I have been working with thermochemistry at school for a while. I have been introduced to the two terms: formation enthalpy and bond enthalpy. However, I have not quite understood the difference between the two and would be very glad if someone could explain it!
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$\begingroup$ Is there a connection between form. entalphy and bond entalphy? For example if we have: C + O2 => CO2. Can the formation entalphy be calculated if one knew the bond entalphy for each bond? $\endgroup$ – SSTT Nov 26 '15 at 18:24
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$\begingroup$ There is a connection of sorts, but not a direct one. As to your example: what bonds exactly do you include in "each bond"? $\endgroup$ – Ivan Neretin Nov 26 '15 at 18:35
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$\begingroup$ The double bond in the O2 and the 2 double bonds between the C and O in CO2 $\endgroup$ – SSTT Nov 26 '15 at 19:31
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$\begingroup$ OK, but that's not enough: you have to consider the bonds in elementary carbon as well. $\endgroup$ – Ivan Neretin Nov 26 '15 at 19:47
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$\begingroup$ Ok, so let's say you take that in account as well. Then how would the form. entalphy be calculated? $\endgroup$ – SSTT Nov 26 '15 at 20:10
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OK, here is the difference: formation enthalpy refers to the formation of a compound from elementary compounds, while bond energy is applicable when considering the formation of a molecule from isolated atoms. The relation between the two is pretty much explained by the Hess's law: the enthalpy of a reaction does not depend on its pathway, but only on the initial and final states.
Now consider the following two pathways for your example.
Pathway 1: we burn graphite in oxygen, just like $\ce{C + O2 -> CO2}$. There is some change of enthalpy associated with this reaction.
Pathway 2: we vaporize graphite to get atomic carbon (see how many C-C bonds are broken, take that into account), atomize $\ce{O2}$ to get elementary oxygen (again, some bonds are broken), and finally produce $\ce{CO2}$ (this time some bonds are formed). Now if you'll look up the energies of all bonds involved and carefully add/subtract them all together, you'll have a decent approximation of the reaction enthalpy obtained in the pathway 1.
(Normally it works the other way around, that is, we use reaction enthalpies to calculate bond energies, but... whatever.)
answered Nov 26 '15 at 20:54
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$\begingroup$ The term "elementary compound" is erroneously used. A compound is made of two or more types of atoms. Thus, an element cannot be a compound. $\endgroup$ – Tan Yong Boon Jan 4 '18 at 0:40
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Formation Enthalpy: energy it takes to form a compound from elements
Bond Enthalpy: an approximation of the energy it takes to form a compound from anything by calculating the bond energies(bond energies of the reactants - bond energies of the products)
