02 Alkaline Earths - The Alkaline Earths and the Halogens...

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The Alkaline Earths and the Halogens – Two Families in the Periodic Table Chad K. Bush 19 April 2006 Tina Masterson
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Bush 1 Abstract In this lab we will work with several solutions of various Group 2 alkaline earths and Group 7 halogens. In this way we will explore periodic trends in the relative solubilities of alkaline earth salts and the relative oxidizing power of the halogens and then use these periodic trends to identify an unknown solution of some alkaline earth halide. Introduction Elements are arranged in the Periodic Table in order of increasing atomic number first, and then into vertical columns of elements that have similar properties because of the repetition of electron configurations and other factors. The elements in each vertical column are referred to collectively as a family or group. In this experiment, we studied the properties and trends of elements in Group 2 (the alkaline earths) and Group 7 (the halogens). Alkaline earths are moderately reactive metals and include beryllium, magnesium, calcium, strontium, barium and radium. For this experiment, we did not use beryllium because of its toxicity or radium because of its radioactivity. All alkaline earths exist in solution as 2+ cations (for e.g., Mg 2+ , Ca 2+ , etc.). We can form salts of these alkaline earths by mixing them in solution with anions (such as CO 3 2- , SO 4 2- , IO 3 - , etc.), and if these salts are insoluble a precipitate will form. For example considering M 2+ as a generic alkaline earth cation in solution: M 2+ (aq) + SO 4 2- (aq) MSO 4 (s) if MSO 4 is insoluble (1a) M 2+ (aq) + 2 IO 3 - (aq) M(IO 3 ) 2 (s) if M(IO 3 ) 2 is insoluble (1b) We would expect that the solubilities of the salt of the alkaline earth cations with any one anion (such as CO 3 2- , SO 4 2- , IO 3 - , etc.) will exhibit a smooth trend consistent with the order of the cations in their period in the Periodic Table.
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Bush 2 Halogens are group 7 elements such as fluorine, chlorine, bromine, iodine, and astatine. In this experiment, we will not use fluorine for its high reactivity or astatine because of its radioactivity. They, like alkaline earths, are a very reactive period, but they tend to gain an electron and form 1- anions. Because of this property, halogens tend to be oxidizing agents (reagents that take electrons from other reactants, and thereby oxidize other reactants [and in the process become reduced themselves]). This sort of reaction takes the following form when X 2 is a halogen and X - its halide ion and Y 2 another halogen and Y - . X 2 (aq) + 2 Y - (aq) 2 X - (aq) + Y 2 (aq) (2) We will be able to tell if such a reaction that takes the form of equation 2 occurs by the color of the solution. In water and in organic solvents (such as hexane), the halogens have characteristic colors. Halide ions are colorless in water and insoluble in organic solvents. For example, Bromine (Br 2 ) in hexane (C 6 H 14 , which is lighter than and insoluble in water) is orange, while Cl 2 and I 2 have different colors. If Br 2
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