Nov. 4/5, 1999 NAME____________________________

 

Chemistry 103 Section 1

HOUR EXAM #2

IMPORTANT NOTE--For partial credit be sure to show your reasoning. Please attempt all parts of all problems--no partial credit can be given for blank spaces. If you need more paper, write on the back of the page and indicate clearly that you have done so.

The structures shown do not necessarily show all lone pair electrons or true molecular shapes.!! Questions are not arranged in order of difficulty.

There will be no partial credit on Problem 1.

 

(Possibly Useful )Physical Constants and Equations

NA = 6.022 x 1023 R = 3.29 x 10 15 Hz

Mass of an electron 9.1 x 10-28 g RH = 2.179 x 10-18 J

Mass of a proton 1.67 x 10-24 g h = 6.63 x 10 -34 J.s/photon

Mass of a neutron 1.67 x 10-24 g c = 3.0 x 10 8 m/s

E = hn E= hc/l E = -RH/n2

H = E + PV Q = m Cp (DT)

Density of water 1.00 g/cm3 Specific heat of water = 4.18 J/g °C

 

Grading Scale (25 basic points, 50 HW level, 25 synthetic)

< 25 0

25-50 1

50-75 2 (2.0, 2.3, 2.7)

75-88 3 (3.0, 3.3, 3,7)

89-100 4

average __71.5____

Congratulations if your score improved by 10 points or more! Keep up the good work! If your score declined, figure out why; please don't rest on your laurels. If you are struggling remember that only working with a tutor is usually ineffective. You still need to study at least an hour a day (no telephones, computers, friends, or other distractions). Try to figure out what you don't understand and come see me. Please bring me probems and concepts you don't understand. (or those for which you would like futher information). I will make appointments outside of my office hours if you have scheduling problems.

Test-taking stategies--READ the PROBLEM. Many of you wasted time describing elaborate bonding schemes in Prob. 3 and 4. I only asked you about the central atom. Many of you had difficulty drawing Lewis Structures because you didn't understand that most of the non-central atoms are bonded to the central atoms. You need to practice, practice, practice on HW problems until this becomes automatic.

 

Problem 1 (20 points) (basic and HW levels)

Indicate whether the quantity in Column A is greater than (>), equal to (=), or less than (<) the quantity in column B. You may write the words, the symbols or both. No partial credit will be awarded on the parts of this Problem. Part (a) is done for you as an example.

Column A Column B

(a) The number of debates won by Bill Bradley, a democratic presidential candidate

is greater than

>

The number of debates contested by George W. Bush, a republican presidential candidate.

(b) The number of nodes (nodal planes) in a pi, p, bond.

> ( 1 vs. 0)

The number of nodes (nodal planes) in a sigma, s, bond.
     

(c) The energy required to remove an electron from Boron (B)

< (periodic trend)

The energy required to remove an electron from Neon (Ne)
     

(d) The dipole of HF

 

> (F is more electronegative than Br)

The dipole of HBr
     

(e)The H-N-H angle in NH3

> (this has to do with lone pair vs bonding pair repulsions creating deviations from ideal tetrahedral geometry)

The H-O-H angle in H2O
     

(f) The temperature increase caused by the complete combustion of one tablespoon of peanut butter in a bomb calorimeter containing of 1000 mL of water in a well insulated container.

> (some heat would leak out of the poorly insulated container)

The temperature increase caused by the complete combustion of one tablespoon of peanut butter in a bomb calorimeter containing of 1000 mL of water in a poorly insulated container.

 

Problem 2 (20 points--parts a-c) (HW and synthetic levels)

Compare the bonding in formic acid (HCOOH) and that of its conjugate base, the formate ion (HCOO-). The central atom is highlighted and bonds to 1 H and 2 O’s in each case. Formic acid is found in ants.

a) Draw Lewis structures for the acid and base forms.

 

acid H-O-CH=O both O's have 2 lone pairs

 

base O-CH=O - remember to count extra electron for - charge. . C makes 2 single and one double bond. One O has 2 lone pairs and the other has 3.

BUT, there are two equivalent resonance structures involving the C-O bond.

 

(see real bulletin board for drawings)

 

 

b) Describe the hybridization and types of bonds formed by the central carbons.

both carbons are sp2 hybridized with trigonal planar geometry. They each make 3 sigma and one pi bond. The base has a delocalized pi bond

 

  1. Compare the carbon-oxygen bonds in the acid and base forms. Order the four C-O bonds by length and strength.

    2. shortest, strongest acid C=O

    medium length, strength base C-O, C=O resonance (BO 1.5)

    longest, weakest C-O acid

     

    Problem 3 (25 points--parts a-e) (hw and synthetic levels)

    Two species which are thought to be important in atmospheric chemistry of the ozone layer are OBrO and CCl2F2.

    Central atoms are underlined.

    a)Draw a Lewis Structure for OBrO, and an accurate drawing and description of its shape.

    Br has a lone pair, a single electron and two bonds to O (I wasn't picky about single, double or resonance). The electron pair shair is tetrahedral, and the molecule is bent. The single electron acts as a pair.

     

     

    b)Draw a Lewis Structure for CCl2F2, an accurate drawing of its shape, and explain why it is or is not polar.

    C makes 4 single bonds so it's tetrahedral. Because F is more electronegative than Cl it is polar. There are no isomers (cis or trans ) here. No matter where you draw the 2 F's they are 109 degrees apart.

     

     

     

     

    c)The bond dissociation energy of a typical C-F bond is 440 kJ/mol. Calculate the maximum wavelength of light which can photodissociate a molecule of CCl2F2 (a chlorofluorcarbon) by breaking a single C-F bond.

    E = Na hc/l (many of you failed to use Avogadro's number)

    l = Na h c / E =[6.02 x 10^23 photons/mol x 6.63 x 10^-34 J/s photon x 3.0 m/s ]/ 440,000 J = 2.7 x 10^-7 m or 270 nm ( in the UV)

     

    d) Suppose that the bond in c) is broken homolytically so that one electron on the bonding pair remains on the CCl2F fragment and the other stays with the detached F. Draw Lewis structures for the products of dissociation of two molecules of CCl2F2.

    Would you expect these products to react further? If so, draw Lewis structures for one of the final products.

    Get two free radicals F. and .CCl2F (read problem carefully for details here)

    These will recombine to form F-F or Cl2FC-CFCl2 (remember NO2 free radical problem?)

     

     

  2. Write the electronic configuration of the detached F.

[He] 2s2 2p5

 

 

Problem 4 --parts a-i. (25 points) (basic , HW, and syn levels)

One of the first drugs to be used in the treatment of acquired immune deficiency syndrome (AIDS) is azidothymidine (AZT).

In the distorted drawing below, nitrogen's unhybridized lone pair electrons are indicated by "p". However, not all lone pairs are shown. Two copies of the drawing are provided for your convenience.

  1. Indicate all of the carbons which are sp3 hybridized.

    2. All C's making 4 single bonds
  2. Indicate all of the carbons which are sp2 hybridized.

    3. All C's making one double and 2 single bonds.

    c) Indicate all of the atoms which are sp hybridized.

    Only middle N in N3 at bottom. Add lone pairs to O's and N's to satisfy octets first.

  3. Label all of the s bonds.

Everyone did this fine

e) Label all of the p bonds.

Fine again

Still Problem 4

f) Which rings are (or could be) planar?

 

Just the top one with 6 atoms having sp2 geometry (class last Mon. and Wed. was supposed to help with this). The bottom ring has atoms with sp3 geometry and will not be flat.

 

g) What is the N-N-N bond angle? Draw the N-N-N bonding orbitals and indicate the geometry around the central N.

The central N makes 2 double bonds and the octet rule is satisfied. It does not have lone pairs or a single electron. It must be linear and use sp orbitals. In each direction is makes a sigma and a pi bond. The two pi bonds are perpendicular to each other.

(see real drawing on bulletin board)

 

 

 

h) What is the C-O-H bond angle?

O has two lone pairs so the angle is a little less than 109 degrees (like water)

  1. Can the two rings rotate relative to each other?

Yes, they are linked by a sigma bond.

Problem 5 (10 points) (HW level)

A lecturer plans to demonstrate an endothermic process by placing a digital thermometer probe in 1400 mL of water in an insulated container. Ammonium chloride, NH4Cl, is then to be added and stirred. Approximately what mass of NH4Cl should be added to lower the temperature from 25 to 10 °C? The heat of solution of NH4Cl is + 14.7 kJ/mol.

Why is your result approximate?

Strategy

  1. Find how much heat needs to be absorbed to cause the temperature decrease. We will assume that only the 1400 mL of water decreases in temperature and ignore the heat absorbed by the salt.
  2. Find the number of moles of salt required for the heat found in the first part
  3. Use the molar mass of the salt to find its mass.

Q = m Cp DT= 1400 g x 4.18 J/degree g x 15 degrees = 88 kJ absorbed

moles = 88 kJ x 1mol/ 14.7 kJ = 5.97 moles

mass = 5.97 moles x 53.5 g / mole = 320 grams.

Our result is approximate because we have ignore the specific heat of the salt. Some of you did more complicated calculations to take this into account.