Materials Included In Kit

Additional Materials Required

Safety Precautions

Nitric acid is a corrosive liquid and vapor and a strong oxidizer; it is harmful if inhaled. Keep away from heat, sparks and open flames. Silver nitrate is a corrosive solid and is toxic by ingestion; it causes skin burns and eye damage and will stain skin and clothes. Acetone is a flammable liquid; do not use near flames and other sources of ignition. Carry out this experiment in a fume hood or well-ventilated lab. Avoid contact of all chemicals with eyes, skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The solution remaining after the silverclad copper wire has been removed may contain residual silver nitrate. Use sodium chloride to precipitate silver chloride according to Flinn Suggested Disposal Method #11. The aqueous rinse solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Acetone may be recycled for future use by extraction and/or distillation. Pure acetone is a characteristic (flammable) hazardous waste subject to RCRA disposal guidelines. Leftover copper wire may be reused or packaged for landfill disposal according to Flinn Suggested Disposal Method #26a. The silver metal may be recycled to produce silver nitrate for use in future experiments. See Procedure A in Flinn Suggested Disposal Method #11. The procedure requires the use of concentrated nitric acid in a fume hood and is a multi-step process. Several hours are needed to complete the process.

Lab Hints

• The experiment as written can be completed within a typical 2-hour lab period.
• The procedure for this experiment has been written to make the most efficient use of lab time. Under the reaction conditions described in the procedure, the mole ratio of copper and silver can be determined and is in excellent agreement with theory. We have found that 30 minutes reaction time is optimum (99.9% yield of Ag based on Cu), at 81% conversion of AgNO₃ to Ag. As the reaction time is increased to 1 hour, the percent yield of Ag based on AgNO₃ increases to 96% at the expense of Cu (the latter yield drops to 94%). It appears that a side reaction may take place at longer reaction times. Complete conversion of silver nitrate to silver metal generally requires 45 min to 1 hour of reaction time at room temperature.
• Transform chemistry into art by viewing the progress of the reaction under a microscope or using a document camera and projector. Cut a small sliver of copper foil or copper wire and place it on a microscope slide. Add a few drops of 0.1 M silver nitrate, and watch as delicate crystals of silver grow in tree-like fashion on the piece of copper. Beautiful needles and branches will continue to grow for 10–15 minutes. This is an excellent demonstration to do during the 30-minute reaction time.
• Almost any single replacement reaction can be adapted for use in this experiment. Excellent results are obtained for the reaction of iron with copper(II) sulfate as well as for the reaction of aluminum with copper(II) chloride.
• This experiment addresses the basic concept of mole ratios and advanced concepts relating to limiting and excess reactants. Is it possible for all of the copper wire to react in the experiment as written? Which reagent is the limiting reactant? Calculate the theoretical yield and percent yield of silver metal produced in the reaction as a function of both copper metal and silver nitrate.
• The word stoichiometry is derived from the Greek words stoicheion, meaning element, and metrike, measurement. The term was first introduced in 1792 by J. B. Richter, a German scientist, to describe the relative amounts of acids and bases in neutralization reactions.
• Several student groups may share the acetone rinse beakers. Place two rinse beakers containing about 125 mL of acetone in the hood or in a central location for ease of use.

Answers to Prelab Questions

Copper(II) chloride (CuCl₂; 0.98 g) was dissolved in water and a piece of aluminum wire (Al; 0.56 g) was placed in the solution. The blue color due to copper(II) chloride soon faded and a red precipitate of solid copper was observed. After the blue color had disappeared completely, the leftover aluminum wire was removed from the solution and weighed. The mass of the leftover aluminum wire was 0.43 g.

1. Calculate the number of moles of (a) copper(II) chloride and (b) aluminum that reacted.
   {14032_PreLabAnswers_Equation_8}
2. What is the mole ratio of copper(II) chloride to aluminum metal? Express to the nearest whole number ratio.
   {14032_PreLabAnswers_Equation_9}
   The nearest whole number ratio of CuCl₂ to Al is 3:2.
3. What happened to the aluminum metal that was consumed in this reaction? Write the formula of the most probable aluminum-containing product.

   Aluminum metal was oxidized to aluminum (Al³⁺) ions upon reaction with Cu²⁺ ions; the ions dissolved in the aqueous solution. The most probable aluminum-containing product is AlCl₃.

4. Write a balanced chemical equation for the single replacement reaction of copper(II) chloride with aluminum.

   3CuCl₂(aq) + 2Al(s) → 3Cu(s) + 2AlCl₃(aq)

5. Would it be possible for the entire piece of aluminum wire (0.56 g) to react with the copper(II) chloride used in the procedure described? Explain your reasoning using the concept of the limiting reactant.

   The number of moles of Al in 0.56 g of Al(s) is equal to 0.56 g/26.98 g/mole, or 0.0208 moles. This is 2.8 times the number of moles of CuCl₂. According to the mole ratio, however, the maximum amount of Al that can react is 2(0.0073)/3 or 0.0049 moles. Copper chloride is therefore the limiting reactant—not all of the Al will react.

Sample Data

Laboratory Report

Answers to Questions

1. Calculate the mass and moles of copper wire that reacted in this experiment.

   Mass (Cu) = 1.77 g – 1.54 g = 0.23 g
   Moles (Cu) = 0.23 g x (1 mole/63.55 g) = 0.00362 moles

2. Calculate the mass and moles of silver metal produced in the reaction.

   Mass (Ag ) = 2.09 g – 1.31 g = 0.78 g
   Moles (Ag) = 0.78 g x (1 mole/107.87 g) = 0.00723 moles

3. Determine the mole ratio—the ratio of the number of moles of silver to the number of moles of copper. Note: Round the result to the nearest whole number.

   Mole ratio (Ag:Cu) = 2.0:1
   The ratio may be rounded to give a whole-number mole ratio of 2:1 for the amount of silver produced per mole of copper consumed. This represents a 0% error!

4. Use the silver/copper mole ratio to write the balanced chemical equation for the reaction of copper and silver nitrate.

   Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

5. Did all of the silver nitrate react in this experiment? Calculate the percent yield of silver based on the amount of AgNO₃ used in the experiment and explain your reasoning.

   The original number of moles of AgNO₃ in the aqueous solution was 1.52 g/(169.87 g/mole) = 0.00895 moles.
   The number of moles of silver produced was 0.0072 moles (see Question 2).
   The percent yield of Ag(s) based on the number of moles of AgNO₃ = (0.0072/0.00895) x 100%, or 81%. It appears that not all of the AgNO₃ reacted, since no other by-products were observed in the reaction.

6. What factors might account for the answer to Question 5?

   Several factors may be responsible for the reduced yield of silver:

   • There was a loss of silver product during the filtration and recovery steps.
   • The copper wire became coated with a thick layer of silver, preventing contact between the copper wire and the silver nitrate solution. The unexposed copper did not react.
   • The reaction did not proceed to completion. A longer reaction time is needed.
7. Identify the limiting and excess reactants in this experiment.

   The maximum amount of copper that can react is one-half the number of moles of AgNO₃, or 0.0045 moles, corresponding to 0.29 g. This is far less than the total mass of Cu wire. AgNO₃ is the limiting reactant and Cu is present in excess.

8. Silver is a precious metal. The price of silver fluctuates daily as it is traded on the open market. Look up the current market value of silver in the financial section of the daily newspaper or on the Internet and record the price. Note: The price of metals is usually quoted per Troy ounce, where 1 Troy ounce = 31.1 grams.

   The current market price of silver is about $17.30 per Troy ounce.

9. Calculate the current market value of the silver produced in this experiment.

   The amount of silver produced in this experiment was 0.78 g.

   {14032_Answers_Equation_10}
   Forty-three cents!

Introduction

The reaction of copper wire with silver nitrate in aqueous solution provides a beautiful display of chemistry in action—delicate silver crystals begin to grow on the wire surface and the color of copper(II) ions gradually appears in solution. What relationships govern the relative quantities of reactants and products in this chemical reaction?

Concepts

• Stoichiometry
• Balanced chemical equation
• Mole ratio
• Limiting reactant
• Percent yield
• Single replacement reaction

Background

Stoichiometry is the branch of chemistry that deals with the numerical relationships and mathematical proportions of reactants and products in a chemical reaction. One of the most important lessons of stoichiometry is that the amounts of the reactants and products in a chemical reaction are related to one another on a mole basis. Chemical reactions are normally represented by balanced chemical equations. The coefficients in a balanced chemical equation summarize the relative number of moles of each reactant and product involved in the reaction. The ratios of these coefficients represent the mole ratios that govern the disappearance of reactants and appearance of products. Knowing the mole ratios in a balanced chemical equation is essential to solving stoichiometry problems.

The reaction of copper metal with silver nitrate solution is a single replacement reaction, represented by the following unbalanced chemical equation.

The values of the coefficients a, b, c and d can be determined or verified experimentally by measuring the mass of copper wire that reacts and the mass of metallic silver that is produced in Equation 1.

In order to determine the mole ratios for the reaction, the stoichiometry of the reaction and the actual experimental procedure must be examined to identify which material is the limiting reactant. The limiting reactant is the reagent that is used up in the reaction and on which the overall yield of product depends. The limiting reactant in any reaction can be determined by calculating the starting number of moles of each reactant. The balanced equation is then used to determine which starting material will “run out” first or, in other words, limit the amount of product formed. Consider, for example, the reaction between hydrogen gas and oxygen gas to produce water. If 10.0 grams of H₂ are mixed with 10.0 grams of O₂, which one will “run out” first and act as the limiting reactant? First, determine the number of moles of each reactant: Then determine which reactant will limit the amount of product formed. Consider H₂ first—4.95 moles of H₂ could theoretically produce 4.95 moles of H₂O since for every two moles of hydrogen that react, two moles of water can be generated: Similarly, 0.313 moles of O₂ could theoretically produce 0.626 moles of H₂O, since two moles of water are generated per mole of oxygen: Therefore, if all of the H₂ reacts, 4.95 moles of H₂O could theoretically form, while only 0.626 moles of H₂O can be obtained from the available O₂. Oxygen is therefore the limiting reactant in this example. The O₂ will “run out” while some of the H₂ remains in excess.

The theoretical yield is the maximum amount of product that can be obtained when the limiting reagent (LR) is completely consumed: This is seldom actually obtained due to side reactions, losses and other complications. The actual yield of product is often expressed as a percentage of the theoretical yield and is called the percent yield:

Experiment Overview

The purpose of this experiment is to determine the number of moles of reactants and products in the reaction of copper and silver nitrate and calculate their mole ratio. The mole ratio relating the disappearance of copper and the formation of silver metal will be used to write the balanced chemical equation for the reaction.

Materials

Prelab Questions

Copper(II) chloride (CuCl₂; 0.98 g) was dissolved in water and a piece of aluminum wire (Al; 0.56 g) was placed in the solution. The blue color due to copper(II) chloride soon faded and a red precipitate of solid copper was observed. After the blue color had disappeared completely, the leftover aluminum wire was removed from the solution and weighed. The mass of the leftover aluminum wire was 0.43 g.

1. Calculate the number of moles of (a) copper(II) chloride and (b) aluminum that reacted.
2. What is the mole ratio of copper(II) chloride to aluminum metal? Express to the nearest whole number ratio.
3. What happened to the aluminum metal that was consumed in this reaction? Write the formula of the most probable aluminum- containing product.
4. Write a balanced chemical equation for the single replacement reaction of copper(II) chloride with aluminum.
5. Would it be possible for the entire piece of aluminum wire (0.56 g) to react with the copper(II) chloride used in the procedure described above? Explain your reasoning using the concept of the limiting reactant.

Safety Precautions

Nitric acid is a corrosive liquid and a strong oxidizer. It will cause skin burns and severe eye damage. Silver nitrate is a corrosive solid and is toxic by ingestion; it will stain skin and clothes. Acetone is a flammable liquid; avoid contact with flames and other sources of ignition. Avoid contact of all chemicals with eyes, skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Obtain a clean and dry 50-mL beaker. Zero (tare) the balance with the beaker and carefully add 1.4–1.6 g of silver nitrate crystals to the beaker. Caution: Use a spatula to transfer the solid. Do not touch the silver nitrate and carefully clean up any silver nitrate spills in the balance area or on the benchtop.
2. Measure and record the exact mass of silver nitrate in the data table.
3. Fill the beaker to the 30-mL mark with distilled water and stir the solution with a stirring rod until all of the solid has dissolved. Rinse the stirring rod with distilled water.
4. Cut a 25-cm piece of copper wire and loosely coil it into the shape shown in Figure 1.
   {14032_Procedure_Figure_1}
5. Find the initial mass of the copper wire to the nearest 0.01 g and record it in the data table.
6. Use a wooden splint to suspend the copper wire in the silver nitrate solution, as shown in Figure 1. The copper wire should not be touching the bottom or sides of the beaker.
7. Carefully add 3 drops of 3 M nitric acid to the beaker. Do NOT stir the solution.
8. Allow the beaker to sit undisturbed on the lab bench for 30 minutes. Try not to jostle or shake the suspended copper wire in any way.
9. Observe the signs of chemical reaction occurring in the beaker and record all observations.
10. While the reaction is taking place, measure and record the mass of a piece of filter paper. Set up a funnel and flask for gravity filtration.
11. After 30 minutes, lift the wooden splint to raise the copper wire above the solution. Gently shake the wire to allow the crystals to fall back into the liquid. Use a slow stream of distilled water from a wash bottle to carefully rinse and remove any crystals that stick to the wire.
12. When all of the silver has been removed, lift the copper wire out of the beaker and place it in the acetone rinse beaker. The acetone will clean the wire surface and allow it to dry more quickly.
13. Remove the copper wire from the acetone rinse beaker and allow it to air dry for 2–3 minutes. Gently blot with a paper towel.
14. Measure and record the final mass of the copper wire. Observe and record the appearance of the leftover wire.
15. Wet the filter paper in the funnel (see step 10) with distilled water, then pour the contents of the reaction beaker through the funnel to collect the silver metal.
16. Rinse the beaker with several portions of distilled water to ensure all of the solid is transferred to the filter paper.
17. When the filtration is complete, carefully remove the filter paper from the funnel and place it on a labeled watch glass.
18. Dispose of the waste solution in the filter flask as directed by the instructor.
19. Store the watch glass containing the filter paper and silver product in a secure location, as directed by your instructor.
20. Allow the solid to dry 24–48 hours, as needed. The filter paper and silver metal may also be dried in a lab oven at a low setting (50–60 °C) for one hour.
21. When the solid is dry, measure and record the final mass of the filter paper plus silver metal. Record any observations.

Student Worksheet PDF

14032_Student1.pdf