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CH 103: PERCENT HYDROGEN PEROXIDE

CH 103: PERCENT HYDROGEN PEROXIDE. In today’s experiment you will be given a solution that has an unknown concentration of hydrogen peroxide (H 2 O 2 ). You will measure this concentration by determining the volume of oxygen (O 2(g) ) produced from the catalytic decomposition of H 2 O 2(aq) .

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CH 103: PERCENT HYDROGEN PEROXIDE

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  1. 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.

  2. 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)?

  3. 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?

  4. 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.

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

  6. 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.

  7. 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

  8. 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?

  9. 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.

  10. 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.

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