Assignment:
Various chemical questions
1. Answer the following problems about gases.
a) The average atomic mass of naturally occurring neon is 20.18 amu. There are two common isotopes of naturally occurring Neon as indicated in the table below.
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Isotope
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Mass (amu)
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Ne-20
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19.99
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Ne-22
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21.99
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(i) Using the information above, calculate the percent abundance of each isotope.
(ii) Calculate the number of N2-22 atoms in a 12.55 g sample of naturally occurring neon.
b) A major line in the emission spectrum of neon corresponds to a frequency of 1.34 x 10 14 s1. Calculate the wavelength, in nanometers, of light that corresponds to this line.
c) In the upper atmosphere, ozone molecules decompose as they absorb ultraviolet (UV) radiation, as shown by the equation below, Ozone serves to block harmful ultraviolet radiation that comes from the sun.
O3(G)→UV O2(G)+O(G)
A molecule ofO3(g) absorbs a photon with a frequency of 1.00 x 1015 s-1.
(i) How much energy, in joules, does the O3(g) molecule absorb per photon?
(ii) The minimum energy needed to break and oxygen-oxygen bond in ozone is 387 kJ mol-1. Does a photon with frequency of 1.00 x 1015 s-1 have enough energy to break this bond? Support your answer with a calculation.
Question-2
N2+3F2(G)→2NF3(g) ΔH°298= -264 KJ mol-1 ; ΔS°298 = -278 J K-1 mol-1
The following questions relate to the synthesis reaction represented by the chemical equation in the box above.
(a) Calculate the value of the standard free energy change, ΔS°298, for this reaction
(b) Determine the temperature at which the equilibrium constant Keq, for the reaction is equal to 1 .00 (Assume that ΔH° and ΔS° are independent of temperature).
(c) Calculate the standard enthalpy change, ΔH° That occurs when 0.256 mol sample of NF3(g) is formed from N2(g) and F2(g) at 1.0 atm and 298K.
The enthalpy change in a chemical reaction, is the difference between energy absorbed in breaking bonds in the reactants and energy released by bond formation, in the products.
(d) How many bonds are formed when two molecules of NF3 are produced according to the equation in the box above?
(e) Use both the information in the box above and the table of average bond enthalpies below to calculate the average enthalpy of the F - F bond.
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Bond
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Average Bond Enthalpy
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NºN
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946
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N-F
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272
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F-F
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?
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3. Answer the following questions that relate to the analysis of chemical compounds.
(a) A compound containing the elements C, H, N and O is analyzed. When a 1.2359 g sample is burned in excess oxygen, 2.241 g of C02(g) is formed. The combustion analysis also showed that the sample contained 0.0648 g of H.
(i) Determine the mass, in grams, of C in the 1.2359 g sample of the compound.
(ii) When the compound is analyzed for N content only, the mass percent of N is found to be 28.84 percent. Determine the mass, in grams, of N in the original 1.2359 g sample of the compound.
(iii) Determine the mass, in grams of O in the original 1.2359 g sample of the compound.
(iv) Determine the empirical formula of the compound.
(b) A different compound, which has the empirical formula CH2Br, has a vapor density of 6.00 g L-1 at 375 K and 0.983 atm. Using these data, determine the following.
(i) The molar mass of the compound.
(ii) The molecular formula of the compound.
Answer question 4 below. The section II score weighting for this question is 10 percent.
4. For each of the following three reactions in part (i) write a balanced equation for the reaction and in part (ii) answer the question about the reaction, In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances extremely ionized. Omit formulas for any ions or molecules chat are unchanged by the reaction. You may use the empty space at the bottom of the 'next page for scratch work but only the equations that are written in the answer boxes provided will be graded,
Example:
A strip of Magnesium is added to a solution of silver(1) nitrate
(i) Balanced equation
Mg+2 Ag+→Mg2+2 Ag
(ii) Which substance is oxidized in the reaction
(a) A solution of sodium hydroxide is added to a solution of lead(II) nitrate.
(i) Balanced equation
(ii) If 1.0 L volumes of 1.0 M solutions of sodium hydroxide and lead(II) nitrate are mixed together, how many moles of the product(s) will be produced? Assume the reaction goes to completion.
(b) Excess nitric acid is added to solid calcium carbonate.
(i) Balanced equation
(ii) Briefly explain why statues made of marble (calcium carbonate) displayed outdoors in urban areas are deteriorating.
(c) A solution containing silver(1) ion (un oxidizing agent) is mixed with a solution containing iron(II) ion (a reducing agent).
(i) Balanced equation
(ii) If the contents of the reaction mixture described above are filtered, what substance(s), if any, would remain on the filter paper.
Answer Question 5 and Question 6. The Section 11 score weighting for these questions is 15 percent each.
Your responses to these Questions will be graded on the basis of the accuracy and relevance of the information cited. Explanations should be clear and well organized. Examples and equations may be included in your responses where appropriate. Specific answers are preferable to broad, diffusive responses.
5 Fe2+(aq) + MnO4-(aq) + 8 H+(aq) -> 5Fe3+(aq) + Mn2+(aq) + 4 H2O(l)
5. The mass percent of iron in a soluble iron(II) compound is measured using a titration based on the balanced equation above.
(a) What is the oxidation number of manganese in the permanganate ion, MnO4-(aq)
(b) Identify the reducing agent in the reaction represented above.
The mass of a sample of the iron(II) compound is carefully measured before the sample is dissolved in distilled water. The resulting solution is acidified with H2SO4(aq) The solution is then titrated with MnO4-(aq) until the end point is reached.
(c) Describe the color change that occurs in the flask when the end point of the titration has been reached. Explain why the color of the solution changes at the end point.
(d) Let the variables g, M, and V be defined as follows:
g = the mass, in grams, of the sample of the iron(II) compound
M= the molarity of the MnO4-(aq)) used as the titrant
V = the volume, in liters, of MnO4-(aq)) added to reach the end point
In terms of these variables, the number of moles of MnO4-(aq)) added to reach the end point of the titration is expressed a M x V. Using the variables defined above the molar mass of iron (55.85 g mol-1) and the coefficients in the balanced chemical equation, write the expression for each of the following quantities.
(i) The number of moles of iron in the sample
(ii) The mass of iron in the sample, in grams
(iii) The mass percent of iron in the compound
(e) What effect will adding too much titrant have on the experimentally determined value of the mass percent of iron in the compound? Justify your answer.
6. Answer the fol1owing questions, which pertain to binary compounds.
(a) In the box provided below draw a complete Lewis electron - dot diagram for the IF3 rnolecule.
(b) On the basis of the Lewis electron-dot diagram that you drew (a), predict the molecular geometry of the IF3 molecule.
(c) In the SO2 molecule, both of the bonds between sulfur and oxygen have the same length. Explain this observation, supporting your explanation by drawing in the box below a Lewis electron-dot diagram (or diagrams) for the SO2, molecule.
(d) On the basis of your Lewis election-dot diagram(s) in part (c), identify the hybridization of the sulfur atom, in the SO2 molecu1e.
The reaction between SO2(g) and O2(g) to form SO3(g) is represented below
2 SO2(g) + O2(g) ↔ 3 SO3(g)
The reaction is exothermic. The reaction is slow a 25oC; however, a catalyst will cause the reaction to proceed faster.
(e) Using the axes provided in the next page draw the complete potential-energy diagram for both the catalyzed and uncatalyzed reactions. Clearly label the curve that represents the catalyzed reaction.
(f) Predict how the ratio of the equilibrium pressures PSO2/PSO3, would change when the temperature of the uncatalyzed reaction mixture is increased. Justify your prediction.
(g) How would the presence of a catalyst affect the change in the ration described in part (f)? Explain.
6. Answer each of the following in terms of principles of molecular behavior arid chemical concepts.
(a) The structures of glucose, C6H12O6, and cyclohexane, C6H12, are shown below
Identify the type(s) of intermolecular attractive forces in
(i) pure glucose.
(ii) pure cyclohexane
(b) Glucose is soluble in water but cyclohexane is not soluble in water. Explain.
(c) Consider the processes represented below:
Process 1: H2O(l) → H2O(g) ΔHo = +44.0 kJ mol-1
Process 2: H2O(l) → H2(g) + 1/2 O2 (g) ΔHo = +286.0 kJ mol-1
(ii) For each of the two processes, identify the type(s) of intermolecular or intraamolecular attractive forces that must be overcome for the process to occur.
(ii) Indicate whether you agree or disagree with the statement in the box below. Support your answer with a short explanation.
When water boils, H2O molecules break apart to form hydrogen molecules and oxygen molecules.
(d) Consider the four reaction-energy profile diagrams
(i) Identify the two diagrams that could represent catalyzed and an uncatalyzed reaction. Indicate which of the two diagrams represents the catalyzed reaction pathway for the reaction.
(ii) Indicate whether you agree or disagree with the statement in the box below. Support your answer with a short explanation.
Adding a catalyst to a reaction mixture adds energy that causes the reaction to proceed more quickly
2007 AP CHEMISTRY FREE-RESPONSE QUESTIONS (FORM B)
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First
Ionization Energy
kJ mol-1
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Second
Ionization Energy
kJ mol-1
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Third
Ionization Energy
kJ mol-1
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Element 1
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1.251
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2,300
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3,820
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Element 2
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496
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4,560
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6,910
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Element 3
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738
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1,450
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7,730
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Element 4
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1,000
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2,250
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3,360
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6. The table above shows the first three ionization energies for atoms of four elements from the third period of the periodic table. The elements are numbered randomly. Use the information in the table to answer the following questions
(a) Which element is most metallic in character? Explain your reasoning.
(b) Identify element 3. Explain your reasoning.
(c) Write the complete electron configuration for an atom of element 3.
(d) What is the expected oxidation state for the most common ion of element 2?
(e) What is the chemical symbol for element 2?
(f) A neutral atom of which of the four elements has the smallest radius?
Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent.
7. Answer the following questions about the structures of ions that contain only sulfur arid fluorine.
(a) The compounds SF4 arid BF3 react to form and ionic compound according to the following equation.
SF4 + BF3 -> SF3 BF4
(i) Draw a complete Lewis structure for the SF3+ cation in SF3BF4
(ii) Identify the type of hybridization exhibited by sulfur in tht SF3+ cation.
(iii) Identify the geometry o the SF3+ cation that is consistent with the Lewis structure drawn in part (a)(i).
(iv) Predict whether the F-S-F bond angle in the SF3+ cation is larger than, equal to, or smaller than 109.5°. Justify your answer.
(b) The compounds SF4 and CsF react to form an ionic compound according to the following equation.
SF4 + CsF -> CsSF5
(i) Draw a complete Lewis structure for the SF5- anion in CsSF5.
(ii) Identify the type of hybridization exhibited by sulfur in the SF5- anion.
(iii) Identify the geometry of the SF5- anion that is consistent with the Lewis structure drawn in part (b)(i).
(iv) Identify the oxidation number of sulfur n the compound CsSF5.