Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide solution is a corrosive liquid; it causes skin burns and serious eye damage. Acetone is a highly flammable liquid and vapor; do not use near heat, sparks or open flames. Ammonium nitrate solution may be harmful if swallowed. Silver nitrate solution is corrosive to skin and eyes and will stain skin and clothing. The mixed solution in the flask may form a potentially explosive material if left standing and heated or allowed to dry. Do NOT mix the solutions beforehand—add them together in the glass ornament and follow the directions on the next page for disposing of the leftover solution immediately after use. Rinse with copious amounts of water into a container for disposal of silver waste. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

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. Warning: The mixture remaining in the ornament after the silver mirror reaction is complete may become unstable or explosive when concentrated or heated. Collect excess or unreacted Tollens’ reagent in a dedicated beaker or flask by rinsing the solutions with copious amounts of water to avoid the formation of silver imide. Test the combined waste solution for the presence of leftover silver ions by adding concentrated hydrochloric acid. If a cloudy, white precipitate of silver chloride is observed, continue adding hydrochloric acid in small amounts until no further precipitation is evident. Filter the mixture—the silver chloride should be identified as a toxicity characteristic hazardous waste (Flinn Suggested Disposal Method #27f ). The filtrate may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• This experiment may be completed within a typical 2-hour lab period. That will allow enough time for the rinsed ornament to dry completely in air both before and after it has been “silvered.”
• Any glassware that will be silvered must be scrupulously clean for the silver mirror to adhere to the glass surface. Check ornaments for dust or debris and rinse as needed.
• Do not use glass ornaments that have an iridescent or opalescent appearence. These ornaments have an interior coating that will prevent the silver from adhering to the surface.
• See the video “Silver Mirrors” from the Best Practices for Teaching Chemistry Video Series on the Flinn Scientific website for a demonstration of the technique used in this experiment.
• Other types of glassware that are commonly used in silver mirror labs of this type include small test tubes or culture tubes, Florence flasks, etc. If the desired glassware is not clean or new, rinse the glassware first with distilled water, followed by 6 M nitric acid. Pour out the nitric acid cleaning solution, rinse well with distilled water, and then rinse a final time with acetone. Allow to air dry thoroughly before adding the reagents for the Tollens’ test reaction.
• The silver mirror holiday ornaments may be protected from oxidation or mechanical stress by coating the inside of the ornament with clear nail polish.
• The “silver mirror reaction” may be downsized to the microscale level by mixing drops of reagents in the relative amounts and order indicated in the Procedure section. Use very small test tubes (6 x 50 mm) to prepare micro-ornaments.
• The risk of explosion for disposal of Tollens’ reagent is attributed to the potential formation of “fulminating silver” due to the formation of solid silver imide. This hazard may be avoided by following the safety precautions and disposal instructions—rinse with copious amounts of water. Do not concentrate and never heat the solution or leftover reaction mixtures.
• All monosaccharides (e.g., glucose, fructose) and most disaccharides (e.g., maltose and lactose) are reducing sugars. The most well-known exception to this general rule is sucrose (“table sugar”). Sucrose is a nonreducing disaccharide that is composed of one glucose unit joined to one fructose unit via the loss of water. Because the monosaccharides are joined at their carbonyl carbon atoms, sucrose does not have an aldehyde functional group that can be oxidized. All polysaccharides (e.g., starch, cellulose) are nonreducing sugars.
• Special mirrors that require perfectly reflective surfaces with few imperfections are made by coating glass with the metal from the vapor phase. The vapor is obtained by electrical vaporization of silver under high vacuum conditions.

Answers to Prelab Questions

1. The diameter of a glass ornament sphere is 2⅝ inches. Convert the diameter of the sphere to centimeters and calculate its radius.

   2⅝" = 2.625 in x 2.54 cm/in = 6.67 cm
   Radius = 3.33 cm (rounded to 3 significant figures)

2. Use the following equation to calculate the approximate outer surface area (SA) of the ornament: SA = 4πr².

   SA = 4(3.14)(3.33 cm)² = 139 cm²

3. Assuming that the glass thickness of the ornament is 1.6 mm, estimate the percent difference in the surface area calculation for the inside versus the outside of the ornament sphere.

   Surface area for the inside of the sphere = 4(3.14)(3.33 – 0.16)² = 126 cm²

   {14022_PreLabAnswers_Equation_2}
4. Oxidation–reduction reactions involve the loss and gain of electrons, respectively. Based on the mole ratios in Equation 1 for the reduction of Ag⁺ cations to Ag metal, how many electrons are gained by the aldehyde when it is oxidized to the carboxylate function group? Hint: Electrons must be balanced and “cancel out” in an oxidation–reduction reaction.

   Reduction of Ag⁺ to Ag metal involves the gain of one electron. The mole ratio for the reaction, however, requires two moles of Ag⁺ ions per mole of aldehyde. The principle of electron balance, therefore, implies that oxidation of an aldehyde to a carboxylate ion involves the loss of two electrons.

Sample Data

Laboratory Report

Answers to Questions

Laboratory Report

1. Using the measured circumference of the glass ornament, calculate the radius (in cm) and the surface area (cm²) of the ornament. (The formula for the circumference of a sphere is 2πr.)

   Circumference = 20.8 cm. Radius = 3.3 cm.
   Surface area = 4πr² = 138 cm².

2. Calculate the mass and the number of moles of silver lining the inside of the glass ornament.

   Mass of silver = 0.22 g. Number of moles = 0.22 g/107.9 g/mole = 0.0020 mole.

3. The density of silver is 10.5 g/cm³. What is the volume of silver metal lining the inside of the glass ornament?

   Volume of silver = 0.22 g/10.5 g/cm³ = 0.021 cm³.

4. Assume that the volume of silver in the ornament can be estimated using the following equation: Volume = surface area x thickness. Calculate the approximate thickness of the silver lining in centimeters.

   Thickness of the silver lining = V/SA = 0.021 cm³/138 cm² = 1.5 x 10^–4 cm.

5. Convert the thickness of the silver layer to micrometers (1 μm = 1 x 10^–6 m) and also nanometers (1 nm = 1 x 10^–9 m).

   1.5 x 10^–4 cm is equal to 1.5 x 10^–6 m or 1.5 μm.
   1.5 μm × 1000 nm/μm = 1500 nm.

6. The radius (r) of a silver atom is 160 picometers (1 pm = 1 x 10^–12 m). Estimate the thickness of the silver lining in terms of the number (N_Ag) of silver atoms. Assume that the thickness is equal to N_Ag x 2r. Hint: Convert the radius of a silver atom from picometers to centimeters first!

   Radius of silver atom = 160 pm x 10^–12 m/pm x 100 cm/m = 1.6 x 10^–8 cm.
   Number of silver atoms (N_Ag ) = thickness/2r = 1.5 x 10^–4 cm/3.2 x 10^–8 cm = 4700 atoms.

7. Balance the following chemical equation for the formation of Tollens’ reagent in this experiment.
   {14022_Answers_Reaction_1}

Introduction

Create a beautiful silver ornament to demonstrate a practical application of an oxidation–reduction chemical reaction. Simply combine four solutions in a glass ornament ball, swirl and voilá—a thin, lustrous silver coating plates out on the inside of the ornament. The process “reflects” the way silver mirrors are actually produced!

Concepts

• Oxidation–reduction
• Tollens’ test
• Reducing sugars
• Metric measurements

Background

Mirrors, also called “looking glasses,” have been known since ancient times. The earliest mirror artifacts, dating back more than three thousand years, have been found in China and the Middle East. These mirrors were made by hammering and polishing thin sheets of metal, such as bronze, copper or tin until the metal surface was smooth and flat. Glassmaking was developed in ancient Rome, and glass mirrors first appeared in about the 1st century AD. In the 1600s, craftsmen in Italy perfected a method of lining glass with a thin sheet of reflecting metal. The mirrors made this way were beautiful, but also very expensive—the pinnacle of this art of mirror-making is represented by the “Hall of Mirrors” at the Palace of Versailles (France). In 1835, the German chemist Justus von Liebig invented a silvering process to plate a sheet of glass with a thin layer of silver metal by reducing silver ions with a sugar solution. This cheaper chemical method of lining glass with a “silver mirror” ushered in the modern era of producing mirrors for common household uses.

The silver mirror reaction invented by Liebig will be used in this lab to make a silver holiday ornament. The overall reaction is a classic oxidation–reduction reaction between silver complex ions and dextrose in ammonia solution. Dextrose or glucose (“blood sugar”) is a simple carbohydrate. It is an example of a reducing sugar, so-named because it is capable of reacting with and reducing mild oxidizing agents such as Ag⁺ or Cu²⁺ ions. In this experiment, dextrose molecules, which are represented by the formula R–CHO, reduce Ag(NH₃)₂⁺ complex ions to form silver metal. The silver plates out as a thin coating on the inside of the glass ornament (Equation 1).

The aldehyde [R–C(H)=O] functional group in dextrose (see Figure 1) is oxidized to a carboxylate functional group (R–CO₂^–) in the process. The reduction of silver–ammonia complex ions is a general reaction that is characteristic of organic compounds containing the aldehyde functional group. Tollens’ test, which is based on this reaction, is a simple qualitative test used in organic chemistry to detect aldehydes. A “positive test result” is easy to identify—a silver mirror forms on the inside of a test tube or flask if an aldehyde is present.

Experiment Overview

The purpose of this experiment is to prepare a “silver mirror” holiday ornament by mixing silver–ammonia complex ions with dextrose inside a glass ornament ball. The silver complex ions needed for the reaction will be generated by mixing dilute silver nitrate, ammonium nitrate and sodium hydroxide solutions. By measuring the mass of silver in the ornament, you can calculate the thickness of the silver mirror and also estimate the number of atoms in the silver layer.

Materials

Prelab Questions

1. The diameter of a glass ornament sphere is 2⅝ inches. Convert the diameter of the sphere to centimeters and calculate its radius.
2. Use the following equation to calculate the approximate outer surface area (SA) of the ornament: SA = 4πr².
3. Assuming that the glass thickness of the ornament is 1.6 mm, estimate the percent difference in the surface area calculation for the inside versus the outside of the ornament sphere.
4. Oxidation–reduction reactions involve the loss and gain of electrons, respectively. Based on the mole ratios in Equation 1 for the reduction of Ag⁺ cations to Ag metal, how many electrons are gained by the aldehyde when it is oxidized to the carboxylate function group? Hint: Electrons must be balanced and “cancel out” in an oxidation–reduction reaction.

Safety Precautions

Sodium hydroxide solution is a corrosive liquid; it causes skin burns and serious eye damage. Acetone is a highly flammable liquid vapor; do not use near heat, sparks or open flames. Ammonium nitrate solution may be harmful if swallowed. Silver nitrate solution is corrosive to skin and eyes and will stain skin and clothing. The mixed solution in the flask may form a potentially explosive material if left standing and heated or allowed to dry. Do NOT mix the solutions beforehand—add them together in the glass ornament and follow the instructor’s directions for disposing of the leftover solution immediately after use. Rinse with copious amounts of water into a container for disposal of silver waste. 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 lab.

Procedure

1. Obtain a plain glass ornament. Wrap a piece of string around the circumference (widest part) of the ornament and mark the length of string.
2. Using a metric ruler, measure the marked-off length of string to the nearest 0.1 cm and record the length in the data table.
3. Gently grasp the “ornament holder” and carefully remove it from the top of the ornament ball. Set the ornament holder aside. Caution: The glass ornament is fragile—do not exert pressure.
4. Measure and record the mass of the glass ornament ball.
5. Using a Beral-type pipet, add about 2 mL of acetone to the ornament ball and swirl the liquid inside the ornament.
6. Pour the acetone into a waste beaker and allow the ornament ball to dry completely in air.
7. Measure 2.5 mL of silver nitrate solution using a graduated cylinder and pour the solution into a clean, dry 50-mL beaker.
8. Rinse the graduated cylinder with distilled water and pour out the rinse water.
9. Measure 2.5 mL of ammonium nitrate solution using the graduated cylinder and pour the solution into the beaker containing silver nitrate. Mix the combined solution using a stirring rod.
10. Rinse the graduated cylinder with distilled water and pour out the rinse water. Measure 5 mL of dextrose solution using the graduated cylinder and pour the dextrose into the completely air-dried ornament ball.
11. Rinse the graduated cylinder with distilled water and pour out the rinse water. Measure 5 mL of sodium hydroxide solution into the graduated cylinder.
12. Add the combined silver nitrate/ammonium nitrate solution from the beaker to the ornament ball, followed immediately by the sodium hydroxide solution.
13. Gently cover the opening of the ornament with a piece of Parafilm and swirl the solution. Rotate the ornament so the solution covers the entire inside surface of the ball. Continue swirling until an evenly distributed, shiny silver coating appears throughout the ornament. The process may take 5–10 minutes.
14. Carefully remove the Parafilm and pour the remaining solution into a labeled waste container. Rinse the ornament thoroughly with distilled water.
15. Using a Beral-type pipet, add about 2 mL of acetone to the inside of the ornament and swirl gently to cover the interior surface. Pour the acetone into a waste beaker and allow the ornament ball to dry completely in air.
16. Measure the mass of the air-dried silver ornament.
17. Carefully replace the ornament holder on top of the silver holiday ornament.

Student Worksheet PDF

14022_Student1.pdf