CHEM 161: Exam 4
Name:
Section:
Page 1
Page 2
Page 3
Page 4
Page 5
Exam Total out of 152
Exam Percentage
Current Course Percentage
(6) 1. Check all of the following statements that are true:
 a. Under constant pressure, the heat of a reaction equals H.
 b. The H˚ for the reaction, N2(g) + 3 H2(g)  2 NH3(g), is a standard enthalpy of formation.
 c. The H˚ for the reaction, 2 K(s) + O2(g)  K2O2(s), is a standard enthalpy of formation.
 d. The H˚ for the reaction, CO(g) + F2(g)  COF2(g), is a standard enthalpy of formation.
 e. Hf =0 for all of the following: Na(s), Ne(g), H2(g), O2(g), F2(g), and Br2(l).
 f. Ammonium carbonate decomposes to form ammonia, steam, and carbon dioxide gas. The
pressure-volume work done by the reaction reduces the overall internal energy of the system.
(5) 2. Check all of the following statements that are true:
 a. Water condensing on an ice-cold class is an exothermic process.
 b. Dry ice subliming to carbon dioxide gas is an endothermic process.
 c. Boiling water is an exothermic process.
 d. If all acid-base neutralization reactions release heat, they are all exothermic reactions.
 e. In an endothermic reaction more energy is required to break the bonds within the reactants
compared to the energy released when the products’ bonds are formed.
(14) 3. Check all of the statements below that are CORRECT:
 a. If CCl4 is nonpolar, then the electronegativity values for carbon and chlorine are equal.
 b. If PF5 is nonpolar, then there are no polar covalent bonds in the PF5 molecule.
 c. If ozone (O3) is polar, the oxygen atoms must have different electronegativity values.
 d. The COCl2 molecule does not require resonance structures to be represented correctly.
 e. The dipole moment in water is greater than the dipole moment in the OF2 molecule.
 f. Resonance structures indicate that a double bond oscillates between different pairs of atoms.
 g. Liquid oxygen is attracted to a magnetic field, so oxygen molecules are paramagnetic.
CHEM 161 Exam 5
page 1 of 5
(56) 4. For each of the following molecules and polyatomic ion:
i. Draw the Lewis structure (with resonance structures if they apply), minimizing formal charges.
ii. Indicate the molecular geometry and bond angle(s) on the central atom in the TABLE below.
iii. Sketch the three-dimensional shape of the molecule and show the dipole for each polar bond.
iv. For each molecule indicate if it is polar or nonpolar.
SOCl2
Lewis Structure
Sketch of the 3D shape with dipoles
ClF3
chlorine trifluoride
Lewis Structure
Sketch of the 3D shape with dipoles
a.
b.
c.
IF5
Lewis Structure
Sketch of the 3D shape with dipoles
d.
BrO2
Lewis Structure
Sketch of the 3D shape with dipoles
Molecular Geometry (Shape)
Bond Angle(s)
Atomic Hybrid
Orbitals
Polar or Nonpolar?
SOCl2
polar
nonpolar
ClF3
polar
nonpolar
IF5
polar
nonpolar
BrO2
CHEM 161 Exam 5
page 2 of 5
5. Consider the formaldehyde molecule, CH2O.
(2) a. Draw the Lewis structure for formaldehyde, CH2O.
(4) b. The shape of CH2O is _____________________________, and its bond angles are _________.
(2) c. The atomic hybrid orbitals in the C atom in CH2O are _________________.
(2) d. There are _______ sigma () bonds and _______ pi () bonds in CH2O.
(6) e. Sketch the 3D shape of the molecule, clearly showing the hybridized orbitals on carbon, the sigma
bonds between the atoms, the unhybridized orbitals and corresponding pi bonds, and any lone pairs
on the atoms. Note that the hybridized orbitals in oxygen are the same as in carbon.
For the problem below, SHOW ALL WORK, giving answers with the correct units and the correct
number of significant figures to receive full credit. Circle your answer.
6. Consider the following reaction:
2 C2H2(g) + 5 O2(g)  2 H2O(g) + 4 CO2(g)
(18) Use Average Bond Energies (on the back of the Periodic Table) to calculate the enthalpy change (H)
for the reaction. (Note the skeleton structure for acetylene, C2H2, is provided below.)
H
C
C
CHEM 161 Exam 5
H
page 3 of 5
(10) 7. Calculate the enthalpy change for this reaction, C2H4(g) + 6 F2(g) 2CF4(g) + 4 HF(g)
using the following data:

H2(g) + F2(g)
H˚ = +537 kJ
C(s) + 2 F2(g)
H˚ = –680 kJ
2 C(s) +
 CF4(g)
2 H2(g) C2H4(g)
H˚ = +52.3 kJ
2 HF(g)
Circle your final answer!
For the problem below, SHOW ALL WORK, giving answers with the correct units and the correct
number of significant figures to receive full credit. Circle your answer.
9. Ammonia reacts with nitrous oxide according to the following thermochemical equation:
2 NH3(g) + 3 N2O(g) 4 N2(g) + 3 H2O(l)
(7)
H˚ = -1012 kJ
a. Calculate the heat released when 50.0 g of nitrous oxide react with excess ammonia.
(10) b. Calculate the enthalpy of formation for nitrous oxide given these enthalpies of formation:
Hf [NO2(g)] = 33.2 kJ/mol
Hf [H2O(g)]= -241.8 kJ/mol
Hf [NH3(g)] = -46.1 kJ/mol
Hf [H2O(l)]= -285.8 kJ/mol
CHEM 161 Exam 5
page 4 of 5
For the problem below, SHOW ALL WORK, giving answers with the correct units and the correct
number of significant figures to receive full credit. Circle your answer.
(12) 10. Diborane (B2H6) is a highly reactive boron hydride once considered as a possible rocket fuel.
Diborane can be synthesized from its elements. When 50.0 g of boron react with 20.0 g of
hydrogen to form diborane, 82.3 kJ of heat are absorbed by the reaction at standard state
conditions. Calculate the standard enthalpy of formation (in kJ/mol) for diborane.
Extra Credit: (8 points)
(3) a. If you attended class on Wednesday, Nov. 24th (the day before Thanksgiving),
give the element symbol or name for the secret element: ____________________. (Spelling counts!)
(5) b. Even though the ozone (O3) molecule contains only oxygen atoms, it is polar. Indicate the formal
charge on each oxygen atom in the molecule, then use formal charges and the molecular geometry of
the molecule to explain its polarity.
CHEM 161 Exam 5
page 5 of 5