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FINAL EXAM ANSWER KEY...

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THE UNIVERSITY OF CALGARY

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FACULTY OF SCIENCE

Q21

Q24

Q22

Q25

CHEMISTRY 201

Mark

Written

Mark

FINAL EXAMINATION Q23

Date:December 10th, 2012

Time: 7:00 pm to 10:00 pm (3 hrs)

FIRST NAME: ___________________ LAST NAME: __________________ ID #: (please print) Lecture Section: Please check one   

L01 L02 L03

MWF MWF TR

1:00pm 2:00pm 9:30am

Dr. V. Mozol Dr. E. Sullivan Dr. E. Sullivan

This test consists of 20 multiple choice questions worth 1 mark each (total 20 marks) and 5 long answer questions (total 40 marks). The total value for the test (multiple choice plus long answer) is 60 marks. All questions must be answered to obtain full marks. The answers to the multiple choice section must be entered on the optical score sheet using a pencil. The answers to the long answer questions must be written in the space provided on the question sheets. Only non-programmable calculators are permitted. Model kits are permitted. A chemical data sheet is provided on the last page of the exam paper. AT THE END OF THE EXAMINATION, HAND IN THE OPTICAL SCORE SHEET AND THE ENTIRE EXAM BOOKLET

Failing to write your name and ID on this booklet and/or failing to encode your Optical Score Sheet correctly will result in the loss of two marks

Chem 201 cont’d

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SECTION I - Computer graded section (Total value 20) Answers to be entered on optical score sheet provided

1. How many valence electrons would an excited atom of oxygen have? a) b) c) d) e)

four five six seven unable to tell as you don’t know where one excites the electron

This question is not great and actually quite open-ended…..you excite a core and you add to the valence, you excite a valence and you keep the valence the same. 2. Which of the following is valid set of quantum numbers for an electron in the valence shell of a fluorine atom? i. ii. iii. a) b) c) d) e)

n 1 2 2

l

0 0 1

ml 0 0 -1

ms +1/2 -1/2 +1/2

i only ii only iii only ii and iii only all of theses choices

What is the electron configuration of a fluorine atom? Which are the electrons in the valence shell? What are valid quantum numbers for these types of electrons (remember the name of the orbitals will immediately give you the quantum numbers n and l !)? 3. Identify which of the following statements is a reasonable explanation for why the first ionization energy of a sulfur atom is less than that of a phosphorus atom. a) b) c) d) e)

the stability of the half-filled subshell in a sulfur atom. the smaller size of a sulfur atom relative to a phosphorus atom. the ease with which a phosphorus atom attains a noble gas electronic configuration. the higher electronegativity of a sulfur atom relative to a phosphorus atom. the increase in stability gained by the unpairing of 3p electrons in a sulfur atom.

Looking at the relative stability of electron configurations is the key to understanding exceptions in periodic trends. Which has the more stable electron configuration P or S?

Chem 201 cont’d

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4. Which of the statements would be true. a)

Removing the second electron from an atom of nitrogen would be more difficult than removing the first electron. b) Removing an electron from any element other than the noble gases would be exothermic. c) The relative atomic radii of elements within a group in the periodic table can be related to their differing effective nuclear charge, Z*. d) Elements with odd numbers of electrons are always more reactive than those with even numbers of electrons. e) The periodic trend of reactivity for the elements is that the most reactive element is the one at the bottom of any group. So questions like these about identifying true or false means you must go through each statement and rationalize what is logical and what is not…..for b) are there ever exothermic ionizations? For c) when does nuclear pull apply to trends when does distance apply to trends? For d) as long as there are UNPAIRED electrons reaction is high. For e) we saw a demo with Cl2, Br2 reacting with O2 where this was not so, why? 5. Which of the following pairs contains isoelectronic species? i. SiO2, SO2 ii. H2O, NH3 iii. NH3, PH3

a) i only b) ii only c) iii only d) ii and iii only e) i and iii only f) So isoelectronic is now a term you can apply to ATOMS but now MOLECULES! Caution that you are no talking only of the valence shell but the entire molecule! 6. Complete the skeleton given below to give the “best” Lewis diagram for a molecule of H2CCHCH2C(O)OH, then identify which of the statements below is false? a) b) c) d) e)

The C3C4 bond is stronger than the C2C3 bond. There are 8 electrons not involved in bonding. This molecule will form hydrogen bonds. This molecule displays resonance. This molecule has only two pi bonds.

It is important to have gotten the correct Lewis diagram.

..

H H H O: H C4 C3 C2 C1 H

.. O ..

H

Chem 201 cont’d

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7. Which of the following would be considered correctly drawn Lewis structures? a)

b)

c)

d) All of the above e) None of the above You need to check for violations of octets or duets! If those were all OK then move to other things you must check for a Lewis diagram….like correct numbers of electrons or correct formal charges.

8. Which of the following compounds would have a seesaw molecular geometry (shape). a) b) c) d) e)

IBr4– TeH4 SnH4 All of the above None of the above

So do you know your VSEPR model geometries well?

9. To form a molecule with an octahedral electron geometry, what set of pure atomic orbitals must be mixed? a) b) c) d) e)

one s and three p one s, three p, and one d one s, three p, and two d two s, six p, and two d two s, six p, and four d

Octahedral geometries must have how many electron groups around them? The number of electron groups will tell you the number of orbitals to be mixed. Then you must mix orbitals in a valence shell from lowest to highest energy.

Chem 201 cont’d

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10. An atom with a linear molecular geometry . . . a) must have sp hybridization b) must have sp3d hybridization c) must have sp3d2 hybridization d) could have any of the above hybridizations e) will not be hybridized. A lot of work as you have to know your geometries, see if there are groups that are 180o from each other, then ask whether you can make those bonding groups and the remaining groups non-bon ding groups. 11. Which of the following diagrams correctly shows the 3D arrangement of the atoms in allene, H2C=C=CH2 (hint: consider the π bonding framework)?? H H H H H a)

C H

C

b)

C

C C H

H

H

C C

H

C

H

H

H

H C C

d)

c)

C

H

C

e) none of the above

H

The two pi bonding overlaps have to be perpendicular to each other as they each involve an unhybridized p orbital on the central carbon atom. 12. Given the following sigma framework for a molecule of phosgene, COCl2, which of the orbitals of oxygen will be responsible for its pi interaction? i.

2px

ii.

2py

iii.

sp2

a) i only b) ii only c) iii only d) i and iii only e) all of theses choices The key is recognizing that the pi interaction must involve the unhybridized p orbitals that are perpendicular to those shown or that they come out of the page at you!

Cl

..

z

..

y

C

:

O

.. ..

Cl For clarity the lone pairs of Cl and the orbitals they are in are not shown.

Chem 201 cont’d

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13. Given the energy of a CCl bond is 338 kJ/mol and the energy of a C-H bond is 414 kJ/mol what is the wavelength of light in meters that could be used to break the CCl bond in a molecule of dichloromethane, CH2Cl2? a) b) c) d) e)

5.61 x 10-19 m 6.87 x 10-19 m 3.54 x 10-7 m 2.89 x 10-7 m None of the above as light cannot be used to break bonds

An application of E = hc/λ λ …..but you must remember that this equation works for a single photon and not for a mole of photons. 14. What are the intermolecular forces that would exist between a molecule of hexane and a molecule of butanal? i. ii. iii.

Dipole-Induced dipole Dipole-Dipole Hydrogen Bonding

a) i only b) i and ii only c) iii only d) ii and iii only e) all the above You want only to look at forces between the two molecules NOT THOSE THAT WOULD EXIST BETWEEN THEMSELVES. 15. Consider the two molecules, ethylene glycol HOCH2CH2OH, and glycerol, HOCH2CH(OH)CH2OH, which of the following is true a) ethylene glycol has the higher viscosity b) ethylene glycol has the higher boiling point c) glycerol has the lowest surface tension d) both have the same type and amount of intermolecular forces e) glycerol has the lower diffusion Does any one have stronger intermolecular forces?……..then evaluate what stronger forces mean in terms of viscosity, boiling point, surface tension and diffusion.

Chem 201 cont’d

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16. You have prepared and isolated a solid in a manner similar to experiment #4. You calculate a yield of 85%. You measure a melting point of 155oC. The literature value for the melting point of the compound is 122-124oC. Which of the statements below is/are true? i. The melting point indicates the compound you made is impure. ii. The melting point indicates that you did not successfully make your compound. iii. A high yield means the compound you made is pure. a) i only b) ii only c) iii only d) i and ii only e) I and iii only Melting points determine purity. What is meant by melting points that match literature, are slightly lower than literature are above literature? If you have an impure sample do yield calculations mean anything? If you have not made your compound do yield calculations mean anything?

17. Whiskey lactone is an important ingredient in the aroma of whiskey, it’s structure is shown to the right. On the structure there are ____ sp3 hybridized carbons, ____ sp2 hybridized carbons and ___ sp hybridized carbons. a) b) c) d) e)

6, 1, 4 7, 2, 0 8, 2, 1 8, 1, 0 9, 2, 0

Which type of hybridized carbons will have only single bonds, which will have double and which triple? Watch out for variations of this question which might ask for ATOMS in general not just carbon atoms!!!!!

Chem 201 cont’d

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18. Which of the following are resonance structures.

i.

ii.

iii.

a) i only b) ii only c) iii only d) ii & iii only e) All of the above What is resonance? More importantly can atoms move? 19. All of the following statements concerning molecular orbital (MO) theory are correct EXCEPT a) The Pauli exclusion principle must be obeyed. b) Hund's rule must be obeyed. c) Aufbau’s rule does not have to be obeyed. d) Each molecular orbital, when filled, has one electron spin +½ and one electron spin -½. e) Atomic orbitals do not need to be filled before a fully occupied molecular orbital is formed. Make the molecular orbital energy diagram for O2 and check it out! Best way to ensure you understand.

Chem 201 cont’d

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20. Consider the following molecular orbitals formed from the combination of two hydrogen 1s orbitals. Choose the correct statement.

I a) b) c) d) e)

II

I is a σ bonding orbital lower in energy than II, which is a σ* antibonding orbital I is a σ* antibonding orbital lower in energy than II, which is a σ bonding orbital I is a σ bonding orbital higher in energy than II, which is a σ* antibonding orbital I is a σ* antibonding orbital higher in energy than II, which is a σ bonding orbital I is a π* antibonding orbital higher in energy than II, which is a σ bonding orbital

So what characterizes a sigma bond versus a pi bond? What characterizes bonding versus antibonding? Which is lower in energy a bonding orbital or an antibonding orbital?

******END OF MULTIPLE CHOICE*******

Chem 201 cont’d

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SECTION II: To be graded manually (Total value 40) To obtain full marks for a question all work/reasoning must be shown. QUESTION 21 VALUE 4 MARKS 21. a) In experiment #5, you looked at the relative boiling points of a series of constitutional isomers with the molecular formula, C4H10O. All contain the same functional group. Line drawings of them are shown below. Place the name of each structure in the line provided. Name:________1-butanol________

Name:___2-methyl-1-propanol____

Name:________ 2-butanol ________

21. b) Constitutional isomers possessing a DIFFERENT FUNCTIONAL GROUP also exist for this molecular formula, C4H10O. Draw one. So to change the functional group to another one destroy the bonding that is most characteristic of it group…..for an alcohol this would be the OH bond. The different functional group would be an ether group. Simplest ether to form is when you exchange the H with a CH3 from the other end of the 1-butanol chain. 21. c) Would the compound you drew have a higher , lower X , or the same above compounds studied in the lab (choose 1 only).

boiling point as the

21. d) In one sentence explain your reasoning for 21. c). The change in functional group leads to only the weaker types of intermolecular forces being present. ie. an alcohol has hydrogen bonding, dipole-dipole and London dispersion. Changing to an ether leads to a loss of hydrogen bonding. Change in a physical property relates to a change in what for a molecule?

Chem 201 cont’d

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QUESTION 22 VALUE 12 MARKS 22. a) Below is shown a line drawing. In the box to the right, draw out the VSEPR molecular shape for the molecule, showing all atoms.

22. b) Use the skeleton of your VSEPR shape to draw the orbitals and the orbital overlaps for the sigma and pi frameworks in the appropriate boxes below. In your diagrams be sure to include: -the names of all orbital overlaps -all axes necessary to show the orientation of the sigma orbitals with respect to the pi orbitals -Show all electrons in orbitals and orbital overlaps.

Sigma framework

Pi framework

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QUESTION 22 CONTINUED 22. c) The structure on the right is a ____conformational_____ isomer of the first structure shown in 22. a).

22. d) Do both molecules, shown in 22. a) and 22. c), have the same type(s) of intermolecular interactions when talking about pure samples of each? (Check off the appropriate answer) X Yes No

22. e) Do either of the molecules display resonance? (Check off the appropriate answer) Yes

22. f) Explain your answer to 22. e) with a diagram. One can make resonance structures for these molecules, however, for a molecule to “display” resonance there must be two or more equivalent “best” Lewis diagrams. The resonance structureS one obtains by moving valence electrons are minor structures due to the formation of unfavourable formal charges. What does it mean to “display” resonance as opposed to ‘are there resonance structures for these species’?

X No

Chem 201 cont’d

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QUESTION 23 VALUE 4.5 MARKS 23. Shown below is monosodium glutamate (MSG).

23. a) The hybridization of the nitrogen atom is ____sp3____. How many electron domains around it? 23. b) Identify any chiral centers with a star (*). What is a chiral carbon? 23. c) Circle and name the functional groups that exist in MSG. Make sure you are familiar with those you need to know. 23. d) Does MSG have an optical isomer? What is needed to have an optical isomer?

X Yes

23. e) Would you expect MSG to be soluble in water? (Check off the appropriate answer) Interactions should be strong between molecules X

Yes

No

No

23. f) BRIEFLY explain your answer to 23. e). Solubility in water requires strong interactions to be occurring between the water molecules and the ions of MSG. You will definitely have strong ion-dipole, hydrogen-bonding and dipole-dipole forces occurring between these species. What is required to have solubility (hint see above)?

Chem 201 cont’d

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24. a) Complete the energy diagram to the right, for the valence shell of an atom of nitrogen. What are the valence orbitals? How many electrons are there in each orbital?

Energy

QUESTION 24 VALUE 15.5 MARKS

24. b) In the box on the right draw the VSEPR structure for a molecule that has a molecular formula of CH5N and contains the amine functional group. • The hybridization of the nitrogen atom is __sp3__ How many electron groups are around N or what is the VSEPR geometry? • The molecular shape around the nitrogen atom is _trigonal pyramidal_.

24. c) Show how Valence Bond Theory proposes how the energy diagram you drew in 24. a) would change as the nitrogen atom hybridized. How many of the atomic orbitals undergo hybridization, as they do this their energy must change! How will it change?

Energy

Is shape defined by both bonding and non-bonding groups?

CH5N + H+ → CH5NH+ Show how the energy diagram you drew in 24 c) will change. Has the hybridization changed? Have the number of electrons around N changed?

Energy

24. d) If the nitrogen atom on the amine is protonated, or the species CH5N undergoes the following reaction:

Chem 201 cont’d

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QUESTION 24 CONTINUED 24. c) Draw the VSEPR molecular shape for a molecule with a molecular formula C3H6O2, which contains an ester functional group.

•

Identify the hybridization of atoms where necessary.

24. d) Place δ+/δ– on the atoms of the most polar bonds in the structure. Identify the bond angles defined by these bonds?

24. e) Give the IUPAC name of the compound you have drawn _______methyl ethanoate_________.

Chem 201 cont’d

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QUESTION 25 VALUE 4 MARKS 25. In a question on an exam a student was asked to draw the Molecular Orbital Theory diagram for F22+ knowing that the bond axis is x. The student then had to use their diagram to predict the bond order in F22+ . The student drew the following: ________ σ*2px π*2pz______ ______ π*2py

Energy

π2pz ______ ______ π2pz ________ σ2px ________ σ*2s ________ σ2s

25. a) What bond order would the student have predicted for F22+ using the drawing above? Show all your work. B.O. 2 What does bond order represent? How will the number of bonding and anti-bonding electrons be important to determining bond order?

25. b) Is the diagram right

or wrong X ?

25. c) You must give feedback to the student. Explain why the answer is right or wrong. The number and placement of orbitals is correct but the name of the pi bonding orbital on the right is wrong (it should be assigned as 2py rather than 2pz). The number of electrons is correct their placement is...