Materials Included In Kit

Additional Materials Required

Safety Precautions

Zinc chloride solution and granular zinc are severe skin irritants. Zinc metal dust can be flammable; dust may be present at the bottom of the bottle of granular zinc. Do not use zinc dust in this procedure. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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 zinc chloride solution may be poured off of the granular zinc and be disposed of according to Flinn Suggested Disposal Method #26b. The granular zinc can either be reused or discarded in the solid waste according to Flinn Suggested Disposal Method #26a.

Lab Hints

Clean pennies are necessary in this lab because a smooth surface is needed for the “silver” and “gold” to plate. Pennies can be cleaned with the salt/vinegar solution as described in the following: 1. Weigh out and place 2.5–3 g of sodium chloride and 15 mL of vinegar in a clean 100-mL beaker. 2. Clean two pennies by placing them in the sodium chloride/vinegar solution until they are shiny. 3. Remove the pennies using tongs, and rinse them thoroughly with water. Dry thoroughly with a towel. Note: Do not handle the clean pennies with your hands. The oils from your skin may interfere with the zinc-plating reaction. Alternatively, the pennies can be cleaned by soaking them in dilute hydrochloric acid solution. The pennies can then be scrubbed with steel wool before starting, if necessary.|Pre-1982 pennies are 95% copper and 5% zinc; post-1982 pennies are 97.6% zinc, coated with a thin electroplating of copper. Either pre- or post-1982 pennies can be used for this lab as long as they are very clean and shiny. Challenge your students to use this knowledge to alloy the penny without undergoing the zinc treatment.|This experiment has been revised from its original procedure due to safety considerations. The original procedure, which used zinc metal and 3 M sodium hydroxide solution presented a possible fire hazard for disposal. The method described here is safer to perform and eliminates the fire hazard.

Teacher Tips

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the lab activity.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry—Big Ideas 2 and 3

Enduring Understandings and Essential Knowledge
The type of bonding in the solid state can be deduced from the properties of the solid state. (2D)
2D2: Metallic solids are good conductors of heat and electricity, have a wide range of melting points, and are shiny, malleable, ductile, and readily alloyed.

Chemical reactions can be classified by considering what the reactants are, what the products are, or how they change from one into the other. Classes of chemical reactions include synthesis, decomposition, acid−base, and oxidation−reduction reactions. (3B)
3B3: In oxidation−reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons is oxidized, and the species that gains electrons is reduced.

Learning Objectives
2.25 The student is able to compare the properties of metal alloys with their constituent elements to determine if an alloy has formed, identify the type of alloy formed, and explain the differences in properties using particulate level reasoning.
2.26 Students can use the electron sea model of metallic bonding to predict or make claims about the macroscopic properties of metals or alloys.
3.8 The student is able to identify redox reactions and justify the identification in terms of electron transfer.

Science Practices
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
4.3 The student can collect data to answer a particular scientific question.
5.1 The student can analyze data to identify patterns or relationships.
5.2 The student can refine observations and measurements based on data analysis.
5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.

Answers to Prelab Questions

1. A student is preparing to study alloys on lab day. Look at Figure 1 in the Background section.
   1. Are the electrons stationary or mobile? Relate this to the statement “metals are good conductors.”

      The electrons are mobile throughout this representation of metallic bonding. Heat and electricity can easily travel through metals because the electrons are mobile.

   2. Can the metal ions easily move around? What does this mean when arguing that a metal is malleable and ductile?

      The metal ions are able to easily move around; therefore, the metal can be pounded and shaped—malleability.

2. Reason through the following particulate-level diagrams. Answer Parts a–d.
   {12382_PreLabAnswers_Figure_3}
   1. What type of alloys do these diagrams represent?

      Diagram a represents an interstitial alloy, and diagram b represents a substitutional alloy.

   2. Explain why these representations of metallic bonding are not stable?

      In diagram a, both atoms in this interstitial alloy are the same size. The atoms in the ‘in between’ spaces are forced and distort the structure. This representation weakens the alloy; increased bond lengths are weaker.
      
      In diagram b, the substituted atoms have a smaller radius. The bond lengths are longer and therefore decrease the attraction.

   3. Redraw correct representations of diagrams a and b.
      {12382_PreLabAnswers_Figure_4}
   4. Observe the new representations from 2c. How would the densities of each compare to the densities of the original metals?

      The density in the interstitial alloy representation would decrease because the interstitial atoms are so small. The density in the substitutional alloy would stay the same or have little change because the added element contains a similar atomic radii and occupies the same amount of space.

3. Consider a property pure metals possess that makes them more desirable than their alloys.

   Because pure metals contain electrons that move around more easily, they can be better conductors of electricity than alloys.

4. Determine the type of alloy formed in the following scenarios.
   1. Two different metals with similar atomic radii.

      Substitutional alloy

   2. Two different metals with vastly different atomic radii.

      Interstitial alloy

   3. A metal and a nonmetal, where the nonmetal has a much smaller atomic radii than the metal.

      Interstitial alloy

5. Provide an example of each of the following scenarios.
   1. An alloy formed from being molten.

      Red gold for jewelry making is composed of gold and copper. They are molten together.

   2. An alloy formed at room temperature.

      When gallium and indium metals are rubbed together, an alloy is formed at room temperature.

6. Students should come to the conclusion that the silver penny should be heated to make the alloy brass. Using tongs, place the silver penny on the hot plate until the penny turns a golden color. Using a heat-resistant glove or tongs, flip the penny every 30 seconds to avoid burning.

   Use tongs to remove the penny from the hot plate and immediately dip the penny into a fresh beaker of distilled water. The penny will be extremely hot and should be handled with tongs until it has cooled for several minutes.
   
   Guide students to search scholarly resources to determine if brass is an interstitial alloy or a substitutional alloy. Brass is a substitutional alloy because the copper and zinc metal ions have similar atomic radii, and the copper ions replace the zinc ions in the penny.
   
   See Lab Hints for percent compoition of pennies. You can challenge the students to alloy pennies without treating it with zinc coating. Place the penny on the hot plate and make observations.

References

Szczepankiewicz, Steven H. Journal of Chemical Education; 1995; 72, 386–388.

AP^® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013.

Introduction

Prepare for the AP^® Chemistry exam this year by studying metallic bonding, specifically focusing on the macroscopic properties of alloys versus pure metals in this activity. The introduction of elements like carbon or nickel to a base metal, copper or iron forms an alloy. The end products contain unique, useful properties not found in the pure metals. From eating utensils to tire rims, humans have been putting alloys to good use for thousands of years. Get excited about making your very own alloy from a penny on wet lab day! A thorough homework assignment solidifies the concepts needed for wet lab. The activity will also delve into redox processes, further preparing you for the exam.

Concepts

• Alloys vs. pure metals
• Metallic bonding
• Oxidation–reduction

Background

Pure metals typically have a small number of valence electrons available for bonding. The valence electrons appear to be free to move among all of the metal atoms, which therefore must exist as positively charged cations. Metallic bonding describes the attractive forces that exist between closely packed metal cations and free-floating valence electrons in an extended three-dimensional structure (see Figure 1).

The unique physical properties of malleability, ductility, high melting point and conduction in metals are a result of the structural features of metallic bonding depicted in Figure 1. A more thorough overview of metallic physical properties due to this type of bonding can be explained with band models, also known as molecular orbital models (refer to your AP^® Chemistry textbook).

An alloy is a mixture of two or more metals (or a metal and a non-metal fused together, often molten) dissolved in each other, so they differ from pure metals in that they contain more than one type of atom. The properties of an alloy are often very different than the properties of its components. There are two types of alloys: interstitial—different, smaller metal or nonmetal atoms are added between the spaces of the existing metal atoms and substitutional—different metal or non-metal atoms replace the existing metal atoms and are of similar size (see Figure 2). “Alloying” a metal may significantly change the pure metal’s physical properties. Because weak bonds exist between atoms in metallic bonding, malleability is possible. For example, because the empty spaces in an interstitial alloy fill up, the alloy can become more rigid rather than more malleable. In fact, most metals in their pure form are not as useful as their alloyed form due to their malleability. Steel, for example, is an interstitial alloy made from iron and carbon. Pure iron is soft, making it easily malleable and ductile. At the particulate level, the metal atoms easily move about. Upon the addition of carbon, the carbon-iron bonds strengthen the metallic bonding, making the end product stronger and more resistant to malleability and ductility. Examples of some alloys and their components are listed in Table 1.

Experiment Overview

Complete the following homework set before lab day. The dry portion of this experiment—the homework set—solidifies the concepts of metallic bonding and alloys. To determine the unknown on lab day, bring in your own shiny penny sample. A guided procedure leads you through coating the penny with a new metal. Then, determine with your partner the best and most efficient method of alloying the penny and determine the identity of the alloy.

Prelab Questions

Complete the Prelab Homework Assignment (Student PDF).

Safety Precautions

Zinc chloride solution and granular zinc are severe skin irritants. Zinc metal dust can be flammable; dust may be present at the bottom of the bottle of granular zinc. Do not use zinc dust in this procedure. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Student Worksheet PDF

12382_Student1.pdf