H. Graden Kirksey Memphis State Un~versity Memphis, Tennessee 38152
The Oxidation Number ~ ' Silicon f An introductory experiment
Students can determine the oxidation numbers of some elements using an electrolytic cell. In experiments of this type one determines the mass of a n element deposited electrolytically during the passage of a measured amount of electric charge. Elements whose oxidation numbers can be determined in this way are copper, lead, nickel, zinc, hydrogen, and oxygen,l but there are others whose oxidation number cannot he determined by this method. The purpose of this experiment is to show an alternative method for the determination of the oxidation number of a n element. Consider the problem of determining the oxidation number of silicon. Silicon does not form soluble salts which dissolve in water as monoatomic ions, and there is no compound of silicon from which it can be deposited in an electrolytic cell under normal laboratorv conditions. Clearlv. " . students cannot determine the oxidation number of silicon directly from an electrolysis experiment. Another method must be devised. If silicon is placed in an aqueous sodium hydroxide solution, no reaction is observed for several minutes, but if the solution is heated, hydrogen gas is evolved as silicon dissolves in the basic solution. Consequently, under appropriate conditions, both an electric current and silicon are capable of causing hydrogen to be evolved from an aqueous solution of sodium hydroxide. Since 1.6 x 10-'9 A-s of charge is required to cause one atom of hydrogen to be evolved from a sodium hydroxide solution during the electrolysis of water, it is logical to say that the number of silicon atoms required to replace a measured number of hydrogen atoms from an identical solution must be chemically equivalent in charge to 1.6 x 10-19 A-s times the number of hydrogen atoms replaced. The hypothetical charge per atom of silicon, in units of 1.6 x 10-19 A-s, is its oxidation number. Experiment Five to ten pieces (50.3 g each) of silicon are dried in a crucible over a low flame for 5 min before measuring the mass of crucible and contents to +5 mg. The silicon is placed in a 20 x 150mm test tube containing about 1.5 g of technical sodium hydrox-
ide, and enough water is added to make the total volume about 5 ml. The test tube is fitted with a one-hole rubber stopper equipped with a delivery tube so that hydrogen gas may be collected by water displacement in a 1-1bottle. The test tube containing the reaction mixture is placed in a 100-ml beaker of gently boiling water. About 20 minis required to collect a 900-ml sample of hydrogen. At this time the test tube containing the reaction mixture is placed in a beaker of cool water to slow the reaction to a negligible rate. The unreacted silicon is isolated by washing several times with tap water and decanting the liquid phase, care being taken not to pour small pieces of silicon out with the wash liquid. A drop of phenolphthalein in each successive washing will clearly show students the benefit of repeated washings and indicate when all the basic solution has been washed from the silicon. The wet silicon is transferred back to the same crucible with the help of a polyethylene wash bottle, dried as before, and the change in mass recorded. The volume of hydrogen collected is measured by displacement with water from a large graduated cylinder. Results This experiment was f i s t conducted by a class of nonscience majors. These students were not prepared to calculate the density of hydrogen from the ideal gas law. The value of 8.04 x 1 0 - Q / m l was given the class as the density of hydrogen under the conditions a t which their samples were collected (736 mm and 296°K). This value is probably correct to *3% for all samwles collected. The av;rage oiidation number of silicon calculated from 24 determinations was +3.87 with a standard deviation of 0.22. There is little doubt that the principal experimental error occurs when students inadvertently pour small hits of silicon out with the wash liquid when washing the silicon sample after reaction. Since typical data show a loss in mass of silicon of 0.5 g and a volume of hydrogen collected near 900 ml, the loss of 15 mg of silicon during the washing would cause the calculated oxidation number to be low by 3%. 'Physical Science Group, "Physical Science 11," Prentice-Hall. Inc., Englewood Cliffs, New Jersey, 1972, p. 19-30.
Volume 51. Number 6.June 1974
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399
