Materials Included In Kit

Additional Materials Required

Safety Precautions

Calcium and magnesium are reactive, flammable solids and possible skin irritants. Use forceps or a spatula to handle these metals. Hydrochloric acid causes skin burns and eye irritation. Ammonium carbonate solution may cause skin and eye irritation. Barium compounds may be harmful if swallowed or inhaled. Avoid contact of all chemicals with eyes and skin. 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 hands thoroughly before leaving the laboratory.

Disposal

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. There will be unreacted magnesium metal remaining in the metal activity reaction plates, even in the HCl well. Do NOT rinse the contents of the reaction plates down the drain. Students may use a Beral-type pipet to remove and rinse the contents of the reaction plate into a central “Metal Waste” beaker containing 1 M hydrochloric acid. This “Metal Waste” solution may then be neutralized according to Flinn Suggested Disposal Method #24b only after all of the metal has reacted. The reaction plate contents from the solubility study may require licensed hazardous waste disposal due to the presence of barium. Mixed aqueous solutions containing barium salts should be collected in a heavy-metal waste container designated for licensed hazardous waste according to Flinn Suggested Disposal Method #27f.

Lab Hints

• Dispense only small pieces of calcium turnings and magnesium ribbon. Ideally, one small piece of calcium turnings should have a mass of about 0.05 g. Magnesium turnings may be substituted for ribbon, but do NOT use magnesium powder. Instructors may prefer to dispense these active metals from a central location.
• The concentration of hydrochloric acid in the metal activity study allows students to observe vigorous but still safe reactions with both calcium and magnesium. Do NOT increase the concentration of acid. Reaction of calcium with more concentrated acid is violent, unsafe and potentially explosive.
• Aluminum does not react with 0.5 M HCl. In order to demonstrate a positive reaction with aluminum, an acid concentration greater than about 3 M would be needed. If desired, the reaction of aluminum metal with more concentrated hydrochloric acid may be demonstrated to show that aluminum also exhibits typical metal activity when the acid concentration is high enough to penetrate and dissolve the impermeable aluminum oxide coating.
• Review the optional burning match test to identify hydrogen.
• A minimum number of solubility tests have been included. Other typical solubility tests for alkaline earth metal cations involve using ammonium oxalate, ammonium hydroxide and ammonium chromate to precipitate oxalate, hydroxide and chromate salts, respectively. Ammonium oxalate gives the same precipitate pattern with alkaline earth metals as does ammonium carbonate. Similarly, ammonium chromate gives the same solubility pattern as potassium iodate. Chromate compounds are known carcinogens and are not recommended for use in this experiment. Ammonium hydroxide can be used to show a positive result for magnesium ions (in the current procedure, magnesium is identified by negative test results with all three testing solutions). Hydroxides are the exception to the rule in the solubility pattern of alkaline earth metals—magnesium hydroxide and barium hydroxide form precipitates, while calcium and strontium hydroxide do not.
• Do not substitute sodium carbonate for ammonium carbonate solution in the solubility study. The high pH of sodium carbonate solution will cause Mg(OH)₂ to precipitate. Ammonium carbonate solution is less basic (pH = 8–9).
• Magnesium chloride does not make a good unknown in the metal solubility tests. Prepare three unknowns A, B, and C— containing calcium, strontium and barium chloride, respectively—and randomly assign them to students.
• The following prerequisite knowledge and skills are assumed for the Laboratory Report. Students should be able to recognize charges on common ions and write correct formulas for ionic compounds. Students may not be familiar with the classification of chemical reactions and may not have experience writing complete or net ionic equations.
• This lab provides a good opportunity to tell the story of Marie Curie and her discovery of radium. It is a heroic story of dedication, patience and persistence. Starting with one ton of a special uranium-depleted ore, Marie Curie carried out a series of exhausting large-scale precipitation reactions. She obtained 100 milligrams of a new, radioactive element— radium, the heaviest member of the alkaline earth metals. The separation worked because of the periodic trend identified in the solubility of alkaline earth metal compounds, which decreases as you go down the column in the periodic table. Radium chloride is ever-so-slightly less soluble than barium chloride. Working with concentrated hydrochloric acid, Marie Curie carried out successive, repeat fractional crystallization reactions with a mixture of barium and radium chlorides from the ore. With each successive crystallization, the precipitate gradually became enriched in the amount of the less soluble radium fraction, until finally almost pure radium chloride was obtained.

Answers to Prelab Questions

1. Read the entire Procedure and the recommended Safety Precautions. Do you think extra pieces of calcium or magnesium metal should be disposed of down the drain? Why or why not?

   No! Solids that do not dissolve in water should never be disposed of down the drain—they will clog the plumbing. In addition, however, these metals may react violently with water. Disposing of reactive metals down the drain may start fires or even explosions.

2. The ionization energy of an element is defined as the amount of energy required to remove an electron from an atom. The following table gives the ionization energy (in units of kilojoules per mole) for five metals, listed in alphabetical order.
   {14028_PreLabAnswers_Table_3}
   1. Locate each of these metals on the periodic table and arrange them in the same order of rows and columns as in the periodic table.
      {14028_PreLabAnswers_Table_4}
   2. Describe the periodic trend in the ionization energy of elements within a group.

      Ionization energy decreases as you go down a group in the periodic table.
      Mg > Ca > Sr

   3. Describe the periodic trend in the ionization energy of elements within a period.

      Ionization energy increases from left to right across a period in the periodic table.
      (Na < Mg and K < Ca)

3. Formation of barium sulfate, which is almost completely insoluble in water, is used as the basis of a classic procedure for analyzing the amount of barium in a sample. Write a chemical equation for the double replacement, precipitation reacton of an aqueous solution of barium nitrate with concentrated sulfuric acid.

   Ba(NO₃)₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HNO₃(aq)

Sample Data

Activity of Metals

Answers to Questions

1. Which Group 2 metal, magnesium or calcium, is more active? Cite your evidence.

   Ca is more reactive than Mg. Ca reacted with both water and HCl (sometimes violently in the case of HCl), whereas Mg reacted only with HCl, and then its reaction was much less vigorous than in the case of Ca.

2. Which period 3 metal, magnesium or aluminum, is more active? Cite your evidence.

   Mg is more reactive than Al. Mg reacted noticeably with HCl, while Al exhibited little or no evidence of reaction (a few bubbles here and there).

3. Rank the three metals tested in Part A from most active to least active.

   Ca > Mg > Al

4. Write a general statement describing the periodic trend in metal activity within a group (vertical column) of the periodic table.

   Metal activity increases as you go down a group in the periodic table (i.e., as the atomic weight of the element increases).

5. Write a general statement that describes the periodic trend in metal activity within a period (horizontal row) of the periodic table.

   Metal activity decreases from left to right across a period in the periodic table.

6. Locate the following metals on the periodic table: magnesium, potassium and sodium. Based on your answers to Questions 4 and 5, rank these metals in order of their expected activity, from most active to least active.

   K > Na > Mg

7. Litmus paper changes color in acidic (red) and basic (blue) solutions. The word alkaline is a synonym for basic. Give two reasons why the Group 2 metals are called alkaline earth metals.

   Litmus paper changed from red to blue (basic) when it was dipped into the water solution after reaction with calcium metal. This indicates that calcium reacted with water to form an alkaline solution. The name alkaline earth metal thus reflects two facts: they produce alkaline solutions and they are important components of the Earth’s crust and oceans.

8. Which alkaline earth metal formed the most precipitates? The fewest?

   Barium formed precipitates with all of the testing solutions, while magnesium did not form a precipitate with any of the solutions tested.

9. Write a general statement describing the periodic trend in the solubility of alkaline earth metal compounds.

   The solubility of alkaline earth metal compounds decreases as you go down the column in the periodic table (i.e., the solubility of their compounds decreases as the atomic weight of an alkaline earth metal increases).

10. Use the solubility pattern observed for the known and unknown alkaline earth compounds to deduce the identity of the unknown alkaline earth ion. Explain your reasoning.

    Answers will vary. The unknown metal described in the sample data is strontium. It showed the same solubility pattern as strontium chloride with each of the three testing solutions. Thus, strontium iodate is soluble, while strontium carbonate and strontium sulfate are not.

11. Using Equation 1 in the Background section as an example, write a chemical equation for each precipitate-forming reaction that was observed for strontium. Include the abbreviations (aq) and (s) to show what compound is responsible for the precipitate in each case.

    SrCl₂aq) + (NH₄)₂CO₃(aq) → SrCO₃(s) + 2NH₄Cl(aq) SrCl₂(aq) + Na₂SO₄(aq) → SrSO₄(s) + 2NaCl(aq)

Introduction

The periodic table is the most recognized symbol of chemistry around the world. It is a valuable tool that allows scientists not only to classify the elements but also to explain and predict their properties. Similarities and differences among the elements give rise to so-called periodic trends, both across rows and within columns of the periodic table. Recognizing periodic trends in the physical and chemical properties of the elements is key to understanding the full value of the periodic table.

Concepts

• Periodic table
• Periodic trends
• Group vs. period
• Alkaline earth metals
• Activity series
• Double replacement reaction

Background

The modern periodic table lists more than 114 elements, of which 92 are naturally occurring. Of these 92 elements, the eight most abundant elements together account for more than 98 percent of the mass of the Earth’s crust, oceans and atmosphere. Two of the eight most abundant elements on Earth are calcium and magnesium, which are present in both mountains and minerals, seawater and seashells. Calcium and magnesium are members of the Group 2 family of elements, the alkaline earth metals. Elements that share similar properties are arranged together within vertical columns, called groups or families, in the periodic table.

The alkaline earth metals—beryllium, magnesium, calcium, strontium, barium and radium—are a reactive group of metals. Because they combine easily with many other elements, the alkaline earth elements are not found on Earth in the form of their free metals. They exist in nature in the form of ionic compounds, such as calcium carbonate, CaCO₃. Calcium carbonate occurs naturally in limestone and marble as well as seashells.

The alkaline earth metals react with water, acids and bases and many nonmetals, including oxygen, sulfur and the halogens. The ease with which a metal reacts is called the activity of the metal. By comparing how fast or how vigorously different metals react, it is possible to rank the metals in order from most active to least active. This ranking—called the activity series of the metals—shows clear periodic trends, both within a group and across a period of elements in the periodic table.

Periodic trends are also observed in the solubility of alkaline earth metal compounds. Although their compounds with most halogen anions are water-soluble, alkaline earth metal compounds with other anions do not always dissolve in water. The solubility of alkaline earth metal compounds with different anions can be tested by carrying out so-called double replacement reactions. Reaction of calcium chloride with sodium carbonate, for example, leads to an exchange of anions between the two metals to give calcium carbonate, which is insoluble in water and precipitates out as a solid when the two solutions are mixed. The chemical equation for this reaction is shown in Equation 1, where the abbreviations (aq) and (s) refer to aqueous solutions and solid precipitates, respectively.

Experiment Overview

The purpose of this experiment is to identify periodic trends in the activity and solubility of the alkaline earth metals. The trend in metal activity within a group (Mg vs. Ca) and across a period (Mg vs. Al) in the periodic table will be determined by comparing the reactions of these metals with water and acids. The solubility of magnesium, calcium, strontium, and barium compounds will also be studied and used to identify an unknown alkaline earth metal.

Materials

Prelab Questions

1. Read the entire Procedure and the recommended Safety Precautions. Do you think extra pieces of calcium or magnesium metal should be disposed of down the drain? Why or why not?
2. The ionization energy of an element is defined as the amount of energy required to remove an electron from an atom. The following table gives the ionization energy (in units of kilojoules per mole) for five metals, listed in alphabetical order.
   {14028_PreLab_Table_2}
   1. Locate each of these metals on the periodic table and arrange them in the same order of rows and columns as in the periodic table.
   2. Describe the periodic trend in the ionization energy of elements within a group.
   3. Describe the periodic trend in the ionization energy of elements within a period.
3. Formation of barium sulfate, which is almost completely insoluble in water, is used as the basis of a classic procedure for analyzing the amount of barium in a sample. Write a chemical equation for the double replacement, precipitation reacton of an aqueous solution of barium nitrate with concentrated sulfuric acid.

Safety Precautions

Calcium and magnesium are reactive, flammable solids and possible skin irritants. Use forceps or a spatula to handle these metals. Hydrochloric acid causes skin burns and eye irritation. Ammonium carbonate solution may cause skin and eye irritation. Barium compounds may be harmful if swallowed or inhaled. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Always wash hands thoroughly before leaving the laboratory.

Procedure

Activity of Metals

1. In a weighing dish or small beaker, obtain 2 small pieces of calcium turnings.
2. Obtain 2 small pieces of magnesium ribbon, approximately 1-cm each, and a short piece of aluminum foil.
3. Place a 24-well reaction plate on top of a sheet of white paper, as shown. Note that each well is identified by a unique combination of a letter and a number, where the letter refers to a horizontal row and the number to a vertical column.
   {14028_Procedure_Figure_1}
4. Use a pipet to add 20 drops of distilled water to wells A1–A3.
5. Test the water in wells A1–A3 with a piece of red litmus paper and record the initial color for this “litmus test.”
6. Use forceps to add one piece of calcium to well A1.
7. Use forceps to add one piece of magnesium ribbon to well A2.
8. Tear off a 2-cm piece of aluminum foil and roll it into a loose ball. Add the aluminum metal to well A3.
9. Observe each well and record all immediate observations. If no changes are observed in a particular well, write NR (no reaction) in the data table.
10. Test the water in wells A1–A3 with a piece of red litmus paper and record any color changes.
11. Continue to watch each well for 1–2 minutes. Record any additional observations comparing the rates of reaction.
12. Use a pipet to add 20 drops of 0.5 M HCl to wells C1–C3 (the first three wells in Row C). Measure the initial temperature of the solutions in wells C1–C3 and record the values as an “observation” in the data table.
13. Use forceps to add one piece of calcium turnings to well C1.
14. Use forceps to add one piece of magnesium ribbon to well C2.
15. Tear off a 2-cm piece of aluminum foil and roll it into a loose ball. Add the aluminum metal to well C3.
16. Observe each well and record all immediate observations. If no changes are observed in a particular well, write NR in the data table.
17. Using a thermometer, measure the temperature of each solution in wells C1–C3. Record the temperature of each solution as an observation in the data table.
18. Is there evidence that a gas is being produced in wells C1–C3? Test the combustion property of the gas by bringing a lit match to the space just above each well C1–C3. Record any observations for this “match test” in the data table.
19. Continue to watch each well for 1–2 minutes. Record any additional observations comparing the rates of reaction.
20. Dispose of the well contents as instructed. Thoroughly rinse the reaction plate with distilled water before using the plate in the second part of this experiment.

Solubility of Alkaline Earth Metal Compounds

1. Place the 24-well reaction plate on top of a sheet of black paper.
2. Referring to the data table as a guide, use a pipet to add 20 drops each of the appropriate alkaline earth metal solution to the following wells. Note: Use a fresh or clean pipet for each solution.
   • Magnesium chloride to wells A1–C1
   • Calcium chloride to wells A2–C2
   • Strontium chloride to wells A3–C3
   • Barium chloride to wells A4–C4
3. Use a clean pipet to add 20 drops of the unknown alkaline earth metal solution to each well A5–C5.
4. Referring to the data table as a guide, use a clean pipet to add 20 drops each of the appropriate testing solution to the following wells.
   • Ammonium carbonate to wells A1–A5
   • Sodium sulfate to wells B1–B5
   • Potassium iodate to wells C1–C5
5. Record observations as follows: if a solid forms in a well, write PPT (precipitate) in the appropriate circle in the data table. If no solid is observed, write NR.
6. Follow the instructor’s directions for disposing of the contents of the reaction plate.

Student Worksheet PDF

14028_Student1.pdf