CHEMISTRY AP* CHEMISTRY EXAM PREP



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CHEMISTRY AP* CHEMISTRY EXAM PREP

About the Exam Total length: 3 hours and 5 minutes 90 minutes = 75 multiple-choice questions 95 minutes = 6 free-response questions

Part A = Short Answer Question, calculator-based, 55 min. Includes one equilibrium question. Part B = Short Answer Question, 40 min. Includes one reaction question.

50% of total score = multiple choice 50% of score = essays

Time Strategies for Multiple Choice Spend the MOST time on questions you know. If you can eliminate one or more answer choices quickly, guess and move on. If you have no clue, leave it blank. Or, jot down the question number and go back if you have time. • • • • •

At time = 25 minutes: 30 questions finished At time = 50 minutes: 30 questions finished At time = 75 minutes: 90 questions finished Next 10 minutes: return to skipped questions Last 5 minutes: pick one tough one that you could get if you just relaxed. Or, use this time to check your work.

Time Strategies for Free Response Questions You have about 18 minutes to spend per problem on Part A. • • • • •

Glance at all 3. Tackle the easiest. Complete it in 10–12 minutes. Tackle the second easiest. Take 15–20 minutes. Hit the hardest. Spend 20 minutes or so. Leave 5 to 10 minutes at the end to review calculations, clarify sloppy handwriting, check units.

On Part B, you have 40 minutes to answer 3 questions. These will be more conceptually-oriented than calculation-based. At 10 minutes per question, plus 10 minutes for review, you’ll be in good shape.





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CHEMISTRY AP* CHEMISTRY EXAM PREP

13. In the reaction below, name the bases and their conjugate acids. NH3 (aq) + H2O(l)  NH4 + (aq) + OH - (aq) Base ________________ Conjugate Acid _______________ Base ________________ Conjugate Acid _______________ This acid/base system is of a __________________type (name of investigator who characterized this type of acid/base system) because_________________________________________________________ _______________________________________________________________________________________. 14. True or False: Carbon generally has four ionic bonds. 15. The emf of a cell, also called the cell potential, is defined as _______________________________ ____________________________________________________________________________________.





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CHEMISTRY AP* CHEMISTRY EXAM PREP

1. There are many tables of periodic elements with lots of data. On simple tables with only one small number above the chemical symbol, what does that small number represent? the atomic number. 2. True or False: Knowing the atomic mass of carbon is enough information to determine electron configuration. The atomic number is enough information. 3. What are three characteristics of elemental sodium that indicate it is a metal? Soft, conducts electricity, very reactive 4. List three different types of bonds or bond characteristics. Covalent, Ionic, metallic, hydrogen bonding, delocalized, resonant, double, triple, aromatic, bond angle, bond length, stable, unstable… 5. What is required for a solution to conduct electricity? A conductor such as ions, a potential, a source of electrons 6. What kind of experiment would be conducted if you used a distillation column in the lab? What would be its purpose? Distillation or fractionating. Separation of mixtures by boiling point. 7. What is the difference between electron affinity and electronegativity? Electron affinity is the willingness of an atom to accept an electon. Electronegativity is the atom's nucleus' ability to attract a shared pair of electrons. 8. True or False: Hydrogen bonding is very important in industrial uses, but not in nature. Hydrogen bonding is very important in nature. Among many other appearances, hydrogen bonding is involved in the morphology of several molecules such as enzymes and DNA; it allows some insects to walk on water and is instrumental in the solubility of many biologically important ions. 9. True or False: The ideal gas law is too idealistic to be useful. While there are conditions under which the ideal gas law requires modification to give useful information, as long as the experiment is conducted near STP only the most reactive gases will follow the ideal gas law very well. 10. There is one mole in one molar volume. Kind of a trick question. There are 22.4 L/molar volume.





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CHEMISTRY AP* CHEMISTRY EXAM PREP

11. Rate experiments are required to determine rate law and reaction order. 12. Catalysts are included in the rate law as a reactant. 13. In the reaction below, name the bases and their conjugate acids. NH3 (aq) + H2O(l)  NH4 + (aq) + OH - (aq)

Base NH3 Base OH -

Conjugate Acid NH4 + Conjugate Acid H2O

The acid/base system is of a Brønsted-Lowry type because the system is one of proton transfers between the acids and bases. 14. True or False: Carbon generally has four ionic bonds. Carbon generally has four covalent bonds. 15. The emf of a cell, also called the cell potential, is defined as the difference between the standard cell potential of the cathode and the standard cell potential of the anode. That is, “cathode minus anode”.





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Worksheet

CHEMISTRY AP* CHEMISTRY EXAM PREP

Free Response Question I Calculate the standard free energy change and the equilibrium constant Kp for the following reaction at 25oC. What oxidation changes take place (if any)? Is it spontaneous? CO(g) + 3H2(g) CH4(g) + HO(g) 5 minutes

10 minutes

15 minutes





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Worksheet

CHEMISTRY AP* CHEMISTRY EXAM PREP

Free Response Question II In the analytical determination of arsenic, a solution of the arsenic as arsenious acid, potassium iodide and a small amount of starch is electrolyzed. Free iodine is produced and immediately oxidizes the arsenious acid to hydrogen arsenate ion. When the reaction is complete, the iodine will combine with the starch to provide a deep blue color. During one analysis, it took 63.3 seconds for a current of 11.2 mA to reach the endpoint. What does the phase “analytical determination” mean? What are some of the requirements of an analytical determination? How many grams of arsenic is present? I2(aq) + H3AsO3(aq) + H2O(l)  2I2(aq) + HAsO42-(aq) + 4H+ 5 minutes

10 minutes

15 minutes





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Worksheet

CHEMISTRY AP* CHEMISTRY EXAM PREP

Answers

Free Response Question Calculate the standard free energy change and the equilibrium constant Kp for the following reaction at 25oC. CO(g) + 3H2(g)  CH4(g) + H2O(g) From tables of Standard Gibbs Free Energy: CHEMISTRY AP* CHEMISTRY EXAM PREP

kJ o ∆GCH 4 = @ 50.80 fffffffff mol kJ o ∆G H2O = @ 228.6 22.86 fffffffff mol ­­ kJ o ∆GCO = @ 137.2 fffffffff mol

kJ o f f f f f f f f f by definition ∆G by definition H2 = 0 mol A o o o ∆GRxn = P n∆GRxnts @ P m∆GRxnts o

`

a

∆GRxn [email protected] 50.80 @ 228.6 @ @ 137.2 [email protected] 142.2 o

∆GRxn [email protected] 142.2

kJ f f f f f f f f f

TEACHING SYSTEMS

kJ f f f f f f f f f mol

14

mol

equilibrium constant calculation: o

@ ∆G o f f f f f f f f f f f f f ln k p = @ ∆G f f f f f f f f f f f f f ln k = RT p RT ln k p [email protected]

J @ 142,200 ffffffffff J f f f ffffffffff f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f @ 142,200 mol f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f f mol ` a J f f f f f f f f f f f f f f ln k p [email protected] ` a 8.31 298fffK Jfffffffff f f 8.31 298 K K mol K mol

ln k p = 57.42 ln k p = 57.42





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Worksheet Answers

CHEMISTRY AP* CHEMISTRY EXAM PREP

taking the antilog:

k p = 57.42 k p = 8.68 x10

24

k pffffff k c = `ffffffff a∆n RT

24

8.68 x10 f f f f f k c =Hfffffffffffffffffffffffffffffffffffffffffffffff I2 f g a atm f f f f f f f f f f` J 0.821 Lffff 298 K K K mol

What oxidation changes take place (if any)? Is it spontaneous? CO(g) + 3H2(g) CH4(g) + H2O(g) Reactants with C

Ion Products with As Charge 0 c H4 =

Ion Charge

C

O

=

?

-2

=

0

?

4(+1)

=

0

+2

-2

=

0

?

+4

=

0

-4

+4

=

0

Carbon has been reduced from +2 to -4 oxidation state. A change of 6 electrons. Looking at the reaction, it makes sense as hydrogen going from a zero oxidation state to +1 in water a total change of six electrons. The reaction is spontaneous because ∆Go is much more negative than -10 kJ/mol.





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Worksheet Answers

CHEMISTRY AP* CHEMISTRY EXAM PREP

Free Response Question II In the analytical determination of arsenic, a solution of the arsenic as arsenious acid, potassium iodide and a small amount of starch is electrolyzed. Free iodine is produced and immediately oxidizes the arsenious acid to hydrogen arsenate ion. When the reaction is complete, the iodine will combine with the starch to provide a deep blue color. During one analysis, it took 63.3 seconds for a current of 11.2 mA to reach the endpoint. What does the phase “analytical determination” mean? What are some of the requirements of an analytical determination? How many grams of arsenic are present? I2(aq) + H3AsO3(aq) + H2O(l) 2I2(aq) + HAsO42-(aq) + 4H+ Analytical determination means that the reaction will go to completion so that a computation of the original quantity in the sample can be made, accurate to within the limits of the method and of error. The analysis is called “quantitative” as opposed to “qualitative”, which would be descriptive in nature only. By using proper lab techniques and an appropriate reaction that is capable of completion, the quantity of a substance can be determined. This is a redox reaction. We need to determine what is happening to our species of interest. Write the balanced equation. One method uses a chart to calculate oxidation states for the Arsenic. I2(aq) + H3AsO3 (aq) + H2O(l)  2 I- (aq) + HAsO4 2Reactants with As H3 3x1 3 3

As ? ? +3

O3 3 x (-2) -6 -6

= = = =

Ion Charge 0 0 0 0

Ion Charge

Products with As H 1x1 1 1

As ? ? +5

O4 4 x (-2) -8 -8

= = = =

-2 -2 -2

The oxidation change for Arsenic in this reaction is (+5) – (+3) = 2 electrons transferred per As atom.





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Worksheet

CHEMISTRY AP* CHEMISTRY EXAM PREP

Answers

Using: 1 Amp = 1 and

Coulomb sec

96500 moles of electrons transfered per Coulomb

Current x time x Faradays constant = moles of electrons transferred then substitute

(

(11.2mA)(63.3 sec)

1A 1000mA

)

( )( C

1 sec A

1mol

e 96500C

) = 7.36 x 10 moles transferred -6

From the change in oxidation number for the arsenic above

for every mole of atomic Arsenic that reacted, 2 moles of electrons were transferred Therefore:

7.36 x 10-4 mole e-

2mole e atomAs

(

74.92g molAs

)

-4

= 2.75 x 10 gm As

quantity in sample = 2.75 x 10 g As -4





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Check Your Knowledge

CHEMISTRY AP* CHEMISTRY EXAM PREP

1. What gas is produced from each lightning strike? How does that help us and our planet? Write a balanced chemical equation for the reaction. (3 points)

2. From just one element in your body, we could produce about 9000 graphite lead pencils. What is that element? (1 point)

3. An ounce of gold can be stretched into wire 50 miles long. The same ounce can then be flattened into a sheet the area of a tennis court. What properties of metals were just demonstrated? Why is gold used in this question? (3 points)

4. You are cleaning the bathroom and did not read the warning on one of the packages not to mix cleaning agents. You just poured two solutions together in a dry tub and you notice the bubbling and vapors being released. What have you done? What should you do? Choose all the useful ideas and the order in which you would implement them. Explain what is not a good idea about the others. (9 points) A) B) C) D) E) F)

Call the fire department. Dump a bucket of water on the chemicals to flush them down the drain. Plug the tub and start running the water. Put on gloves and try to sponge the liquids up. Open a window. Mop up the liquids with old towels.





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