CH 103: PERCENT HYDROGEN PEROXIDE In today’s experiment you will be given a solution that has an unknown concentration of hydrogen peroxide (H2O2). You will measure this concentration by determining the volume of oxygen (O2(g)) produced from the catalytic decomposition of H2O2(aq). What is a catalyst? A catalyst speeds up a chemical reaction. It is recycled during the reaction. It is not permanently created. And it is not permanently consumed. In today’s experiment yeast will be the catalyst.

A REVIEW OF LEWIS STRUCTURES What is the Lewis structure for hydrogen peroxide (H2O2)? What is the Lewis structure for hydroxide ion (OH-)? Does hydroxyl radical ( ) have a complete octet? No; therefore, it is very reactive. Similarly, hydroperoxyl radical ( ) does not have a complete octet and is very reactive. What is the Lewis structure for oxygen (O2)? What is the Lewis structure for water (H2O)?

THE CATALYTIC DECOMPOSITION OF H2O2 The reaction mechanism for the catalytic decomposition of H2O2 has 5 steps. Use your Lewis structures to account for every valence electron in this mechanism. Is the catalyst recycled? Yes, it either losses or gains an electron in each of the first 4 steps. Therefore, the catalyst is not permanently changed. Do the highly reactive hydroxyl radicals ( ) cancel? Yes. Radicals often have an incomplete octet; therefore, they are too reactive to form stable products. Do the highly reactive hydroperoxyl radicals ( ) cancel? Yes. What is the net reaction?

EXPERIMENTAL APPARATUS The rubber tubing connecting Flask B with Beaker D must be filled with water. The water level in Flask B must come to just below the short inlet tube. The volume of O2(g) produced by the catalytic decomposition of H2O2 equals the volume of water collected in Beaker D.

THE IDEAL GAS LAW The Ideal Gas Law will be used to convert this volume of O2(g) to moles of O2(g).

THE IDEAL GAS LAW The pressure of an ideal gas is 745 mm Hg (745 Torr). Its temperature is 45° C. And its volume is 514 mL. Calculate the moles of this gas. Convert the pressure to atmospheres. Convert the temperature to Kelvin. Convert the volume to liters. Calculate the moles of ideal gas.

DALTON’S LAW OF PARTIAL PRESSURES Dalton’s Law of Partial Pressures says that the total pressure (Ptotal) exerted by a mixture of gases in a container equals the sum of individual pressures for each gas. Ptotal = PA + PB + PC + … John Dalton

DALTON’S LAW OF PARTIAL PRESSURES In today’s experiment the total press (Ptotal) in flask B equals the pressure of oxygen (PO2) plus the pressure of water vapor (PH2O). This Ptotal also equals the barometric pressure (Pbar). Ptotal = Pbar = PO2 + PH2O PO2 = Pbar – PH2O The decomposition of a H2O2 solution displaces 315 mL of water at 14° C. The vapor pressure of water at 14° C is 12.0 mm Hg or 12.0 Torr. The atmospheric pressure is 746 mm Hg or 746 Torr. What is the pressure of O2(g)? PO2 = Pbar – PH2O = 746 mm Hg – 12.0 mm Hg = 734 mm Hg How many moles of O2(g) was produced?

SAFETY Give at least 1 safety concern for the following procedures that will be used in today’s experiment. Using H2O2. This is an irritant. Wear your goggles at all times. Immediately clean all spills. If you do get H2O2 in your eye, immediately flush with water. Using glassware. Glass can easily break and form shards. Wear your goggles at all times. Your laboratory manual has an extensive list of safety procedures. Read and understand this section. Ask your instructor if you ever have any questions about safety.

SOURCES McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. Park, J.L. 2004. ChemTeam: Photo Gallery Menu. Available: http://dbhs.wvusd.k12.ca.us/webdocs/Gallery/GalleryMenu.html [accessed 16 November 2006]. Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company. Schumb, W.C., C.N Satterfield, R.L. Wentworth. 1955. Hydrogen Peroxide. New York, NY: Reinhold Publishing Corporation.