Materials Included In Kit

Additional Materials Required

Prelab Preparation

Preparing Unknowns for Part B

This kit contains at least 25% extra of each of the alkaline earth metal solutions needed for Part B (barium chloride, calcium chloride, magnesium chloride, and strontium chloride). To prepare unknowns for student use, transfer 25 mL of each solution to separate bottles marked with appropriate unknown labels (A, B, C, D). Be sure to record the identity of each unknown in your notes.

Safety Precautions

Calcium and magnesium are reactive, flammable solids and are possible skin irritants. Calcium may react violently with water and acids. Dispense only small sample sizes of these metals and use forceps or a spatula to handle them. Hydrochloric acid is toxic by ingestion and inhalation and is corrosive to skin and eyes; avoid contact with body tissues. Strontium and barium compounds are toxic by ingestion. Potassium iodate solution is moderately toxic and a strong irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Consult current Safety Data Sheets for additional safety, handling and disposal information.

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 reaction plates, even in the HCl well. Do NOT allow students to rinse the contents of their reaction plates in Part A down the drain. Students should 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. Dispose of this “Metal Waste” solution down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b only after you are sure all of the metal has reacted. The reaction plate contents from Part B can be washed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Excess barium chloride solution can be disposed of by converting it to insoluble barium sulfate according to Flinn Suggested Disposal Method #27h. Excess potassium iodate can be disposed of by reduction according to Flinn Suggested Disposal Method #12a.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs. The lab work for this experiment can reasonably be completed in one 50-minute lab period.
• The concentration of hydrochloric acid in Part A allows students to observe vigorous but still safe reactions with both calcium and magnesium. 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 by the teacher to show that aluminum also exhibits typical metal activity when the acid concentration is high enough. Do NOT allow students to test the higher HCl concentration. Reaction of calcium with more concentrated acid is violent, unsafe and potentially explosive.
• Review with students the nature of the optional burning match test to identify hydrogen. The test results are not needed to answer the Post-Lab Questions or to achieve the experiment objectives.
• The following assumptions have been made in designing the experiment objectives and writing appropriate Post-Lab Questions. (a) Students should be able to recognize charges on common ions and write correct formulas for ionic compounds. (b) Students may not be familiar with the classification of chemical reactions and may not have experience writing complete or net ionic equations. Teachers who have already covered these topics may wish to assign additional post-lab questions to build on prior learning.
• This lab provides a good opportunity to tell the heroic story of Marie Curie and her discovery of radium. 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 Part B—the solubility of alkaline earth metals 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 many 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. Extra pieces of calcium or magnesium metal may NOT be disposed of down the drain? Why not? Solids that do not dissolve in water should never be disposed of down the drain—they will clog the plumbing. Note: These metals may react violently with water. Disposing of these 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 (in units of kilojoules per mole) for five metals, listed in alphabetical order. 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.
   1. 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).

   2. 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)

      {12021_PreLabAnswers_Table_2}

Sample Data

Data Table A. Activity of Metals

Answers to Questions

1. Which group IIA 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 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.

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 describing 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 IIA 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. In Part B, which alkaline earth metal formed the most precipitates? The fewest?

   Barium formed precipitates with all of the testing solutions; magnesium did not form precipitates with any of the testing solutions.

9. Write a general statement that describes 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—solubility decreases as the atomic weight of the alkaline earth metal increases.

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

    The unknown metal described in the Sample Data is strontium. It showed the same solubility pattern as strontium chloride with all three testing solutions. Thus, strontium iodate was soluble, strontium carbonate and strontium sulfate were not.

11. (Optional) Write a chemical equation for each precipitate-forming reaction that was observed for strontium in Part B. Include the abbreviations (aq) and (s) to show what compound is responsible for the precipitate in each case.

    SrCl₂(aq) + Na₂CO₃(aq) → SrCO₃(s) + 2NaCl(aq)
    SrCl₂(aq) + Na₂SO₄(aq) → SrSO₄(s) + 2NaCl(aq)

Introduction

The periodic table is the most recognized symbol of chemistry across 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
• Alkaline earth metals
• Double replacement reaction

Background

The modern periodic table lists more than 112 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 IIA 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, marble and 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 halide anions are all 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. In Part A, the reactions of magnesium, calcium and aluminum with water and acids will be compared in order to determine the trend in metal activity within a group (Mg vs. Ca) and across a period (Mg vs. Al) in the periodic table. In Part B, the solubility of magnesium, calcium, strontium and barium compounds will be studied and used to identify an unknown alkaline earth metal.

Materials

Prelab Questions

1. Read the entire Procedure and the recommended Safety Precautions. Extra pieces of calcium or magnesium metal may NOT be disposed of down the drain. Why not?
2. The ionization energy of an element is defined as the amount of energy required to remove an electron from an individual atom. The following table gives the ionization energy (in units of kilojoules per mole) for five metals, listed in alphabetical order. Locate each of these metals on the periodic table and arrange them in order of rows and columns as in the periodic table.
   1. Describe the periodic trend in the ionization energy of elements within a group.
   2. Describe the periodic trend in the ionization energy of elements across a period.
      {12021_PreLab_Table_1}

Safety Precautions

Calcium and magnesium are reactive, flammable solids and possible skin irritants. Use forceps or a spatula to handle these metals. Hydrochloric acid is toxic by ingestion and inhalation and is corrosive to skin and eyes; avoid contact with body tissues. Strontium and barium compounds are toxic by ingestion. Potassium iodate solution is moderately toxic and a strong irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. 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 in the following figure. 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.
   {12021_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” in Data Table A.
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 in Data Table A. 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 for this litmus test in Data Table A.
11. Continue to watch each well for 1–2 minutes. Record any additional observations comparing the rates of reaction in Data Table A.
12. Use a pipet to add 20 drops of 0.5 M HCl to wells C1–C3. Measure and record the initial temperature of the solutions in well C1–C3 in Data Table A.
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 in Data Table A. If no changes are observed in a particular well, write NR in the data table.
17. After 2 minutes, measure the temperature of each solution in wells C1–C3. Record the final temperature of each solution in Data Table A.
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 Data Table A.
19. Continue to watch each well for 1–2 minutes. Record any additional observations comparing the rates of reaction in Data Table A.
20. Dispose of the well contents as instructed by your teacher. Rinse the reaction plate with distilled water before proceeding to Part B.

Part B. Solubility of Alkaline Earth Metal Compounds

21. Place the 24-well reaction plate on top of a sheet of black paper.
22. Referring to Data Table B as a guide, use a pipet to add 20 drops of alkaline earth metal solutions to the appropriate wells, as follows:
    • Magnesium chloride to wells A1–C1
    • Calcium chloride to wells A2–C2
    • Strontium chloride to wells A3–C3
    • Barium chloride to wells A4–C4
23. Use a clean pipet to add 20 drops of the unknown alkaline earth metal solution to wells A5–C5.
24. Referring to Data Table B as a guide, use a clean pipet to add 20 drops of testing solution to the appropriate wells, as follows:
    • Sodium carbonate to wells A1–A5
    • Sodium sulfate to wells B1–B5
    • Potassium iodate to wells C1–C5
25. Record observations in Data Table B 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 (no reaction) in the appropriate circle in the data table.
26. Dispose of the contents of the reaction plate as instructed by your teacher.

Student Worksheet PDF

12021_Student1.pdf