当前位置：搜档网 › ap06_sg_chemistry

ap06_sg_chemistry

AP? Chemistry

2006 Scoring Guidelines

The College Board: Connecting Students to College Success

The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed of more than 5,000 schools, colleges, universities, and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and 3,500 colleges through major programs and services in college admissions, guidance, assessment, financial aid, enrollment, and teaching and learning. Among its best-known programs are the SAT?, the PSAT/NMSQT?, and the Advanced Placement Program? (AP?). The College Board is committed to the principles of excellence and equity, and that commitment is embodied in all of its programs, services, activities, and concerns.

? 2006 The College Board. All rights reserved. College Board, AP Central, APCD, Advanced Placement Program, AP, AP Vertical Teams, Pre-AP, SAT, and the acorn logo are registered trademarks of the College Board. Admitted Class Evaluation Service, CollegeEd, connect to college success, MyRoad, SAT Professional Development, SAT Readiness Program, and Setting the Cornerstones are trademarks owned by the College Board. PSAT/NMSQT is a registered trademark of the College Board and National Merit Scholarship Corporation. All other products and services may be trademarks of their respective owners. Permission to use copyrighted College Board materials may be requested online at:

https://www.sodocs.net/doc/143713187.html,/inquiry/cbpermit.html.

Visit the College Board on the Web: https://www.sodocs.net/doc/143713187.html,.

AP Central is the official online home for the AP Program: https://www.sodocs.net/doc/143713187.html,.

Question 1

1. Answer the following questions that relate to solubility of salts of lead and barium.

(a) A saturated solution is prepared by adding excess PbI 2(s ) to distilled water to form 1.0 L of solution

at 25°C. The concentration of Pb 2+(aq ) in the saturated solution is found to be 1.3 × 10?3 M . The

chemical equation for the dissolution of PbI 2(s ) in water is shown below.

PbI 2(s ) →←

Pb 2+(aq ) + 2 I ?(aq ) (i) Write the equilibrium-constant expression for the equation.

2+[Pb ][I ]sp K ?=

One point is earned for the correct expression.

(ii) Calculate the molar concentration of I ?(aq ) in the solution.

By stoichiometry, [I ?] = 2

× [Pb 2+] , thus [I ?] = 2 × (1.3 × 10?3) = 2.6 × 10?3 M One point is earned for the correct concentration.

(iii) Calculate the value of the equilibrium constant, K sp . K sp = [Pb 2+][I ?]2 = (1.3 × 10?3)(2.6 × 10?3)2

= 8.8 × 10?9

One point is earned for a value of K sp that is consistent with the answers in parts (a)(i) and (a)(ii).

(b) A saturated solution is prepared by adding PbI 2(s ) to distilled water to form 2.0 L of solution at 25°C.

What are the molar concentrations of Pb 2+(aq ) and I ?(aq ) in the solution? Justify your answer. The molar concentrations of Pb 2+(aq ) and I ?(aq ) would be the same as in the 1.0 L solution in part (a)

(i.e., 1.3 × 10?3 M and 2.6

× 10?3 M , respectively). The concentrations of solute particles in a saturated solution are a function of the constant, K sp , which is independent of volume. One point is earned for the concentrations (or stating they are the same as in the solution described in part (a)) and justification.

Question 1 (continued)

does not change, does the molar concentration of Pb2+(aq) in the solution increase, decrease, or remain the same? Justify your answer.

[Pb2+] will decrease.

The NaI(s) will dissolve, increasing [I?]; more I?(aq) then combines with Pb2+(aq) to precipitate PbI2(s) so that the ion product [Pb2+][I?]2 will once again attain the value of 8.8 × 10?9 (K sp at 25°C).

One point is earned for stating that [Pb2+] will

decrease.

One point is earned for justification (can involve a Le Chatelier argument).

(d) The value of K sp for the salt BaCrO4 is 1.2 × 10?10. When a 500. mL sample of 8.2 × 10?6M

Ba(NO3)2is added to 500. mL of 8.2 × 10?6M Na2CrO4, no precipitate is observed.

(i) Assuming that volumes are additive, calculate the molar concentrations of Ba2+(aq) and

CrO42?(aq) in the 1.00 L of solution.

(ii) Use the molar concentrations of Ba2+(aq) ions and CrO42?(aq) ions as determined above to show why a precipitate does not form. You must include a calculation as part of your answer.

The product Q = [Ba2+][CrO42?]

= (4.1 × 10?6M)(4.1 × 10?6M)

= 1.7 × 10?11

Because Q = 1.7 × 10?11 < 1.2 × 10?10 = K sp, no precipitate forms.One point is earned for calculating a value of Q that is consistent with the concentration

values in part (d)(i).

One point is earned for

using Q to explain why no precipitate forms.

Question 2

CO(g ) +

2O 2

(g ) → CO 2(g ) 2. The combustion of carbon monoxide is represented by the equation above.

(a) Determine the value of the standard enthalpy change, rxn

H ?D , for the combustion of CO(g ) at 298 K using the following information.

C(s ) +

O 2(g ) → CO(g ) 298H ?D = ?110.5 kJ mol ?1

C(s ) + O 2(g ) → CO 2(g ) 298

H ?D = ?393.5 kJ mol ?1

(b) Determine the value of the standard entropy change, rxn S ?D , for the combustion of CO(g ) at 298 K using the information in the following table.

Substance 298

S D (J mol ?1 K ?1)

CO(g ) 197.7 CO 2(g ) 213.7 O 2(g )

205.1

Question 2 (continued)

G ?D , for the reaction at 298 K. Include units with your answer.

rxn G ?D = rxn H ?D ? T rxn S ?D

= ?283.0 kJ mol ?1 ? (298 K)(?0.0865 kJ mol ?1 K ?1)

rxn

G ?D = ?257.2 kJ mol ?1 One point is earned for substituting

the values from parts (a) and (b)

into the equation. One point is earned for the answer

(with sign and units).

(d) Is the reaction spontaneous under standard conditions at 298 K ? Justify your answer.

Yes, the reaction is spontaneous because the value

of rxn

G ?D for the reaction is negative (?257.2 kJ mol ?1).

One point is earned for an answer with justification (consistent with the

answer in part (c)).

(e) Calculate the value of the equilibrium constant, K eq , for the reaction at 298 K.

Question 3

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 CO2(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.

1.2359 g sample × 0.2884 = 0.3564 g N One point is earned for the correct answer.

(iii) Determine the mass, in grams, of O in the original 1.2359 g sample of the compound.

Because the compound contains only C, H, N, and O,

mass of O = g sample ? ( g H + g C + g N )

= 1.2359 ? (0.0648 + 0.6116 + 0.3564) = 0.2031 g

One point is earned for the

answer consistent with the answers in parts (a)(i) and (a)(ii).

(iv) Determine the empirical formula of the compound.

Question 3 (continued) Divide all mole quantities by the smallest number of moles:

0.05092 mol ÷ 0.01269 mol = 4.013 0.06429 mol ÷ 0.01269 mol = 5.066 0.02544 mol ÷ 0.01269 mol = 2.005 0.01269 mol ÷ 0.01269 mol = 1.000

? Empirical formula is C4H5N2O

One point is earned for dividing by the smallest number of moles.

One point is earned for the empirical formula consistent with the ratio of moles

calculated.

(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

4. Write the formulas to show the reactants and the 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.

General Scoring: Three points are earned for each reaction: 1 point for correct reactant(s) and 2 points for correct product(s). Designation of physical states is not required.

(a) Solid potassium chlorate is strongly heated.

KClO3→ KCl + O2

(b) Solid silver chloride is added to a solution of concentrated hydrochloric acid.

AgCl + Cl?→ [AgCl2]?

HC2H3O2 + OH?→ H2O + C2H3O2?

(d) Ammonia gas is bubbled into a solution of hydrofluoric acid.

NH3 + HF → NH4+ + F?

(e) Zinc metal is placed in a solution of copper(II) sulfate.

Zn + Cu2+→ Zn2+ + Cu

(f) Hydrogen phosphide (phosphine) gas is added to boron trichloride gas.

PH3 + BCl3→ H3PBCl3

Note: PH3BCl3 also acceptable as a product.

(g) A solution of nickel(II) bromide is added to a solution of potassium hydroxide.

Ni2+ + OH?→ Ni(OH)2

(h) Hexane is combusted in air.

C6H14 + O2→ CO2 + H2O

5. Three pure, solid compounds labeled X, Y, and Z are placed on a lab bench with the objective of

identifying each one. It is known that the compounds (listed in random order) are KCl, Na2CO3, and MgSO4. A student performs several tests on the compounds; the results are summarized in the table below.

Compound pH of an Aqueous

Solution of the

Compound

Result of Adding

1.0 M NaOH to a

Solution of the

Compound

Result of Adding

1.0 M HCl Dropwise to

the Solid Compound

X> 7 No observed reaction Evolution of a gas Y7No observed reaction No observed reaction

Z 7 Formation of a white

precipitate

No observed reaction

(a) Identify each compound based on the observations recorded in the table.

Compound

X ______ Na2CO3 _______

Compound Y ______ KCl _________

Compound Z _____ MgSO4 _________

One point is earned for one correct identification, and a

second point is earned for a second correct identification.

(No points are earned if all three identifications are the same compound;

no second point is earned if two identifications are the same compound.)

(b) Write the chemical formula for the precipitate produced when 1.0 M NaOH is added to a solution of

compound Z.

Mg(OH)2One point is earned for the correct formula.

part of your explanation.

CO32? reacts with water to form OH?.

CO32?(aq) + H2O(l) → OH?(aq) + HCO3?(aq)One point is earned for identifying CO32?

as a base.

One point is earned for a correct equation.

Question 5 (continued)

(d) One of the testing solutions used was 1.0 M NaOH. Describe the steps for preparing 100. mL of

1.0 M NaOH from a stock solution of 3.0 M NaOH using a 50 mL buret, a 100 mL volumetric flask,

distilled water, and a small dropper.

(e) Describe a simple laboratory test that you could use to distinguish between Na2CO3(s) and CaCO3(s).

In your description, specify how the results of the test would enable you to determine which compound was Na2CO3(s) and which compound was CaCO3(s).

A water solubility test would work. Put a small amount of one substance

in a beaker of distilled water. If the substance dissolves readily when

stirred, then it is Na2CO3; if it does not dissolve, it is CaCO3.

A flame test would work. Dip a moistened wire into a sample of

one of the substances and place the wire in the flame of a bunsen

burner. If a bright orange-yellow color is observed, then the sample is Na2CO3; if a brick red color is observed, it is CaCO3.

Note: The student does NOT have to perform a confirmatory test on the other substance if one has already been identified with a test.One point is earned for any reasonable test. One point is earned for interpreting the results that will identify one

compound.

Question 6

6. Answer each of the following in terms of principles of molecular behavior and chemical concepts.

(a) The structures for glucose, C6H12O6, and cyclohexane, C6H12, are shown below.

Identify the type(s) of intermolecular attractive forces in

(i) pure glucose

Hydrogen bonding OR dipole-dipole interactions OR

van der Waals interactions (London dispersion forces

may also be mentioned.)

One point is earned for a correct answer.

(ii) pure cyclohexane

London dispersion forces One point is earned for London dispersion forces.

(b) Glucose is soluble in water but cyclohexane is not soluble in water. Explain.

The hydroxyl groups in glucose molecules can form strong hydrogen bonds with the solvent (water) molecules, so glucose is soluble in water. In contrast, cyclohexane is not capable of forming strong intermolecular attractions with water (no hydrogen bonding), so the water-cyclohexane interactions are not as energetically favorable as the interactions that already exist among polar water molecules. OR

?Glucose is polar and cyclohexane is nonpolar.

?Polar solutes (such as glucose) are generally soluble in

polar solvents such as water.

?Nonpolar solutes (such as cyclohexane) are not soluble in the polar solvent.

One point is earned for explaining the solubility of glucose in terms of hydrogen bonding or dipole-dipole interactions with water.

One point is earned for explaining the difference in the polarity of

cyclohexane and water.

One point is earned for any one of the three concepts; two points are earned for any two of the three concepts.

Process 1: H 2O(l ) → H 2O(g ) ?H ° = +44.0 kJ mol ?1

Process 2: H 2O(l ) → H 2(g ) + 21

O ()2

g ?H ° = +286kJ mol ?1

(i) For each of the two processes, identify the type(s) of intermolecular or intramolecular attractive

forces that must be overcome for the process to occur.

In process 1, hydrogen bonds (or dipole-dipole interactions) in liquid water are overcome to produce distinct water molecules in the vapor phase.

In process 2, covalent bonds (or sigma bonds, or electron-pair bonds) within water molecules must be broken to allow the atoms to recombine into molecular hydrogen and oxygen.

One point is earned for identifying the type of intermolecular force involved

in process 1. One point is earned for identifying the type of intramolecular bonding involved in process 2.

(ii) Indicate whether you agree or disagree with the statement in the box below. Support your answer

with a short explanation.

When water boils, H 2O molecules break apart to form hydrogen molecules and oxygen molecules.

I disagree with the statement. Boiling is simply Process 1, in which only intermolecular forces are broken and the water molecules stay intact. No intramolecular or covalent bonds break in this process.

One point is earned for disagreeing with the statement and providing a

correct explanation.

(d) Consider the four reaction-energy profile diagrams shown below.

(i) Identify the two diagrams that could represent a catalyzed and an uncatalyzed reaction pathway for

the same reaction. Indicate which of the two diagrams represents the catalyzed reaction pathway for the reaction.

Diagram 1 represents a catalyzed pathway and diagram 2 represents an uncatalyzed pathway for the same reaction.One point is earned for identifying the correct graphs and indicating which represents which pathway.

(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.

I disagree with the statement. A catalyst does not add energy, but provides an alternate reaction pathway with a lower activation energy.One point is earned for disagreeing with the statement and providing

an explanation.

Question 7

7. Answer the following questions about the structures of ions that contain only sulfur and fluorine.

(a) The compounds SF4 and BF3react to form an ionic compound according to the following equation.

SF4 + BF3→ SF3BF4

(i) Draw a complete Lewis structure for the SF3+ cation in SF3BF4.

One point is earned for the correct Lewis

structure (the structure must include lone pairs of

electrons, which may be represented as dashes).

(ii) Identify the type of hybridization exhibited by sulfur in the SF3+ cation.

sp3One point is earned for the correct hybridization.

(iii) Identify the geometry of the SF3+cation that is consistent with the Lewis structure drawn in part (a)(i).

Trigonal pyramidal One point is earned for the correct shape.

(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.

The F–S–F bond angle in the SF3+cation is expected to be slightly smaller than 109.5° because the repulsion between the nonbonding pair of electrons and the S–F bonding pairs of electrons “squeezes” the F–S–F bond angles together slightly.

One point is earned for stating that the angle is smaller, with justification.

Question 7 (continued)

(b) The compounds SF4and 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

One point is earned for the correct Lewis structure

(the structure must include lone pairs of electrons, which

may be represented as dashes).

(ii) Identify the type of hybridization exhibited by sulfur in the SF5? anion.

sp3d2One point is earned for the correct hybridization.

(iii) Identify the geometry of the SF5?anion that is consistent with the Lewis structure drawn in part (b)(i).

Square pyramidal One point is earned for the correct shape.

(iv) Identify the oxidation number of sulfur in the compound CsSF5.

+4One point is earned for the correct oxidation number.

Question 8

8. Suppose that a stable element with atomic number 119, symbol Q, has been discovered.

(a) Write the ground-state electron configuration for Q, showing only the valence-shell electrons.

8s1One point is earned for the electron configuration.

(b) Would Q be a metal or a nonmetal? Explain in terms of electron configuration.

It would be a metal (OR an alkali metal). The valence electron would be held only loosely. One point is earned for the correct answer and explanation, which must include reference to the

valence electron.

smallest? Explain in terms of electronic structure.

It would have the largest atomic radius in its group because its valence electron is in a higher principal shell.One point is earned for the correct answer and explanation; the size must refer to number of

electron shells.

(d) What would be the most likely charge of the Q ion in stable ionic compounds?

One point is earned for the correct charge.

(Must be consistent with configuration in part (a).)

(e) Write a balanced equation that would represent the reaction of Q with water.

2 Q(s) + 2 H2O(l) → 2 Q+ (aq) + 2 OH?(aq) + H2(g)One point is earned for H2 as a product.

One point is earned for balancing

the equation.

(f) Assume that Q reacts to form a carbonate compound.

(i) Write the formula for the compound formed between Q and the carbonate ion, CO32? .

Q2CO3One point is earned for the formula consistent with the charge given in part (d).

(ii) Predict whether or not the compound would be soluble in water. Explain your reasoning.

It would be soluble in water because all alkali metal carbonates are soluble.One point is earned for the answer consistent with the identification of Q.

ap06_sg_chemistry

相关文档

最新文档