[ashleighmarie08]

I am having trouble starting these problems so unfortunately I can't really show that I already attempted them. I'm not sure where to begin!? 
You don't have to give me the answers, just show me the steps to work them. Thanks in advance for your time.

1. Use bond energies and the fact that the heats of formation of OF2 and OCl2 are 24.7 and 80.37 kJ/mol, respectively to determine the O-F and O-Cl bond energies.

2. a. Use a table of bond energies to determine the atomization energy of PCl3 and the average P-Cl bond energy in PCl3.

b. Use the P-Cl bond energy and the atomization energy of POCl3 to determine the P-O bond energy in POCl3

3. Use Bond energies to estimate the heats of formation of the following gases:
NF3
H2N-NH2
F2C=CH2*
CH3Cl*
*Hint: You must use the heat of formation of carbon gas and bond energies to determine this energy.

4. The combustion reaction of chrysene (C18H12) at 298K.
C18H12(s) + 21O2(g) --> 18 CO2(g) + 6 H2O(l)

a. What is ∆Hcomb of chrysene if ∆Ecomb = -8942.6 kJ/mol?
b.What is the heat of formation of chrysene?

[Yggdrasil]

A good place to start on some of these is to write down the standard reactions for the formation of the compounds.  To estimate reaction enthalpies from bond energies, you basically add up the bond energies from all of the bonds that are formed and subtract the bond energies for all of the bonds that are broken.