Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110

Chem 110, Dr. Paselk

Hour Exam 2

 Name
Fall 2002

(100 pts)
 Lab

Answer Key

This examination is scheduled for fifty (50) minutes. At the end of this fifty minute period you will have five (5) minutes to bring your examination to the front of the examination room.

 

1. (10 @ 2 ea) True/False. Indicate whether the following statements are True (T) or False (F).

Diamagnetism is due to paired electrons in an atom or molecule. _______T________

An atom with dsp3 hybridization will have a trigonal bipyramidal electronic geometry. _______T_______

Hybrid orbital theory is based on a localized electron model. _______T________

The Group VA elements are an excellent example of the trend of increasing metallicity with increasing atomic size in a group. _______T________

The inert pair effect may explain the behavior of Group VIIIA elements. _______F________

 

2. (21 @ 3 ea) Answer the following in the spaces provided.

How many allotropes exist for carbon under normal conditions? _______3________

Give the formula of the strongest of the chlorine oxyacids _______HClO4________

Would you expect the oxide of Sn or Sb to be the stronger base? _______Sn_______

What is the formula for white phosphorus? ______P4_________

Would you expect a Te oxide to be acidic, basic, or amphoteric? __amphoteric__

List two properties typical of metals generally. __luster, good conductor (heat and electricity)____

Give an oxidation state for Sb explained by the inert pair effect. _______+3________

 

3. (8) Sketch a molecular orbital diagram for lithium fluoride showing how the MO's arise from the orbitals of the bonded atoms, including all bonding electrons from each atom before and after bond formation.

Li atom

LiF molecule

F atom

E
   s*  

2s
   
     
     
     
     
    2p
     s  

4. (10) a. Draw a Molecular Orbital Energy-level diagram for molecular oxygen, including all valence electrons from each atom.

 s2p*  . .
 p2p*
 p2p
 s2p
 s2s*
 s2s

b. What is the bond order for oxygen? Indicate your reasoning or show a calculation based on your diagram for credit.

(8 - 4)/2 = 2

c. Based on your diagram, is this molecule diamagnetic or paramagnetic? Explain (short sentence or phrase).

paramagnetic (unpaired electons)

5. (12) Fill in the table below to describe the hybridization and bonding seen for the central atom in each structure.

Molecule Hybridization Sigma bonds Pi bonds
SF4  dsp3  4  0
HCN   sp1  2 2
PCl5   dsp3  5 0
H2CO   sp2  3 1

 

6. (12) In one of our demonstrations the instructor heated a yellow powder in a test tube over a Bunsen burner. As the contents were heated a yellowish, watery liquid appeared at the bottom of the tube as the powder melted. With continued heating the liquid gradually turned orange and them reddish as the liquid became very viscous. When poured into cold water the viscous liquid turned to a light yellow elastic solid.

a. Explain the change at the molecular level which gives rise to the viscosity change.

The substance was sulfur. With heat the sulfur melted to a liquid composed of S8 rings (watery), but with further heating the rings open and polymerize to form chains giving a red, viscous liquid.

b. What will happen to the elastic material obtained at the end? Why?

It will become brittle as teh sulfur goes back to the S8 allotrope stable at room temperature.

c. During the demonstration an acrid odor was noted as the experiment proceeded. What is the odiferous gas likely to be? Write an equation for its formation.

Sulfur dioxide

S + O2 Æ SO2

or S8 + 8 O2 Æ 8 SO2

7. (7) Why do the elements of the second period tend to differ so markedly from the other elements of their groups?

They are very small which results in stronger forces on the valence electrons. This results in stronger, more covalent bonds.

In addition, they have no d-orbitals available, so cannot have expanded valence shell bonding.

8. (20) Write the NET ionic equation for the reaction which occurs when the following are mixed. (If there is no reaction, indicate by "NR.")

a. Tin(IV) sulfide is treated with a large excess of sodium hydroxide.

SnS2 + 6 OH- Æ Sn(OH)62- + 2 S2-

b. Bismuth(III) chloride is mixed with hydrogen sulfide in a moderately acidic solution.

3 H2S + 2 Bi3+ Æ Bi2S3(s) + 6 H+

c. Aluminum metal is added to a solution of tin(IV) in 6M hydrochloric acid (SnCl62-).

2 Al(s) + 3 Sn(Cl)62- Æ 2 Al3+ + 3 Sn(Cl)42- + 6 Cl-

d. Sulfuric acid is added to a solution of barium chloride.

HSO4- + Ba2+ Æ BaSO4(s) + H+

e. Sulfur trioxide is added to water.

SO3(g) + H2O Æ H+ + HSO4-


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Last modified 30 October 2006