Copper Cycle Lab: Exploring Five Reaction Types and Analyzing Yield Loss

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Objective

To understand different types of chemical reactions, including precipitation, redox reactions, decomposition, and neutralization reactions, in the context of the copper cycle. The purpose of the experiment is to track a series of reactions involving copper, nitric acid, sodium hydroxide, magnesium, and sulfuric acid. By observing and analyzing these reactions, the goal is to gain insight into the types of reactions, the products formed, and the losses that occur during the reactions.

Abstract

A total of five consecutive reactions with copper were carried out, showing a copper loss of 0.089 grams or 59.33%.

Introduction

The copper cycle experiment involves several chemical reactions to demonstrate fundamental principles. It includes reactions of solid copper with nitric acid, aqueous solution of copper(II) nitrate with sodium hydroxide, heating of copper(II) hydroxide, formation of copper(II) sulfate from copper acid and sulfuric acid, and reaction of copper(II) sulfate with magnesium. These reactions covered different types, such as redox reactions, precipitation, decomposition, and neutralization. The experiment will also include calculating the percent yield to evaluate the efficiency of the reactions. By comparing the actual copper yield obtained at the end of the experiment with the expected yield based on stoichiometry, it will be possible to determine the percent yield of the reaction.

Experimental Procedure

Reaction 1: Copper and Nitric Acid

Obtain a clean medium test tube (15mm x 180 mm).
Weigh a piece of copper wire (0.1-0.2 g) using an electronic balance.
Place the copper wire in the test tube and add 2 mL of 6 M nitric acid in the fume hood. 4. Warm the contents in a hot water bath until the copper dissolves, ensuring no boiling occurs.
Record observations, emphasizing any loss of copper-containing product.

Reaction 2: Copper(II) Nitrate and Sodium Hydroxide

Add 6 M sodium hydroxide drop-wise to the reaction mixture until it becomes basic. 2. Stir well after each drop, observing the formation of a distinct blue precipitate.
Confirm basicity by testing with red litmus paper, ensuring no confusion with the blue precipitate.
Record observations, particularly any potential loss of copper(II) ion-containing products.

Reaction 3: Heating Copper(II) Hydroxide

Gently heat the test tube in a hot water bath until the blue gel precipitate turns into a black suspended precipitate.
Allow the solution to cool to room temperature and then add an equal volume of distilled water.
Mix well, let the solid settle, and decant the clear fluid. Repeat the washing process.
Discard the supernatant, keeping the suspended black solid. Record observations, noting any product losses.

Reaction 4: Copper Oxide and Sulfuric Acid

Add 3 M sulfuric acid dropwise to the suspended black solid.
Ensure complete dissolution of the black solid without excess sulfuric acid.
Record observations, paying attention to any loss of copper-containing products.

Reaction 5: Copper(II) Sulfate and Magnesium

The instructor will provide magnesium ribbon; add it to the clear solution in the test tube.
Agitate the magnesium ribbon constantly to accelerate the reaction.
If magnesium hydroxide forms, add 6 M sulfuric acid. Document all observations, specifying any magnesium hydroxide formation.
Once the reaction is complete, transfer a drop of the solution to a small test tube, add water and concentrated aqueous ammonia.
Observe the formation of a deep blue complex with copper ions.
Wash and collect the solid copper formed, documenting all steps and potential losses.
Dry the copper, record its mass, and calculate the percentage yield.

Explanation and Equation

The reactions include precipitation reactions, acid/base reactions, decomposition reactions, and redox reactions. The primary focus is on understanding the transformations of copper throughout these reactions, starting with solid copper, and concluding with the recovery of the metal.

The cyclical sequence involves five reactions.

Reaction 1

Cu(s) + 4HNO_3(ag) = Cu(NO₃)_2(ag) + 2NO_2(g)+ 2H₂O_(l)

This equation represents the reaction of solid copper with nitric acid, producing copper(I) nitrate and nitrogen dioxide gas.

Reaction 2

Cu(NO₃)_2(ag) + 2NaOH_(ag) = Cu(OH)_2(s) + 2NaNO3_(ag)

This equation shows the reaction between copper(II) nitrate and sodium hydroxide, forming copper(II) hydroxide and sodium nitrate.

Reaction 3

Cu(OH)_2(s) = CuO_(s) + H₂O_(g)

This equation represents the decomposition of copper( hydroxide into copper(D) oxide and water upon heating.

Reaction 4

CuO_(s) + H₂SO_4(ag) = CuSO_4(ag) + H₂O_(l)

This equation demonstrates the creation of copper(I) sulfate from copper oxide and sulfuric acid in a neutralization reaction.

Reaction 5

CuSO_4(ag) + Mg_(s) = Cu_(s) + MgSO_4(ag)

In this reaction, copper sulfate reacts with magnesium, leading to the formation of copper and magnesium sulfate.

Experiment

Reaction 1

The initial molarity of copper(II) nitrate was 0.472M, which is equivalent to 0.472 moles of the substance per 1 liter of water. Given that only 5 mL of copper(II) nitrate was taken, the number of moles of the substance is:

moles(Cu(NO3)2) = 0.005 L ∙ 0.472 moles.L-1 = 0.00236 moles

Since all of the copper(II) nitrate was taken from the first reaction, given the stoichiometric ratio between solid copper and copper(II) nitrate in the first reaction:

moles(Cu) = moles(Cu(NO3)2) = 0.00236 moles

Hence, the initial mass of copper can be determined using the molar mass, viz:

mass(Cu) = 0.00236 moles ∙ 63.55 g.mol-1 = 0.150 g.

Table 1 – Observations During Reactions.

Reaction	Observation
No. 1	The obtained copper(II) nitrate was colored bright blue, and the initial mass was equal to 150 g. The solid metallic copper dissolved completely in nitric acid, and brown gas was observed as an additional feature.
No. 2	After interaction between blue copper(II) nitrate and alkali, the formation of a blue solid precipitate with unequal color distribution (gradient was observed) was observed
No. 3	After heating the blue precipitate of copper(II) hydroxide, a black precipitate of copper(II) oxide was observed; no other signs of reaction were observed.
No. 4	The black solid was mixed with colorless sulfuric acid: the resulting product was turbid blue.
No. 5	The turbid blue copper(II) sulfate solid was mixed with magnesium metal solid, which resulted in a change of metallic coloration to bronze (pure copper), but the color of the solution disappeared.

Conclusion

In the present work, a copper cycle was carried out; the point of using it was to study the stoichiometric relations and chemical continuity between reactions. More specifically, five processes were carried out, each involving copper. The mass of pure copper that entered the process and was collected after the last reaction was used as a quantification of such a cycle. It appeared that the percentage error was as high as 59.33%, indicating that more than half of the copper was lost. Random errors that could have been sources of this loss were discussed.

References

“Experiment 6. Copper Cycle” in Chemistry 113.1 Introduction to Chemical Techniques Laboratory Manual, C. M. Evans, F. H. Watson, and G. L. Findley (Queens College, New York, 2012).