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Bond Enthalpies

Posted: Thu Jan 28, 2016 4:45 pm
by 304638245
What is the difference in using mean bond enthalpies and bond enthalpies of diatomic molecules? For example, the O-O bond has a mean bond enthalpy of 157 kJ/mol but is 496 kJ/mol under bond enthalpies of diatomic molecules. Is it in regards to the bond type because O2 has a double bond? If so will the information always be given in separate tables like that?

Re: Bond Enthalpies

Posted: Thu Jan 28, 2016 5:38 pm
by LinaLi2E
Do you remember the molecular geometry based on VSEPR? Well that is linked to the mean bond enthalpy in a way. For example, CH4, every hydrogen broken off affects the remaining C-H bond. So every C-H requires a slightly different energy to be broken. That is why you use the mean bond enthalpies. While the bond enthalpies for diatomic molecules apply to just diatomic molecules. Diatomic molecule only have one bond (single, double, or triple). When that bond breaks, there are no other bonds remain that could shift, thus the bond enthalpies for diatomic molecules are different. The bond enthalpies for diatomic molecule are what they are, they do not change depending what is already broken and what is not, thus there is no mean for one molecule. So you would use the bond enthalpy for diatomic molecule for O2, and the mean bond enthalpy when there is more than one bond in the molecule.