Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the magnesium ribbon into 2-cm pieces (one for each lab group).
2. Make enough copies of the periodic table so each student group has at least one.
3. Provide a silver waste container for Part 1 disposal.
4. Have students obtain four Beral-type pipets and label them as follows—water (H₂O), silver nitrate (AgNO₃), ammonia water (NH₃) and hydrochloric acid (HCl).

Safety Precautions

Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes; avoid all contact with body tissues. Silver nitrate solution is highly toxic and causes burns; it will stain skin and clothing. Calcium reacts with water to evolve flammable hydrogen gas; skin irritant. Magnesium is a flammable solid. Ammonia water is moderately toxic by ingestion and inhalation, is irritating to eyes and is a serious respiratory hazard. Sodium bromide and sodium iodide are slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Silver nitrate and silver waste solutions should be disposed of according to Flinn Suggested Disposal Method #11. Neutralize excess acid solution and flush down the drain with plenty of water according to Flinn Suggested Disposal Method #24b.

Teacher Tips

• More than enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period.
• With respect to the Part 1 solids—NaCl, NaBr, NaI—be sure to remind students to use only about ½ of a spatula head (or about 0.2–0.3 grams if a balance is available). Students often think that more is better and, in this case, more than the specified amount will not dissolve and will interfere with results.
• If desired, prepare labeled Beral-type pipets ahead of time to avoid wasting laboratory time.

Sample Data

Results. Part 1
To help identify the halide salts in Part 1:

Chloride—A white precipitate of silver chloride (AgCl) is formed upon adding a silver nitrate solution to a chloride. Silver chloride is readily soluble in ammonium hydroxide due to the formation of silver-ammonia complex ions.

Bromide—A cream-colored precipitate of silver bromide (AgBr) is formed upon addition of AgNO₃. It is soluble only in a large excess of ammonia water.

Iodide—A yellow precipitate of silver iodide (AgI) is formed initially, which does not dissolve in ammonia solution.

Answers to Questions

Part 1. Halide Salts

1. What effect does silver nitrate solution have upon each of the halide salt solutions?
   The chemical reaction between each of the halide salt solutions and the silver nitrate produces a solid precipitate, showing that silver salts of chloride, bromide, and iodide are insoluble in water.
2. Write the chemical equations for the reaction of silver nitrate with each of the three halide salt solutions. Label the solid product with (s) and write the name and color of each precipitate.
   AgNO₃(aq) + NaCl(aq) →  NaNO₃(aq) + AgCl(s). Silver chloride = white solid
   AgNO₃(aq) + NaBr(aq) →  NaNO₃(aq) + AgBr(s). Silver bromide = light cream solid
   AgNO₃(aq) + NaI(aq) →  NaNO₃(aq) + AgI(s). Silver iodide = pale yellow solid
3. What effect does ammonia water (ammonium hydroxide) have on each of the precipitates? Write the chemical equations for any reactions that occur.
   The ammonia water does not appear to dissolve the silver bromide or silver iodide. The silver chloride dissolves to form a silver ammine complex ion according to the equation shown below.
   AgCl(s) + 2NH₃ →  Ag(NH₃)₂ ⁺ + Cl^–

4. Which Group II alkaline earth metal is more reactive, magnesium or calcium?
   Calcium is the more reactive element.
5. Look at the position of the two elements on the periodic table. Write a general statement about position of metals and reactivity.
   Calcium is below magnesium on the table; therefore, it seems that metal reactivity increases as one moves further down the table within one column or family of metals.
6. Based on your observations, predict the order of reactivity of the following elements—strontium, barium, and beryllium.
   From least to most reactive—beryllium, strontium, and then barium.
7. Write the balanced chemical equations for the reactions of magnesium and calcium with hydrochloric acid. What is the gas in each case that causes the bubbling and fizzing?
   Mg(s) + 2HCl(aq) →  MgCl2 (aq) + H₂(g)
   Ca(s) + 2HCl(aq) →  CaCl2 (aq) + H₂(g)
8. In this lab, the oxidation reaction of two metals in acid is observed. When oxidation occurs, the solid metal loses electrons to form the aqueous metal cation. For example, in barium, Ba(s) → Ba²⁺ + 2e^–. Energy is needed to cause this oxidation— it is directly related to the ionization energy of the compound. The higher the ionization energy, the harder it is to lose electrons. Based on this information, which metal (calcium or magnesium) do you predict has a higher ionization energy? Explain.
   Magnesium is less reactive than calcium and therefore would be predicted to have a higher ionization energy. More energy is needed to ionize Mg to Mg²⁺ than is needed to ionize Ca to Ca²⁺.

Discussion

The chemical activity of metals generally increases down the columns (families) in the periodic table. For example, compare the reactivity of three Group I alkali metals—lithium, sodium, and potassium. If placed in water, the three will react very differently. Lithium will barely sizzle on the surface of the water; sodium will sizzle violently and possibly ignite on the water (See Flinn’s Safe Swimming with Sodium demonstration, Catalog No. AP8916); while potassium will most likely react explosively when in contact with water. Group II alkaline earth metals react in a similar way—as you move down the family, the reactivity increases. Calcium is much more reactive with hydrochloric acid than magnesium. Strontium and barium would be much more reactive than either magnesium or calcium.

Within a single family of metals, the reactivity of metals is related to their ionization energies. The lower the ionization energy, the more reactive the metal. The first ionization energy for magnesium is 738 kJ/mol, whereas it is only 590 kJ/mol for calcium. The general trend for metals going down a group is increasing reactivity and decreasing ionization energies. The pattern becomes less pronounced as you move from Group 1 toward Group 12, where the opposite pattern begins to be observed. For further information on ionization energy and periodicity, consult your chemistry textbook.

Teacher Handouts

11973_Teacher1.pdf

Introduction

Elements in one family or group of the periodic table tend to have certain characteristics or properties that are similar to others in the same family. Investigate two families of the periodic table—the alkaline earth metals and the halide salts.

Concepts

• Periodicity
• Alkaline earth family
• Halogen family

Background

In 1869, the Russian chemist Dmitri Mendeleev organized the known elements into a table—the Periodic Table. The periodic table today organizes nearly 118 chemical elements and helps scientists categorize, summarize and visualize important chemical data. The periodic table is also the tool that displays, at-a-glance, the similarities and differences among the elements. For example, with few exceptions, atomic weights increase regularly from left to right across the horizontal rows called periods. Mendeleev found that certain elements repeat similar physical and chemical properties at specific intervals in the table. Mendeleev organized these into vertical columns called families or groups.

In order to maintain a consistent periodicity in elemental properties, Mendeleev was forced to leave some “holes” in the periodic table. One of his crowning achievements was that he boldly predicted that elements existed to fill these gaps, even though they had yet to be discovered. In subsequent years, more elements were discovered, isolated and indeed were found to have the physical and chemical properties predicted by Mendeleev. Mendeleev’s brilliant work provided the groundwork for the modern periodic table.

In this laboratory activity, the reactivity and solubility of two families will be studied—the alkaline earth metals and the halide salts—and general periodic trends will be observed.

Materials

Safety Precautions

Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes; avoid all contact with body tissues. Silver nitrate solution is highly toxic and causes burns; it will stain skin and clothing. Calcium reacts with water to evolve flammable hydrogen gas; skin irritant. Magnesium is a flammable solid. Ammonia water is moderately toxic by ingestion and inhalation, is irritating to eyes and is a serious respiratory hazard. Sodium bromide and sodium iodide are slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Halide Salts

1. Label three small test tubes as Cl^–, Br^– and I^–, representing chloride, bromide and iodide ions, respectively.
2. Using an electronic balance, mass out approximately 0.2–0.3 grams of each of the following halide salts:

Sodium chloride (NaCl) as a source of Cl^– ions
Sodium bromide (NaBr) as a source of Br– ions
Sodium iodide (NaI) as a source of I^– ions

Using a spatula, place each salt into its respective test tube. (Note: While it is best to mass out each salt, if a balance is not available, use about ½ of a spatula head of each salt.)

3. Add 25 drops of distilled or deionized water into each test tube. Use a wood stick (a separate one for each tube) to stir the contents until all solid particles have dissolved. (Note: If necessary, add additional water dropwise to dissolve the solid.)
4. Use a labeled Beral-type pipet to add 8–10 drops of 0.1 M silver nitrate solution (AgNO₃) to each test tube, observing carefully as each drop is added. Record observations in the Data Table.
5. Use a labeled Beral-type pipet to add 30 drops of ammonia water (ammonium hydroxide) into each test tube, observing carefully as each drop is added. Stir the contents of each tube with its wood stick. Record observations in the Data Table. Allow any precipitate to settle for 1–2 minutes and record additional observations.
6. Dispose of the solutions in the tubes in a silver waste container as provided by your instructor. Rinse the tubes with water and combine rinsings in the silver waste container.

7. Label two small test tubes as Mg and Ca, representing magnesium and calcium. Set the test tubes in a test tube rack.
8. Use a labeled Beral-type pipet to add about 20 drops of 0.5 M hydrochloric acid solution to each tube.
9. Obtain a 2-cm piece of magnesium ribbon and obtain approximately 0.1 g of calcium turnings.
10. With the tubes setting in the test tube rack, use forceps or tongs to add each metal to the appropriate test tube. (Note: Stirring is not necessary.) Observe each tube and make immediate observations. Record observations in the data table.
11. Continue to watch the reactions for three minutes. Compare the rates of reactions in the two tubes. Carefully feel the bottom of each tube. Be sure to use your senses (except taste!) as observations are made. Record observations in the data table.
12. When the reactions are complete, rinse the contents of the tubes down the drain with plenty of water.

Student Worksheet PDF

11973_Student1.pdf