Chapter 7

Question 1

Localized vs delocalized

Answer 1

localized: one random double bond
delocalized: double bond, single bond, double bond, single bond ; more than one resonance structure

Question 2

Draw the MO diagram for H2

Answer 2

see chapter 7 notes

Question 3

How do you calculate bond order?

Answer 3

(bonding-antibonding)/ 2

Question 4

Draw the MO diagram for He2+

Answer 4

see chapter 7 notes

Question 5

Use MO diagrams to explain the trend in the following bond energies:
B2= 290 KJ/mol
C2 = 600 KJ/mol
N2 = 942 KJ/mol

Answer 5

B2 bond order = 1
C2 bond order = 2
N2 bond order = 3

bond order increases as energy increases

Question 6

The first ionization energy of NO is 891 KJ/mol, that of N2 is 1500 KJ/mol and that of CO is 1350 KJ/mol. Use MO electron configurations to explain why NO ionizes more easily than either N2 or CO.

Answer 6

NO ionizes more easily because removing an electron from an antibond makes the molecule more stable. N2 and CO would be unstabilized if an electron were to be removed.

Question 7

Electrical eye door openers use photoconductors that respond to infrared light with a wavelength of 1.5 um. Which is suitable for photoconductors operating at this wavelength, germanium (Eg=64 kJ/mol) or silicon (Eg= 105 kJ/mol)

Answer 7

E= nhc/landa
= 80000 J/mol or 80kJ

germanium has enough energy
silicon not enough energy

Question 8

For the N and B atoms in the molecule NB, determine the hybridization and the formal charge for each.

Answer 8

N: sp3 and +1
B: sp3 and -1

Question 9

Determine the number of sigma and pi bond in each molecule.

a) H2C=CCl2
b) HOOC-COOH
c) FHC=C=CHF

Answer 9

a) sigma = 5 pi= 1
b) sigma = 7 pi = 2
c) sigma = 6 pi = 2

Question 10

Construct the MO diagram for H2+ and then identify the bond order.

Answer 10

see cahier

bond order= 1/2

Question 11

Which of the following contains one or more localized or delocalized pi bonds? 
H20
O3
HCN
CO3-
C2H4

Answer 11

delocalized: O3, CO32-
localized: HCN, C2H4
* H20 has no double bonds*

Question 12

Conductor vs semiconductor vs insulator

Answer 12

conductor: no band gap, electrons easily promoted to conduction band to conduct electricity
semiconductor: small band gap where electrons can gain enough energy to be promoted into the conduction band to conduct electricity
insulator: does not conduct electricity due to huge band gap where too much energy is required for electrons to be promoted to the conduction band

Question 13

The conductivity of intrinsic semiconductors, such as silicon and germanium, can be increased when small amounts of group 3A or group 5A elements are added, a process called dping. The resulting semiconductors are called either p-type or n-type semiconductors. Classify Ge doped with Ga and Si doped with P.

Answer 13

p-type: Ge doped with Ga
Ge has one more electron than Ga, making Ga more positive

n-type: Si doped with P
* Si has one less electron than P, making P more negative*

Question 14

CdTe is a semiconductor material that has a band of 139 kJ/mol. What is the longest wavelength of light, in nm, that will excite an electron from the valence band to the conduction band?

Answer 14

E=nhc/ landa
landa= nhc/E
= 8.61*10^2 nm

Question 15

For which of the following molecules and ions does the central atom nitrogen have sp3 hybridization?

a) HNO3
b) NBr3
c) NO2
d) NO
e) NOBr

Answer 15

b) NB3

tetrahedral

Question 16

Which of the following solid compounds will have the band structure represented in the right? Black represents electron population, and the energy of the band group corresponds to the visible region of the electromagnetic spectrum.

a) InSb
b) silicon doped with 0.3% by mass arsenic
c) silicon doped with 0.2% by mass aluminum
d) NaCl doped with sulphur anions
e) zinc

Answer 16

• since valence electrons are not completely filled, there is an empty space, this is a positive doping, meaning group 3A will be involved. If it were negative, group 5A would be involved*
  c) silicon doped with 0.2% by mass aluminum

Question 17

Phosgene is a colourless and highly toxic gas with formula CCl2O. What is the hybridization state of the carbon atom?

a) sp
b) sp2
c) sp3
d) sp3d
e) sp3d2

Answer 17

b) sp2

Question 18

If an unknown element X makes 3 sigma bonds and one pi bond to three other atoms and X possesses a lone pair of electrons with zero formal charge in its best lewis structure, what group of the periodic table is X from?

a) 3
b) 4
c) 5
d) 6
e) 7

Answer 18

group 6A due to X having 6 valence electrons