Last updated May 26, 2006
Metals 2
Objective
The goals of this experiment are:
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.
Do not remove the tubes from the fume hood.
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
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
C. Reactions with I2
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
B. Reactions of Silver Halide precipitates with Aqueous Ammonia, NH3
All the next steps involving aqueous ammonia should be performed in the hood.
Part 4. Identification of the Halide Ions in the Presence of Other Halide Ions:
Caution! Sulfuric acid is corrosive. Wash immediately if you get any on you.
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.
Important note: (NH4)2S2O8 oxidizes Br- and I- to Br2and I2 which boil away.
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.
Part 5. Displacement Reactions: Reactions of Halides with Ca2+ Ions
Part 6. Identification of Unknown:
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