Last updated May 26, 2006

 Metals 2

Objective

The goals of this experiment are:

  • to appreciate the unique properties of the halogens, the members of Group 17 arising from a valence configuration of ns2np5 requiring one or more electron to reach a closed-shell configuration.
  • to observe the smooth trends in physical properties.
  • to observe the smooth trends in chemical properties, with the exception of some properties of fluorine.

Introduction

Halogens are economically important elements.

Fluorides are found in many naturally occurring substances such as fluorspar, CaF2, and cryolite, Na3AlF6. One of the major industrial uses for fluorine is in the preparation of uranium hexafluoride, UF6, used in separating uranium isotopes for nuclear power generation.

The electrolysis of sodium chloride produces chlorine, the 9th most industrially important chemical produced in the United States. Most chlorine is used as a bleaching (oxidizing) agent.

Both bromide (Br-) and iodide (I-) ions are found in seawater. An important use of bromine was as the gasoline additive dibromoethane, BrCH2CH2Br, in leaded gasoline. Bromine is used in dyes, as silver bromide AgBr for photographic film, and as NaBr and KBr for sedatives. The iodide ion is concentrated by seaweed and marine animals. KI is added to table salt to provide sufficient iodine for biological synthesis of the thyroid hormone thyroxine for humans.

  

The Group 17 elements are the most reactive group of nonmetals. They have seven electrons in their highest energy level (ns2np5). They often react with metals by accepting an electron to form halide ions (X-). The halogens usually form covalent compounds with other nonmetals and more electronegative metals by sharing one electron in a covalent bond. In both ionic and covalent compounds the halogens exhibit the -1 oxidation state.

The properties of the halogens and halide ions vary with increasing atomic number. Fluorine, F2, is an extraordinarily reactive element. In contrast, iodine, I2, is much less reactive. F2 is a very strong oxidizing agent and F- is a very weak reducing agent. I2, is a weak oxidizing agent and I- is a mild reducing agent.

A. Displacement Reactions

Most metal halides are soluble. Some exceptions are the insoluble compounds AgCl, AgBr, AgI and CaF2. Precipitation of these compounds from solutions of their ions in metathetical reactions is shown below:

Ag+ ( aq) + Cl- (aq) - > AgCl (s) (white)

Ag+ (aq) + Br- (aq)  - > AgBr (s) (cream)

Ag+ (aq) + I- (aq)  - > AgI(s) (yellow)

Ca2+ (aq) + 2 F- (aq) - > CaF2 (s) (white)

 

If several compounds precipitate in a mixture, the color of one may be masked by the color of another. Other tests will be needed to distinguish among the ions. All silver halides tend to decompose and darken in bright light.

B. Oxidation-Reduction Reactions

 The ease of loss of electrons (oxidation) of the halide ions to yield the free halogen:

(2 X- - > X2 + 2 e-) increases from top to bottom of the periodic table; thus, I- ions undergo oxidation with the greatest ease. The ease of oxidation follows the order:

 

 I- > Br- > Cl- > F-

The development of a system for identifying specific anions depends upon selectively varying the strengths of oxidizing agents. For example, iron (III) ions, Fe3+ will not oxidize Br-, Cl-, or F-, but will oxidize I- to I2:

2 Fe3+ (aq) + 2 I- (aq) - > I2 (s) + 2 Fe2+ (aq)

The formation of I2 confirms that the reaction occurs but it is difficult to distinguish the yellow color of I2 in many aqueous solutions. However, nonpolar I2 is soluble in an organic solvent such as cyclohexane. I2 can be extracted into cyclohexane from aqueous systems. The resulting cyclohexane solution is violet colored.

Chlorine, Cl2, oxidizes both I- and Br- ions.

Cl2 (g) + 2 Br- (aq) - > Br2 + 2Cl- (aq)

Cl2 (g) + 2 I- (aq) - > I2 (s) + 2 Cl- (aq)

 

In this experiment, Cl2 will be prepared in solution by the reaction of HCl (aq) with the hypochlorite ions, ClO-, from common household bleach (5% NaClO). This reaction should be carried out in the fume hood.

2 H+ (aq) + Cl- (aq) + ClO- (aq)  - > Cl2 (g) + H2O (l)

 

Excess chlorine further oxidizes I2 to colorless iodate, IO3-.

5 Cl2 (g) + I2 (s) + 6 H2O (l) - > 2 IO3- (aq) + 10 Cl- (aq) + 12 H+ (aq)

 

Persulfate ions, S2O82-, oxidize both I- and Br- ions to the free elements which vaporize from solution when it is boiled.

 

The formation of a white precipitate AgCl when AgNO3 is added confirms the presence of Cl- ions.

 

S2O82- (aq) + 2 Br- (aq) - > 2 SO42- (aq) + Br2 (l)

S2O82- (aq) + 2 I- (aq) - > 2 SO42- (aq) + I2 (s)

Ag+ (aq) + Cl- (aq) - > AgCl (s)

C. Formation of a Complex

Distinguishing one ion from another in a given family depends upon subtle differences in reactivity. If one ion has a stronger tendency to form soluble complex ions than do the others, this property is useful in selectively dissolving one insoluble substance in a mixture of precipitates.

2 M aqueous NH3 dissolves AgCl. But AgBr is only very slightly soluble in 2 M aqueous NH3, while AgI is completely insoluble.

AgCl (s) + 2 NH3 (aq) - > Ag(NH3)2+ (aq) + Cl- (aq)

 

PROCEDURE

 

Solutions of the halide ions in the following concentrations are on the reagent shelf:

A.     1 M NaF

B.     1 M NaCl

C.     1 M NaBr

D.     1 M NaI

E.      1 M NaBr + l M NaI

F.      1 M NaCl + l M NaBr

G.     1 M NaCl + 1 M NaI

H.     1 M NaCl + 1 M NaBr + 1 M NaI

Use DEIONISED WATER to rinse all equipment and to dilute reagents.

Perform each of the following tests on a known solution in tandem with testing your unknown solution - it will save you time!  Always mix solutions well. The formation of any solids, their colors, gas evolution, color changes in solution, and temperature changes indicate chemical changes. Record on your report form

CAUTION! Some compounds used in this experiment are corrosive and toxic. Review lab safety. Wear safety goggles and gloves at all times.

Part 1. Solubility of Halogens in Polar and Nonpolar Solvents:

All the reactions in part 1 should be done in the fume hood.

  1. Clean three small test tubes and label them 1,2 and 3. Place about 1 mL. of chlorine water in tube 1, 1 mL. of bromine water in tube 2 and 1 mL. of iodine water in tube 3.

Do not remove the tubes from the fume hood.

  1. Add about 20 drops of cylcohexane to each test tube and shake the contents vigorously. Record your observations.

Dispose of all your hexanes waste into the "Hexane waste" container in the fume hood.

Part 2. Reactions of Halides with Halogens:

All the reactions in part 2 should be done in the fume hood.

A. Reactions with Cl2

  1. Place 10 drops of 1M NaBr solution in one test tube and 10 drops of 1M NaI solution in another.
  2. Add 10 drops of chlorine water and 20 drops of cyclohexane to each test tube. Shake the test tubes vigorously.
  3. Allow them to settle for at least 2-3 minutes. Observe the color of the cyclohexane layer. By comparison with the results in Part 1 deduce which of the three halogens are present in the solution and whether a reaction has occurred.

Important note: Just because a color is present in the cyclohexane layer, do not assume that a reaction has occurred. The color must also match the color of the halogen expected as the product in the possible reaction.

B. Reactions with Br2

  1. Place 10 drops of 1M NaCl solution in one test tube and 10 drops of 1M NaI solution in another.
  2. Add 10 drops of bromine water and 20 drops of cyclohexane to each test tube. Shake the test tubes vigorously.
  3. Allow them to settle for at least 2-3 minutes. Observe the color of the cyclohexane layer. Use the color of the cyclohexane layer to determine if a reaction occurred between the halogen and the halide ion.

C. Reactions with I2

  1. Place 10 drops of 1M NaCl solution in one test tube and 10 drops of 1M NaBr solution in another.
  2. Add 10 drops of iodine water and 20 drops of cyclohexane to each test tube. Shake the test tubes vigorously.
  3. Allow them to settle for at least 2-3 minutes. Observe the color of the cyclohexane layer. Use the color of the cyclohexane layer to determine if a reaction occurred between the halogen and the halide ion.

Dispose of all your hexanes waste into the "Hexane waste" container in the fume hood.

Part 3. Complex Ion Formation:

A. Reactions of Halide Ions with Ag+ Ions

  1. Clean and dry a 12- well porcelain plate. Add 10 drops of 1M NaF to the first well, 10 drops of 1M NaCl to the second well, 10 drops of NaBr to the third well and 10 drops of NaI to the fourth well.
  2. Add 5 drops of 0.1 M AgNO3 to each well. Note the colors and record your observations. Write balanced equations for each reaction. Save solutions for the next procedure.

B. Reactions of Silver Halide precipitates with Aqueous Ammonia, NH3

  1. Allow the precipitates from the previous step to settle (at least 2 minutes). Use a clean, dry micropipet to draw up as much liquid as possible leaving a few crystals behind. If there is too much residual liquid, the following step will not work. Rinse the pipet if you change wells

All the next steps involving aqueous ammonia should be performed in the hood.

  1.  Add 15 drops of 5 M NH3(aq) to each well, which contains a precipitate.
  2. Stir thoroughly. Rinse and dry the glass rod each time when changing wells. Record your observations and write the appropriate equations.

Part 4. Identification of the Halide Ions in the Presence of Other Halide Ions:

  1. Prepare three clean 15 x 125 mm test tubes. Add 10 mL of distilled water to one test tube and mark the water level with a grease pencil. Discard the water and dry the tube. Place 1 mL of solution F (1M NaCl, 1M NaBr) in one test tube, 1 mL of solution G (1M NaCl, 1 M NaI) in another and 1 mL of solution H (1 M NaCl, 1M NaBr, 1 M NaI) in the third tube.

Caution! Sulfuric acid is corrosive. Wash immediately if you get any on you.

  1.  Add 5 mL of distilled water, 1 mL of 3M H2SO4 and 0.1 g of ammonium persulfate (also known as peroxodisulfate, (NH4)2S2O8) to each test tube.

Important note: Measure these quantities carefully because the addition of too much of these materials will give a false positive test for Cl-.

 Caution! Aviod breathing the vapors of Br2 and I2 from the beakers. They may be irritating to the skin and the mucous membranes. Heat the test tubes in a well-vented hood.

  1.  Add a boling stone or a boiling chip to each test tube and place the three test tubes in a dry 150 mL beaker. Place the beaker with the test tubes on the wire gauze supported by a ring stand in the hood and gently boil the solutions until they are colorless.

Important note: (NH4)2S2O8 oxidizes Br- and I- to Br2and I2 which boil away.

  1. Add enough water to each test tube to make a total of about 10 mL (up to the level of the pencil mark). Add 1 drop of 0.1 M AgNO3 solution to each test tube and mix thoroughly.

Important note: Since Br- and I- ions have been removed, a white precipitate indicates the presence of Cl- ions. If only a faint cloudiness is observed, Cl- ions should not be reported because Cl- are trace impurities in many chemicals.

  1. Record your observations and write appropriate equations.

Part 5. Displacement Reactions: Reactions of Halides with Ca2+ Ions

  1. Repeat Part 3 (A), step 1
  2. Add 10 drops of 2 M Ca(NO3)2 to each well. Mix thorughly, rinsing and drying a stirring rod if you change wells and observe carefully for any evidence of a chemical reaction. Record your observations and write appropriate balanced equations for each reaction.

Part 6. Identification of Unknown:

  1. Obtain a solution of an unknown halide or a mixture of halide salts from your TA.
  2. Carry out the tests similar to those in the preceding sections on your unknown solution in order to determine which halide ion or ions are present in your unknown. Record your observations and conclusions on your report form.

 

 

 

 

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