Oxygen from hydrogen peroxide: A safe molar volume molar mass

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Oxygen from Hydrogen Peroxide A Safe Molar Volume-Molar Mass Experiment John H. Bedenbaugh, Angela 0. Bedenbaugh, and Thomas S. Heard University of Southern Mississippi, Hattiesburg, MS 39406 The manganese dioxide-catalyzed thermal decomposition of potassium chlorate to produce oxygen is the classic molar volume-molar mass experiment that has been found in countless general chemistry laboratory textbooks for decades. However, it is a procedure that is falling into disrepute. The potential hazards inherent in this preparation of oxygen have been listed ( I ) and reports of explosions occurring during this experiment continue to mount (2).By direct suggestion (3) or implication (4) teachers are being advised not to incorporate this procedure into their program of experiments. unfortunately, teachers will find only a very limited number of experiments from which to choose a suhstitute nrocedure that will accomolish the same instructional objectives (5). Convinced of the hazard of usina- potassium chlorate, we wanted to develop a specific replacement procedure (one in which a reactant on decomposition would yield oxygen gas quantitatively) for use by students in determining molar volume or molar mass. We sought a method that would be simple, rapid, inexpensive, and in which we could use locally available chemicals. In 1926 Wikoff and Brown (6)published a procedure for the qualitative preparation of oxygen that is rarely cited today. A cake of compressed bakers' yeast was mixed with water to give a uniform suspension. When a 3% solution of hvdroeen was added to the susoension, a vigorous - neroxide . evolution of oxygen occurred. Bakers' yeast is exceptfonally rich in the enzvme. catalase, which catalyzes the decomposito produce oxygen and water. tion of hydrogen

view of generation system

expanded

gas

-

2H,0,

-

2H,O

Gas generation system and gas collection apparatus

+ O2

Wikoff and Brown vrovosed the reaction as a simple, inex. . pensive way to prepare oxygen, and also suggested an adaptation oftheexverimenr toastudy ofthe rate of liherationof oxygen. ~ o w e i e rthey , pointed out that the yield of oxygen from hydrogen peroxide via this route was not quantitative, hut they did not explain why. Years later Alyea included an updated version of this reaction among several methods of preparing oxygen (7). He used active dry yeast powder instead of moist yeast cake. Much more recently George and Johnson developed a procedure using dry yeast and 3% hydrogen peroxide for the safe, simple production of oxygen for use by elementary school children (8). Because the decomposition of hydrogen peroxide using comoressed veast cake had been reported to be nonquantitative;we decided to determine experimentally whither the reaction involving dry yeast powder is quantitative. In order to do that, however, i t was necessary to develop a method for introducing.the yeast into the hydrogen peroxide solution in a closed system so that the oxygen evolved could be measured. We sought a simple method appropriate for use by students. ~

Procedure Titration of Aqueous Hydrogen Peroxide with Standard Potassium Permanqanate We wanted to determineaccurately the concentration of the commercially available "3%" hydrogen peroxide to he used. Several samples (approximately 2.4 g each) were taken from a bottle of

hydrogen peroxide purchased at a local store, weighed exactly, titrated with standard potassium permanganate, and the results were averaged to determine the concentration to two decimal places. The procedure (using the data from one of the determinations) follows. A 2.3996-g sample of "3%" hydrogen peroxide solution was weighed into a 250-mL Erlenmeyer flask. Then 100 mL of 1 M sulfuricacidwas added, and the solution was titrated with commercia1 standard potassium permanganate (0.1005-0.0995 N) using magnetic stirring. The volume of permanganate solution used was 41.30 mL. The actual concentration of that particular sample af hydrogen peroxide solution was thus determined to he 2.93%. Catalytic Decomposition of Hydrogen Peroxide To Produce Oxygen We used essentially the same type of gas collection apparatus as that described by Peck, Irgolic, and O'Connor (5)but developed the gas generationsystem illustrated in the figure. The pencil should be cylindrical (circular base). It is critical that there be a good match between the holes in the stopper and the diameter of the pencil. Holes may have to he drilled in solid stoppers on site to achieve the fit necessary for easy movement of the pencil through the stopper. Prior to insertion of the ruhher stopper assembly into the test tube, the lower 2 cm of the round pencil was coated with petroleum jelly (Vaseline).Then dry active yeast powder was sprinkled on the coated end of the pencil, which was tapped several times to dislodge any yeast particles not adhering well to the hydrocarhon coating. The stopper assembly was then inserted carefully (so as not to dislodge the yeast) into the mouth of the test tube into which approximately 4.00 g of 3% hydrogen peroxide solution had been weighed. Volume 65

Number 5

May 1988

455

The gas collection apparatus, filled with tap water that had been left overnight in a container to come to room temperature, was connected with a 60-cm length of flexible tubing to the gas generation system, and the closed system was checked for air leaks by raising and lowering the leveling bulb. After it was determined that the system was gas-tight,the tubing was momentarily disconnected at the top of the buret, and the level of water in the buret was adjusted to approximately 1 cm above the zero mark by moving the leveling bulb. Then the free end of the tubing was reattached, thereby closing the system again, and the water level in the huret dropped to slightly below the zero mark. After again adjusting the leveling bulb until its water level matched that of the buret, the water level in the buret was read and recorded. Also recorded were the room temperature and barometric pressure. Reaction was initiated by pushing the pencil into the solution until the top mark on the pencil coincided with the top of the stopper. (Lubrication of this upper area of the pencil facilitated its movement through the stopper.) The yeast adhering to the petroleum jelly was thus brought into contact with hydrogen peroxide, which decomposed to give a smooth, steady evolution of oxygen. To insure good mixing the test tube was shaken gently several times. When the water level became stationary,the reaction was complete. The test tube was then shaken a final time, the water levels in the buret and bulb matchedaeain. " . and thevolumeofwater remainine in the buret read and recorded. Reaction was complete in less than three minutes. Only about 15 minutes was required for all operations involved in each determination. A volume correction (to he subtracted from the volume of oxygen measured) is required to compensate for the increase in gas volume produced by the insertion of the pencil into the liquid. This correction can be determined experimentally by simply measuring the amount of water displaced in the buret when the pencil is pushed the measured distance into the empty test tube with the entire system closed. Alternatively,knowing the diameter of the pencil and the additional length (in centimeters) of the pencil to be inserted, the volume correction can be calculated from the formula: u = rrzh, where h is the distance from mark to mark and, therefore, the distance of the increased intrusion of the pencil into the closed system. We assumed no loss of oxygen generated due to solubility in water. This appears reasonable since the tap water used in the measuring apparatus had not been degassed.

Data Obtained Following the standardization of the hydrogen peroxide solution with Dermanganate, the procedure described was carried out se\:en tnn& to get the data we report. A sample m a s of approximately 4 g was used to produce a volmne of 110-45 ml.1 that would utilipe most ofthecaoacitvof oxwen~, the buret. ~ n o w i ' nthe ~ volume of oxygen collected over water a t room temnerature and nressure from a aiven mass of hydrogen peroxide solutinn, the \,olume of oxvgen at 5'1'1' w3s calr~datrd.This value was romr~aredwith the throrrrical yield of oxygen a t S T P from the.given mass of hydrogen peroxide solution, and the error was calculated. Errors ranged from 0.18-3.13%) the average error was 1.80%. We concluded that, within experimental error, the reaction is quantitative. d

n

~

~

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Three Experiments Possible The ~roceduredescribed is the basis of three experiments that may be done by students to determine: (1) molar volume of oxygen, (2) molar mass of hydrogen peroxide, (3) concentration of hydrogen peroxide in aqueous solutions. A student can make any of these three determinations after measurine the volume of oxvaen obtained from the experimental procedure and notingthat two moles of hydroeen oeroxide vield one mole of oxygen (from the equation for .ihe ieaction): The data obtained in the laboratory are: mass of the hydrogen peroxide solution; room temperature and pressure; and initial and final huret readings. From these readings, after application of the correction factor for pencil insertion,

456

Journal of Chemical Education

one can determine the volume of oxygen collected over water a t room temperature and pressure. After calculating the partial pressure of the oxygen produced in the system, the volume of oxygen a t STP is determined via the usual gas-law calculations. To determine either the molar volume of oxygen or the molar mass of hydrogen peroxide, obviously the student must be given (or must first determine) the concentration of the aqueous hydrogen peroxide solution used in order to calculate themass of hydrogen peroxide from the mass of the hydrogen peroxide solution. The basic equations needed for these experiments are (1) Molar Volume of Oxygen -- 68.02 g H,O, g H,O, mL 0, (STP) x [molar mass of Hz02 = 34.01 glmol] (2) M d a r Mass of Hydrogen Peroxide X gH A -mL 0, (STP) 11,200 mL 0% (3) Concentration of Hydrogen Peroxide in a n Aqueous Solution 34.01 g H,O, (x)(gH,O, soln.) =mL 0, (STP) 11,200 mL 0, Comments This experiment is inherently safe because no heat is involved and no hazardous chemicals are used. Moreover, an effort has been made to make every aspect of the entire procedure safe. Thus a pencil is used to introduce the yeast to avoid cuts arising from pushing a glass rod through the hole in the rubber stopper. Also medicine dropper pipets are used rather than glass tubing because glass cutting is avoided and insertion into the holes of the rubber stoppers is made easier because of the tapered ends of the pipets. I n preparing "unknowns" for students to use in determining concentration of solutions, i t is suggested that various dilutions of 3% hydrogen peroxide he prepared rather than purchasing more concentrated solutions that are not readily available and that may be hazardous when used in this procedure. The short reaction time makes it quite feasible for the student to conduct the experimental procedure three times during a one-hour laboratory period to get an average value and a standard deviation. This is an exceptionally economical experiment. The cost of consumable supplies is only $0.015 per reaction. Thus 20 students could each perform the experiment three times for a total cost of $0.90 for consumables. Summary Dry yeast-induced decomposition of dilute aqueous hydrogen peroxide to produce oxygen is a safe, simple, economical replacement for thermal decomposition of potassium chlorate in molar volume and molar mass experiments. Moreover, the hydrogen peroxide decomposition~procedure can he the basis of an experiment in which the concentration of hydrogen peroxide solutions is determined. Literature Cited

oiPublic Inatructian: Raleigh, NC. 4. Flinn 1985 Cololog/Re/ermc~Monuai: Flinn Scientific Batavia. IL; p 103 6. Peck. L.: Irdic. K.:O'Connor.R. J. Chem. E d u c 1980.57.517.