Question

Why is there a difference in the heats of formation of CF4 and CCl4? I know that the bond length of CCl4(C--Cl) is longer than that of CF4(C--F), does this affect the heat of formation. Heat of formation are -225.709kcal/mol(CF4) and -28.120kcal/mol(CCl4).

Answer 1

Wouldn't you expect the heat of formation of a stronger bond to be different than that of a weaker bond? I would.

Answer 2

Thanks. I also have a similar question about cis and trans isomers.why is the heat of formation lower for a trans isomer? the only thing i could think of is that the dipole moment is 0.But does this have anything to do with the heats of formation?

Answer 3

That sounds like the answer is in the ball park. Also, consider that in the cis isomer, there is a repulsion of the two atoms on the same side that is not there when they are trans to each other. It takes extra energy to overcome the repulsion present in the cis configuration.