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One of the most useful things about studying chemistry is discovering the amount of useful information contained in the periodic table. In today’s experiment, some of the properties of the Group 7A (Group 17) elements, known as halogens, and their compounds will be explored. The solubility properties of the halogens will be used to observe their reactions.
     
The more electronegative an element is, the more it wants an electron. Group 17 atoms that have become ions by gaining an extra electron, such as F^–, Cl^–, Br^–, and I^–, are called halides. Note that NaCl, sodium chloride, is a halide — specifically a chloride. Also note that anions such as halides must always be paired with a cation when found in the formula for a compound. Group 17 molecules that have not gained extra electrons, such as F₂, Cl₂, Br₂, and I₂, are halogens. In this experiment, the relative electronegativities of the halogens will be determined.

If a halide solution containing halide, X^–, is added to a solution of a different halogen, Y₂, there are two possibilities. When element Y is more electronegative than element X, then Y₂ will take the electron from X^–, leaving X₂ as a halogen. On the other hand, when Y₂ is less electronegative than X^–, then no reaction will take place, and Y₂ remains as the halogen. In terms of balanced equations:

       2 X^– + Y₂ 2 Y^– + X₂        (or)        2 X^– + Y₂ No Reaction

The properties of two solvents, water and hexane, will be useful in sorting out what happened. Water is a polar solvent and will solvate polar or ionic molecules. This means that a polar molecule (one that has a dipole) or an ionic compound may dissolve in water. A diatomic molecule is polar if the two atoms have different electronegativities. Identify which of the halogens and halides in the above equations are ionic and which are non-ionic.

      Non-polar solvents solvate non-polar molecules. Hexane is an organic molecule that is nonpolar. Since water is polar and hexane is non-polar, the two do not mix. When added together, two distinct, colorless layers are formed. The denser liquid, water, is on the bottom.

If colored compounds are added to a test tube containing water and hexane, the polarity of the compounds can be determined. If they are non-polar, they will color the hexane layer. If the colored compounds are polar, the color is observed in the water layer.

For the reaction postulated above, 2 X^– + Y₂ 2 Y^– + X₂, if Y₂ is green before any reaction takes place, the hexane layer is green because non-polar compounds will be in the hexane layer. As the reaction proceeds, the green disappears from the hexane layer because the Y₂ molecules are reacting and disappearing. The hexane will then take on the color of X₂. On the other hand, if X is more electronegative than Y, the more electronegative atom already has the electron, so no reaction would occur. Since no reaction occurs, the hexane layer would remain green, the color of Y₂.

Whenever a color change occurs, this is a clue that a reaction is taking place. Three halogens in aqueous (water) solutions will be available today: chlorine, bromine and iodine. Each of these halogens has a distinctly different color in hexane. Therefore, by observing the color of the hexane layer, the halogen that is present can be determined.