a cycle of hydrogen experiment similar to this cycle of copperexercise? Can you design an experimental apparatus for a cycle of oxygen lab?

82