Nov. 7/8, 1996 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.!!

There will be no partial credit on parts of Problem 1.

(Possibly Useful )Physical Constants and Equations

NA = 6.022 x 1023 R = 3.29 x 10 15 Hz E = - R (1/n_l²-1/n_u²)

Mass of an electron 9.1 x 10-28 g

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 DH°_B C-C = 348 kJ/mol DH°_B C=C 612 kJ/mol

Visible light 400 nm to 900 nm UV light 200 to 400 nm

Density of water 1 g/cm3

Problem 1 ________

Problem 2 ________

Problem 3 ________

Problem 4________

Total _________

NAME____________________________

Problem 1 (20 points)

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 votes for Clinton	is greater than >	The number of votes for Dole
(b) The number of electrons allowed in the 2s orbital(s).		The number of electrons allowed in the 3d orbital(s).
(c) The energy of a photon whose wavelength is 600 nm.		The energy of a photon whose wavelength is 300 nm.
(d) The dipole moment of HF		The dipole moment of HBr
(e)The number of Xe lone pairs in XeO3		The number of I lone pairs in ICl4-
(f) The number of hybrid orbitals which results.		The number of orbitals “mixed” to obtain hybrid orbitals.

NAME____________________________

Problem 2 (25 points)

Compare the bonding in formic acid (HCOOH) and that of its conjugate base, the formate ion (HCOO- ). The central atom is highlighted in each case. The H on the left is bound to the central C while the H on the right is bound to O.

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

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

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

NAME____________________________

Problem 3 (25 points)

Ultraviolet light is harmful because its photons are capable of breaking chemical bonds in living organisms. Some of the most harmful UV radiation is normally absorbed by the ozone layer. However, when chlorofluorcarbons rise to the upper atmosphere, they can be photodissociated by UV light and the resulting products may then react to destroy the beneficial ozone.

See front page for useful information.

a)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.

b) Draw the Lewis Sructure of CCl2F2. What is its shape? Is it polar? Justify your answer with a drawing and a sentence.

c) UV light is harmful, in part, because it can damage DNA by joining two separate thymidines (see Problem 4 for structure). This process may ultimately lead to skin cancer. Essentially, photons are absorbed and convert two carbon-carbon (C=C) double bonds to two carbon-carbon (C-C) single bonds and subsequently two new C-C single bonds form to join two thymidines.

How many photons of wavelength 280 nm are required to convert one C=C double bond to a C-C single bond?

NAME____________________________

Problem 4 (30 points)

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". Two copies of the drawing are provided for your convenience.

a) Indicate all of the carbons which are sp3 hybridized.

b) Indicate all of the carbons which are sp2 hybridized.

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

d) Label all of the s bonds.

e) Label all of the p bonds.

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

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

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

i) Can the two rings rotate relative to each other?