Chem 11 - Exam #4 Sample with Answers

Sample Exams are designed to give you an idea of the types of questions you can expect.  Questions may not exactly agree with currently assigned material, so you may need to examine other sample exams.

1. FILL-IN-THE-BLANK each of the following questions.

In general the ionization energy and electron affinity increase from __ Answer: left to right ____ (left to right or right to left) in a period of the periodic table.

Which of the following elements -- Be, B, C, & N -- has the smallest ionization energy? Answer: B (Be accepted)

Which of these elements -- Li, O, Ne, & Na -- has the largest atomic radius? Answer: Na

Ionization energy describes the energy required to remove an electron from a gaseous atom or ion?

Electron affinity is the energy change associated with the addition of an electron to a gaseous atom.

The lattice energy is the change in energy that takes place when separated gaseous ions are packed together to form an ionic solid.

Hybridization is the term used to describe the mixing of native atomic orbitals to form special orbitals for bonding.

Which has the larger atomic radius, P or Sb? Answer: Sb

Which has the larger ionic radius, O^2- or F^- ?  Answer: O^2-

2. Answer the following TRUE/FALSE questions.

_X_T ___F Valence electrons are found on the outermost principal quantum level of an atom.

___T _X_F When a metal reacts with a nonmetal a covalent bond is formed.

___T _X_F A nonpolar covalent bond results from the unequal sharing of a pair of electrons between atoms in a molecule.

_X_T ___F A double bond occurs when two atoms share two pairs of electrons.

___T _X_F The shape of an ammonia molecule is tetrahedral.

_X_T ___F The Lewis structure for CHCl₃ has 9 lone electron pairs.

_X_T ___F The shape of a carbon dioxide molecule is linear.

___T _X_F The size in a series of isoelectronic ions increases as the nuclear charge increases

_X_T ___F Whenever a set of equivalent tetrahedral atomic orbitals is required it will adopt a set of sp3 orbitals.

___T _X_F The hybridization of the B in BH₃ is sp³

___T _X_F In the Lewis structure for SF₄ the central sulfur atom shares 12 electrons.

___T _X_F Since double bonds have two pairs of electrons, double bonds are longer than single bonds because of electron repulsion.

3. Draw Lewis dot structures for each of the following compounds or ions.  Then, give its molecular geometry (if it has a structure we have not studied, write "none")  Formulas for molecules and ions are correct

Answer: I don't have the ability to post these structures right now.....so, check the book or make your best guess

PF₆^-

NI₃

PCl₄⁺

ClO₂

XeF₆

SiH₄

H₃PO₄

BF₃

4. Using the bond energies shown, estimate the heat of combustion for one mole of acetylene:

C₂H₂ (g) + (5/2)O₂ (g) --> 2CO₂ (g) + H₂O(g)

Answer: DH = -1227.5 kJ

Bond Bond Energy (kJ/mol)
C=C (triple bond) 839
C-H 413
O=O 495
C=O 799
O-H 467

5. Given the following information, calculate the bond energy of a single N-F bond.

N₂ bond energy = 941 kJ/mol
F₂ bond energy = 154 kJ/mol

1/2 N₂ (g) + 3/2 F₂ (g) --> NF₃ (g) DH^o = -103 kJ/mol

Answer: Bond energy for a single N-F bond is 268.2 kJ/mol

6. Draw the Lewis structures for each of the molecules listed below. Then, answer the following questions (more than one answer may be correct):

Answer: I don't have the ability to post these structures right now.....so, check the book or make your best guess

BH₃

O₃

NO₂

PCl₅

SF₆

________O₃_____________________________ Which molecule(s) obey the octet rule?

_________ 4 ____________________________ Which molecules have no dipole moment?

_______NO₂  and O₃ __________________________ Which of these molecules show resonance?

7. Given the following information (equations can be reversed, if necessary):

Li (s) --> Li (g) heat of sublimation of Li(s) = 166 kJ/mol
HCl (g) --> H (g) + Cl (g) bond energy of HCl = 427 kJ/mol
Li (g) --> Li⁺ (g) + e^- ionization energy of Li (g) = 520. kJ/mol
Cl (g) + e- --> Cl^- (g) electron affinity of Cl (g) = -349 kJ/mol
Li⁺ (g) + Cl^- (g) --> LiCl(s) lattice energy of LiCl(s) = -829 kJ/mol
H₂ (g) --> 2H (g) bond energy of H₂ = 432 kJ/mol

Calculate the net change in energy for the reaction: 2Li (s) + 2HCl (g) --> 2LiCl (s) + H₂ (g)

Answer: -562 kJ

8. Draw all possible resonance structures for each of the following two ions (atoms must be in the order shown):

Answer: I don't have the ability to post these structures right now.....so, check the book or make your best guess

NCO^-

CNO^-

9. Consider the structure of glycine, the simplest amino acid:

NOTE: CH₂ is C₁, COOH is C₂, COOH is O₁, COOH is O₂

NH₂-CH₂-COOH

a. What is the orbital hybridization for each of the N, C, and O atoms in glycine?

N = sp³
C₁ = sp³
C₂ = sp²
O₁ = sp²
O₂ = sp³

b. What is the total number of bonds in the molecule?  Answer: 10 bonds

c. What is the total number of p bonds in the molecule?  Answer: 1 p bond

These multiple choice questions are beneficial to study.  Answers given at the end of this page.

1. Atoms having greatly differing electronegativities are expected to form:

a.  no bonds
b.  polar covalent bonds
c.  nonpolar covalent bonds
d.  ionic bonds
e.  covalent bonds

2. Choose the compound with the most ionic bond.

a.  LiCl
b.  KF
c.  NaCl
d.  LiF
e.  KCl

3. For the elements Rb, F, and O, the order of increasing electronegativity is:

a.  Rb < F < O
b.  Rb < O < F
c.  O < F < Rb
d.  F < Rb < O
e.  None of these

4. Based on electronegativities, which of the following would you expect to be most ionic?

a.  N2
b.  CaF2
c.  CO2
d.  CH4
e.  CF4

5. Which of the following elements has the highest electronegativity?

a.  Ba
b.  Ca
c.  Si
d.  P
e.  Cl

6. Based on electronegativity differences, which of the following is most likely to be ionic?

a.  BaF2
b.  Cl2
c.  NH3
d.  NO3
e.  CH4

7. How many of the following molecules possess dipole moments?

      BH3, CH4, PCl5, H2O, HF, H2

a.  1
b.  2
c.  3
d.  4
e.  5

8. Which of the following molecules has no dipole moment?

a.  CO2
b.  NH3
c.  H2O
d.  all
e.  none

9.  Choose the statement that best describes the PbCl4 molecule in the gas phase.

 a.  The bond angles are all about 109o.
 b.  The molecule is polar.
 c.  The molecule has a dipole moment.
 d.  The bonds are nonpolar.
 e.  The bond angles are all about 109o, the molecule is polar, and 
     the molecule has a dipole moment.

10. Which of the following has the smallest radius?

a.  Cl-
b.  K+
c.  Cr
d.  Al3+
e.  Ar

11. Which of the following has the smallest ionic radius?

a.  Ca2+
b.  Cl-
c.  Li+
d.  O2-
e.  Be2+

12. Which of the following ionic compounds has the largest lattice energy, i.e., the lattice energy most favorable to a stable lattice?

a.  CsI
b.  LiI
c.  LiF
d.  CsF
e.  MgO

13. Calculate the lattice energy for LiF(s) given the following:

     sublimation energy for Li(s)         +161 kJ/mol
     delta Hf for F(g)                    +77 kJ/mol
     first ionization energy of Li(g)     +520. kJ/mol
     electron affinity of F(g)            -328 kJ/mol
     enthalpy of formation of LiF(s)      -617 kJ/mol

a.  285 kJ/mol
b.  -650. kJ/mol
c.  800. kJ/mol
d.  -1047 kJ/mol
e.  None of these

14. When electrons in a molecule are not found between a pair of atoms but move throughout the molecule, this is called

a.  ionic bonding.
b.  covalent bonding.
c.  polar covalent bonding.
d.  delocalization of the electrons.
e.  a dipole moment.

15. As the number of bonds between two carbon atoms increases, which one of the following decreases?

a.  number of electrons between the carbon atoms
b.  bond energy
c.  bond length
d.  all of these
e.  none of these

16. How many of the following molecules or ions contain double or triple bonds?

      NH3      NH4+       HCN     CO2     NO2

a.  0
b.  1
c.  2
d.  3
e.  4

17.  How many of the following molecules -- SF2, SF4, SF6, SiO2 -- are polar?

 a.  0
 b.  1
 c.  2
 d.  3
 e.  4

18. Which of the following molecules are nonlinear?

      NO2-,     C2H2,     N3-,     HCN,     CO2,     H2O2

a.  C2H2, HCN
b.  CO2, N3-
c.  NO2-, H2O2
d.  N3-, NO2-
e.  all are linear

19.  In the molecule C2H4 the valence orbitals of the carbon atoms are assumed to be

 a.  not hybridized.
 b.  sp hybridized.
 c.  sp2 hybridized.
 d.  sp3 hybridized.
 e.  dsp hybridized.

20. Which of the following molecules contains an sp2-sp2Æ sigma bond?

a.  CH4
b.  C2H2
c.  C2H4
d.  C2H6
e.  none of these

21.  The hybridization of Br in BrF3  is

a.  sp
b.  sp2
c.  sp3
d.  dsp3
e.  d2sp3

22. If four orbitals on one atom overlap four orbitals on a second atom, how many molecular orbitals will form?

a.  1
b.  4
c.  8
d.  16
e.  none of these


TRUE/FALSE

23. In general, a larger atom has a smaller electronegativity.

24. The shape of a carbon dioxide molecule is linear.

25. When an electron pair is shared in the area centered on a line joining the atoms a sigma bond is formed.


SHORT ANSWER

26. When nonmetals chemically combine, they tend to form what type of bond?


FILL-IN-THE-BLANK

27. The __________ of a molecule shows how the valence electrons are arranged among the atoms in the molecule.

28. When several nonequivalent Lewis structures can be drawn for a molecule, __________ is used to determine the most appropriate structure(s).


ANSWER KEY FOR TEST

1. d
2. b
3. b
4. b
5. e
6. a
7. b
8. a
9. a
10. d
11. e
12. e
13. d
14. d
15. c
16. d
17. c
18. c
19. c
20. c
21. d
22. c
23. True
24. True
25. True
26. covalent
27. Lewis structure
28. formal charge