Exp 4: The Identification of an Unknown Solution
It is a given that the unknown solution in this experiment contains a cation belonging to the alkaline-earth family and an anion belonging to the halogen family. These compounds are relatively soluble in water, but there are several other salts of the alkaline-earths that vary in solubility. By carrying out ionic reactions between solutions of each of these cations with solutions of selected anions, such as sulfate, you will have a reference with which to compare your unknown solution's reactions with those same anions.
Halides, the anion form of halogens, can be oxidized to the elemental form. In fact, one kind of replacement reaction is that between a halide and a halogen. A halogen will replace a less active halogen (in it's halide form) from a compound. By carrying out a series of replacement reactions between halogens and halides, you can determine the relative oxidizing strengths of the halogens and use this information to identify the unknown halide in your solution.
I. Relative Solubilities of Alkaline Earths
Now add the nitrate salts of magnesium, calcium, strontium, and barium so that each alkaline earth cation is tested with each of the anions from above. Be sure to note how many drops are added to get any precipitates and the appearance of each. Stop adding after 12 drops if there is still no ppt, but check after a few minutes to see if there is any delayed reaction. Leave the last well of each for the unknown.
II. Relative Oxidizing Powers of the Halogens
To each of 3 more small test tubes add about 1 mL of bromine water and 1 mL cyclohexane. Then add 1 mL of 0.1 M NaCl to the first test tube, and the same amount of NaBr to the second, and the same amount of NaI to the third. Stopper and shake well. Compare the color of the top layer to that in the reference test tubes from above. Decide if Br2 is a stronger oxidizing agent than the halogen that was produced.
Repeat the above procedure twice, first using chlorine water and then iodine water. Again, decide whether or not Cl2 or I2 are stronger oxidizing agents than the halogen that was produced.
III. Identification of the Unknown Alkaline Earth Halide