Chemistry 201 Laboratory Harold Washington College Reactions

Chemistry 201 Laboratory Harold Washington Collegereactions Of Copper

In this laboratory exercise, the focus is on understanding and identifying the three main classes of aqueous reactions: precipitation, acid-base neutralization, and redox reactions. The experiment involves starting with metallic copper wire and observing its transformation through a series of reactions, with an emphasis on reaction types, color changes, gas evolution, temperature changes, and precipitate formation. Additionally, the experiment aims to determine the percent recovery of copper after completing the series of reactions, adhering to the Law of Mass Conservation. Safety precautions, materials, detailed procedures, and data collection are integral parts of the process, culminating in the calculation of the copper's percent yield based on observed and measured data. This comprehensive activity allows for direct observation of fundamental chemical concepts and the practical application of stoichiometry to real-world reactions involving copper.

Paper For Above instruction

The laboratory exercise on the reactions of copper involves a detailed exploration of the three primary categories of aqueous reactions: precipitation reactions, acid-base neutralizations, and redox reactions. Starting with a piece of metallic copper wire, students will observe the changes it undergoes when exposed to various reagents, noting physical and chemical changes that indicate different reaction types. This hands-on approach provides concrete understanding of theoretical concepts covered in the textbook.

Introduction

Understanding chemical reactions is fundamental in chemistry, especially in aqueous solutions where different types of reactions occur. The three main classes—precipitation, acid-base, and oxidation-reduction—are distinguished by their reactants, products, and observable changes. This laboratory experiment offers students insight into these processes through the reduction and recovery of copper from its compounds, emphasizing reaction identification, stoichiometry, and the conservation of mass.

Reaction 1: Formation of Copper Nitrate via Oxidation

The process begins with copper metal reacting with concentrated nitric acid, producing copper nitrate, nitrogen dioxide gas, and water. This is an oxidation-reduction reaction, as evidenced by the transfer of electrons and the change in oxidation state of copper from 0 to +2. The observable changes include a color change in the solution from colorless or pale blue to a darker coloration as copper nitrate forms, the evolution of toxic nitrogen dioxide gas (reddish-brown), and potential temperature change due to exothermicity. The chemical equation for this step is:

Cu(s) + 4 HNO₃(aq) → Cu(NO₃)₂(aq) + 2 NO₂(g) + 2 H₂O(l)

This reaction is classified as an oxidation-reduction process because electrons are transferred from copper to nitric acid, and copper's oxidation state increases from 0 to +2.

Reaction 2: Formation of Copper Hydroxide

Following the initial oxidation, the solution is buffered with sodium hydroxide. The addition of NaOH to copper nitrate results in the formation of copper hydroxide, a blue precipitate, while sodium nitrate remains in solution. This step exemplifies a double displacement or precipitation reaction where insoluble copper hydroxide forms. The chemical equation is:

Cu(NO₃)₂(aq) + 2 NaOH(aq) → Cu(OH)₂(s) + 2 NaNO₃(aq)

Observations include the appearance of a blue precipitate (copper hydroxide), indicating the formation of an insoluble salt, and a change in the solution's color.

Reaction 3: Conversion to Copper Oxide

Heating the copper hydroxide precipitate with more water and gentle heating results in dehydration, turning copper hydroxide into copper oxide, a black solid. The reaction can be represented as:

Cu(OH)₂(s) → CuO(s) + H₂O(g)

This step involves thermal decomposition, observed as black solid formation and possible slight temperature change during heating. The black copper oxide precipitate signifies the conversion of hydroxide into oxide.

Reaction 4: Acidification with Sulfuric Acid and Formation of Copper(II) Sulfate

The addition of sulfuric acid to copper oxide results in copper sulfate, a blue solution, and water. The reaction is a typical acid-base neutralization and salt formation:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Changes include the solution turning blue, indicating copper sulfate formation, and possibly slight exothermic heat evolution.

Reaction 5: Reduction of Copper Ions with Zinc

Adding zinc metal to the copper sulfate solution causes a displacement reaction where zinc reduces Cu²⁺ ions to metallic copper, which precipitates out as a solid, and zinc becomes zinc ions in solution. This redox reaction can be written as:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

The zinc metal is oxidized from 0 to +2 oxidation state, while copper is reduced from +2 to 0, forming a solid copper deposit. Observation includes the formation of reddish copper metal and a colorless zinc sulfate solution.

Gas Formation and Final Recovery

When the reaction slows, gentle heating releases gases, primarily hydrogen. The hydrogen gas is identified through its effervescence and flammability. After gas evolution ceases, the copper precipitate is collected via decanting and filtration, washed, dried, and weighed to determine the percent recovery. The total mass of recovered copper compared to the initial represents the percent yield, highlighting the efficiency and conservation of mass in chemical transformations.

References

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