How to derive radius ratio and calculate reaction energy

1. Jan 11, 2008

aznkid310

Greetings all,

My first problem: I have been asked to derive the minimum radius ratio for coordination numbers 4 and 6. I read the book and looked at the sample problems, but they do not help at all. I realize that it has to do with the geometry of it, but have no idea how to use that.
The book is:
INTRODUTION TO MATERIALS SCIENCE FOR ENGINEERS (6th edition) by James F. Schackelford.

My second problem: How does one calculate the rxn energy for polymerization of styrene (C_3H_6). We are given various bond energies for single carbon - carbon bonds at different lengths. I drew it and looks like: c - c = c w/ H atoms around it. In an example, the picture is: c = c and they broke it down into 2(c - c). how would i do it for 3 carbons? Also, how would I know what the bond length is (since the bond energies are related to them)?

2. Jan 12, 2008

Spirochete

Does rxn energy=enthalpy? If so, you need a table of bond dissociation energies, which tell you how much energy it takes to break different kinds of bonds. Then look at the reaction and see which bonds were broken and which were formed. Enthalpy equals the energy used to break the bonds plus the energy gained by forming the new bonds. So enthalpy=broken minus formed.

I'm not sure how bond length would be required to solve the problem, but that information should be available in a table somewhere.

But I'm no engineer, so chances are the problem is more complicated than this.