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I am perfectly sure than heterolytic fission of a bond needs more energy than homolytic ones, since it requires opposite charge separation. When the bond dissociation energy is listed, which one are they talking about?

Ex: Suppose bond energy of A-B is 500kJ/mol. Is the energy to make A radical + B radical, or A cation + B anion, or A anion + B cation?

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    $\begingroup$ Suppose you read in a book that bond energy is 260kJ/mol, only they don't say which bond was that. You'll get confused and angry at the author, and rightly so. What did they think they were doing? This number is useless unless we know what it refers to! Now, this is the answer to your own question as well. $\endgroup$ – Ivan Neretin Apr 21 '17 at 6:46
  • $\begingroup$ @Ivan Neretin Exactly! $\endgroup$ – Pritt says Reinstate Monica Apr 21 '17 at 6:58
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    $\begingroup$ As a 'rule of thumb' the radicals are usually found at a lower energy than the ions, e.g. NaI, which means that there is more than one dissociation energy depending on what the products are. As @Ivan Neretin writes these should be stated in any table/book. $\endgroup$ – porphyrin Apr 21 '17 at 9:29
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This Wikipedia article summarizes the definition of bond-dissociation well:

Bond-dissociation energy (BDE or D0) is one measure of the strength of a chemical bond. It can be defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at $\pu{0 K}$ (absolute zero). For instance, the bond-dissociation energy for one of the $\ce{C–H}$ bonds in ethane ($\ce{C2H6}$) is defined by the process: $\ce{CH3CH2–H → CH3CH2• + H•}$

Thus the definition of bond-dissociation refers to homolytic fission, and in your example the bond energy of $\ce{A-B}$ is the energy required to make "A radical + B radical".

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