CHM1045
Spring 1999
Dr. Michael Blaber
Exam #3 100 points total
Mon July 19th 1999
-3 points if [Be]+ and [Br]- given (i.e. only 1 electron transfer)
2. Ordinary table salt (NaCl) is an ionic compound of sodium and chlorine. Sodium ion has a radius of 0.97Å, and the chlorine ion has a radius of 1.81Å. What is the magnitude of the attractive potential energy between the two ions in NaCl? (k = 8.99 x 109 J m/C2) (5 points)
2 points for basic equation, 1 point for correct parameters, 2 points for correct answer
3. For each of the noble gases below give an example of a cation that is isoelectronic with the noble gas (5 points)
He Li+ or Be2+ Ne Na+ or Mg2+ Ar K+ or Ca2+ Kr Rb+ or Sr2+ Xe Cs+ or Ba2+
4. Of the following ions, which is the smallest? Cl-, Br-, K+, Ca2+ (5 points)
Ca2+
5. What are the five steps to follow when drawing a Lewis Structure for an ABn type of molecule? Note: the correct sequence is essential! (5 points)
1. Sum valence electrons for all atoms
2. Write the symbols for the atoms, and make appropriate connections with a single bond
3. Complete the octets of atoms bonded to central atom
4. Place any leftover electrons on central atom
5. Try multiple bonds if central atom has less than an octet of electrons
6. Draw the Lewis Structure for the carbonate ion, CO32- (Note: carbonate is an ABn type of molecule, with C as the central atom) (5 points)
-2 if no resonance structures given, -1 if no brackets and charge
7. For the following bonds, indicate whether you would expect the bonds to be ionic, polar covalent, or nonpolar covalent (5 points)
O-O Nonpolar covalent
H-Br Polar covalent
H-F Polar covalent
H-P Nonpolar covalent
Na-F Ionic
8. For each of the following possible Lewis Structure(s) of the PSiO- ion, indicate the formal charges on the atoms and based on the formal charges indicate the most probable structure (8 points)
9. The combustion of ethane (see structure below) with oxygen produces water and carbon dioxide. Write the balanced equation for the combustion of ethane (5 points). Based upon bond enthalpies (see last page), calculate the reaction enthalpy for the combustion of 1mol of ethane (5 points)
C2H6 + 7/2 O2 -> 2 CO2 + 3 H2O or 2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O
Bonds broken: 1 C-C, 6 C-H, 7/2 O=O
348 + (6*413) + (7/2*495) = 4559 kJ (2 points)
Bonds formed: 4 C=O, 6 O-H
(4 * 799) + (6 * 463) = 5974 kJ (2 points)
Overall reaction: bonds broken - bonds formed = 4559 kJ - 5974 kJ = -1415 kJ (1 point)
10. Give the names for the following molecular geometries (5 points)
11. The GeF3- ion is an ABn type molecule (Ge is central atom). Draw the Lewis Structure (2 points). Give the electron pair geometry (2 points) and the molecular geometry (2 points) for this ion.
-1 if no brackets or charge
12. The SeCl4 molecule is an ABn type molecule (Se is central atom). Draw the Lewis Structure (2 points). Give the electron pair geometry (2 points) and the molecular geometry (2 points)
13. The molecule F2SiO is an ABn type molecule (with Si as the central atom). Draw the Lewis Structure (2 points). What is the expected angle between F, Si and O atoms (2 points)? What is the expected angle between F, Si and F atoms (2 points)?
14. Give three examples of molecular geometries for ABn type molecules that would not have an overall dipole even if the individual bonds were covalant polar bonds. Give three examples of molecular geometries for ABn type molecules that would have an overall dipole if the individual bonds were covalent polar bonds (6 points)
No dipole for: trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, linear, square planar
Dipole for: bent, trigonal pyramidal, seesaw, T-shaped, square pyramidal
15. Give the electron pair geometry, expected hybrid orbitals and molecular geometry for the atoms in the following structure (6 points)
16. For the following resonance structure of the NO3- ion, draw a valence orbital diagram for each atom (12 points)
2 points for correct hybrid orbitals, 2 points for correct electron assignments