TOPIC 8. Basic Concepts of Chemical Bonding

1. Calculate the molar enthalpy of formation (DHf) of KF(s) using the tables of Molar Enthalpies of Formation, Ionization Energies, Electron Affinities, and Lattice Energies.

HINT: Look up the values and insert them into the equation:

DHf = DHf(K) + DHf(F) + I(K) + EA(F) + U(KF)

ANSWER: Solving the equation results in DHf of KF(s) = - 547 kJ/mol

 2. Using ONLY the Periodic Table, place the following sets of atoms in the order of increasing electronegativity:

a) S, Cl, Se

b) Li, Be, Na

c) F, O, P

HINT: Electronegativities increase from left to right and from the bottom up.

ANSWERS:

a) Se < S < Cl

b) Na < Li < Be

c) P < O < F

 3. Using ONLY the Periodic Table, determine which one of the following pairs of bonds is more polar:

a) O-F, C-F

b) B-N, B-O

c) O-Se, O-S

HINT: The greater the difference in electronegativities the more polar the bond.

ANSWERS:

a) C-F is more polar than O-F

b) B-O is more polar than B-N

c) O-Se is more polar than O-S

 4. Draw the Lewis structure for each of the following:

a) SiH4

b) SF2

c) N2O (a nitrogen is in the middle)

d) HSO4- (all four oxygens are bonded to the sulfur, and the hydrogen is on one of the oxygens)

e) NH2OH (connected like this: H2N-O-H with two of the hydrogens on the nitrogen and the other on the oxygen)

HINTS: Arrange the available electrons so that there are 8 around each atom. You may have to use double or triple bonds and there may be unshared pairs of electrons on some atoms. Do these in pencil with a big eraser! Keep rearranging the electrons until you've got it right. These become easier to do with practice.

ANSWERS:

 

5. Calculate the formal charges on the following (you will have to write their Lewis structures first).

a) CO2 (the carbon is in the middle)

b) CH3-

c) PO43- (the four oxygens are bonded to the phosphorus)

d) CO

HINTS: Draw the Lewis structure just as you did in the previous question. Then use the equation:

FC = # of valence e- - (½ shared e- + unshared e-)

 

ANSWERS:
6. Based on formal charges, which is the more likely arrangement of atoms NFO or FNO?

HINT: Draw the Lewis structure and determine the formal charges in each arrangement. The smaller set of formal charges is the more likely arangement.

ANSWER: The smallest set of formal charges for NFO (double bond between N and F and single bond between F and O) is -1 for N, +2 for F and -1 for O. The smallest set of formal charges for FNO (single bond between F and N and double bond between N and O) is 0 for F, 0 for N and 0 for O. FNO is the more likely arrangement of atoms.

 7. Draw resonance structures for the following:

a) O3

b) CO32- (the oxygens are all bonded to the carbon)

c) HCO2- (the two oxygens and the hydrogen are bonded to the carbon)

d) SO3 (the oxygens are all bonded to the sulfur)

HINT: Draw all possible valid Lewis structures in each case.

ANSWERS:
8. Draw the Lewis structures for each of the following. Identify the ones that do not obey the octet rule and tell why they do not:

a) SO32- (the oxygens are all bonded to the sulfur)

b) BH3

c) I3-

d) AsF6- (all six fluorines are bonded to the arsenic)

e) O2-

HINTS:

Atoms beyond the second row in the Periodic Table may have 8, 10, or 12 electrons around them.

Atoms with too few valence electrons may have less than 8 electrons around them.

Molecules or ions with an odd number of electrons can not have electron pairs.

ANSWERS:

9. Using the table of Average Bond Enthalpies estimate DHorxn for the following reaction:

    H                        H          H
    |                        |          |
  H-C=N-H(g) + H-O-H(g) à H-C=O(g) + H-N-H(g)

HINTS:

Determine how much energy is needed (positive enthalpies) to break all the bonds in the reactants.

Determine how much energy is given off (negative enthalpies) by the formation of the bonds in the products.

Add these to give the DHorxn.

ANSWER: DHorxn = 22 kJ     (2763 kJ - 2741 kJ)
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