The Lewis structure of the CFC-12 molecule is: [structure would be given]

  1. Give the correct chemical name for this compound.

  2. Describe the geometrical shape of the compound and estimate the Cl-C-Cl angle.

  3. Identify the type of hybridization exhibited by the central carbon atom.

  4. The energy of the C-Cl bond is 327 kJ/mol bonds; the energy of the C-F bond is 485 kJ/mol bonds. Explain this difference.

or, alternatively,

  1. The energy of the C-Cl bond is 327 kJ/mol bonds. Would you predict the energy of the C-F bond to be higher or lower? Explain your answer.

  2. Calculate the frequency, in s-1, of the radiation required to break a C-Cl bond.

  3. Calculate the wavelength (in nm) of the radiation required to break a C-Cl bond.

  4. Radiation of this wavelength falls within which region of the spectrum?

  5. What is the practical significance of the fact that radiation breaks C-Cl bonds in CFCs?

AP CHEMISTRY 1999, SECTION II, PART B, QUESTION 4

Original Question

Write the formulas to show the reactants and products for any FIVE of the laboratory situations described below. Answers to more than five choices will not be graded. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations.

Example: A strip of magnesium is added to a solution of silver nitrate.

  1. Calcium oxide powder is added to distilled water.

  2. Solid ammonium nitrate is heated to temperatures above 300oC.

  3. Liquid bromine is shaken with a 0.5 M sodium iodide solution.

  4. Solid lead(II) carbonate is added to a 0.5 M sulfuric acid solution.

  5. A mixture of powdered iron(III) oxide and powdered aluminum metal is heated strongly.

  6. Methylamine gas is bubbled into distilled water.

  7. Carbon dioxide gas is passed over hot, solid sodium oxide.

  8. A 0.2 M barium nitrate solution is added to an alkaline 0.2 M potassium chromate solution.

(SOURCE: CEEB, 1999b, p. 45)

Suggested Modification



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OCR for page 43
Learning and Understanding: Improving Advanced Study of Mathematics and Science in U.S. High Schools - Report of the Content Panel for Chemistry The Lewis structure of the CFC-12 molecule is: [structure would be given] Give the correct chemical name for this compound. Describe the geometrical shape of the compound and estimate the Cl-C-Cl angle. Identify the type of hybridization exhibited by the central carbon atom. The energy of the C-Cl bond is 327 kJ/mol bonds; the energy of the C-F bond is 485 kJ/mol bonds. Explain this difference. or, alternatively, The energy of the C-Cl bond is 327 kJ/mol bonds. Would you predict the energy of the C-F bond to be higher or lower? Explain your answer. Calculate the frequency, in s-1, of the radiation required to break a C-Cl bond. Calculate the wavelength (in nm) of the radiation required to break a C-Cl bond. Radiation of this wavelength falls within which region of the spectrum? What is the practical significance of the fact that radiation breaks C-Cl bonds in CFCs? AP CHEMISTRY 1999, SECTION II, PART B, QUESTION 4 Original Question Write the formulas to show the reactants and products for any FIVE of the laboratory situations described below. Answers to more than five choices will not be graded. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations. Example: A strip of magnesium is added to a solution of silver nitrate. Calcium oxide powder is added to distilled water. Solid ammonium nitrate is heated to temperatures above 300oC. Liquid bromine is shaken with a 0.5 M sodium iodide solution. Solid lead(II) carbonate is added to a 0.5 M sulfuric acid solution. A mixture of powdered iron(III) oxide and powdered aluminum metal is heated strongly. Methylamine gas is bubbled into distilled water. Carbon dioxide gas is passed over hot, solid sodium oxide. A 0.2 M barium nitrate solution is added to an alkaline 0.2 M potassium chromate solution. (SOURCE: CEEB, 1999b, p. 45) Suggested Modification

OCR for page 43
Learning and Understanding: Improving Advanced Study of Mathematics and Science in U.S. High Schools - Report of the Content Panel for Chemistry The panel would like to see the reaction/equation question tied more closely to phenomena and laboratory observation. In each of the cases below, the original version (a, b, c, etc.) and a suggested revision (a', b', c', etc.) are given. (The panel also is not convinced that requiring ionic equations is the most appropriate strategy.) (a) Calcium oxide powder is added to distilled water. (a') Calcium oxide powder is added to distilled water. Write the equation for the reaction, and indicate whether the final solution will have a pH less than-, equal to-, or greater than 7. (b) Solid ammonium nitrate is heated to temperatures above 300 °C. (b') The vapors from 6 M HCl and 6M NH3 combine to form a white cloud. Write the equation for the reaction. (c) Liquid bromine is shaken with a 0.5 M sodium iodide solution. (c') 5.0 mL of a 0.5 M solution of sodium iodide is shaken with 5.0 mL of a water solution of bromine and 5.0 mL of hexane, C6H14. Describe what you would observe, and write an equation representing the reaction. (d) Solid lead(II) carbonate is added to a 0.5 M sulfuric acid solution. (d') 0.5 M sulfuric acid is added to a small quantity of lead(II) carbonate in a test tube. Describe the first thing you would notice, and write an equation representing the reaction. (e) A mixture of powdered iron(III) oxide and powdered aluminum metal is heated strongly. (e') A mixture of powdered iron(III) oxide and powdered aluminum metal is ignited with a burning piece of magnesium. Sparks, flames, and a pool of molten metal are formed. Write the equation for the reaction. What is the molten metal, and what does this imply about the reaction? AP CHEMISTRY 1999, SECTION II, PART B, QUESTION 6 Original Question Answer the following questions in term of thermodynamic principles and concepts of kinetic molecular theory. Consider the reaction represented below, which is spontaneous at 298 K. CO2(g) + 2 NH3(g) → CO(NH2)2(s) + H2O(l) ∆Ho298 = - 134 kJ For the reaction, indicate whether the standard entropy change, ∆So298, is positive, or negative, or zero. Justify your answer. Which factor, the change in enthalpy, ∆Ho298, or the change in entropy, ∆So298, provides the principal driving force for the reaction at 298 K? Explain. For the reaction, how is the value of the standard free energy, ∆Go, affected by an increase in temperature? Explain.

OCR for page 43
Learning and Understanding: Improving Advanced Study of Mathematics and Science in U.S. High Schools - Report of the Content Panel for Chemistry Some reactions that are predicted by their sign of ∆Go to be spontaneous at room temperature do not proceed at a measurable rate at room temperature. Account for this apparent contradiction. A suitable catalyst increases the rate of such a reaction. What effect does the catalyst have on ∆Go for the reaction? Explain. (SOURCE: CEEB, 1999b, p. 47) Suggested Modification The panel suggests that Section II, Part B, Question 6 of the AP exam, which deals with thermodynamic principles and KMT concepts, could be directed to higher levels of thinking in part (a) by relating a simple system, such as an ice cube melting, to enthalpy, entropy, and free energy. The student could be asked to explain the process using the above terms and appropriate equations. Part (b) could be similar but related to something common, such as the possible oxidation of sucrose, which has a large negative free energy. The student could discuss why the sugar does not spontaneously combust on the kitchen table since the free energy is favorable. Included in the explanation would be descriptions of the differences between thermodynamic and kinetic stability. AP CHEMISTRY 1999, SECTION II, PART B, QUESTION 7 Original Question Answer the following questions, which refer to the 100 mL samples of aqueous solutions at 25ºC in the stoppered flasks shown above (four partially full flasks are shown, each containing an equal volume of 0.10 M solutions of NaF, MgCl2, C2H5OH, and CH3COOH, respectively). Which solution has the lowest electrical conductivity? Explain. Which solution has the lowest freezing point? Explain. Above which solution is the pressure of water vapor greatest? Explain. Which solution has the highest pH? Explain. (SOURCE: CEEB, 1999b, p. 48) (NOTE: On the exam, students were asked to answer either this question or the next question concerning principles of chemical bonding and molecular structure, but not both.) Suggested Modification The panel suggest the following modified question: Modification 1. Do not have all solutions be 0.10 M. Instead use 0.010 M NaF, 0.050 M MgCl2, 0.10 M C2H5OH, and 0.20 M CH3COOH. This change requires a student to think more