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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
  • $\begingroup$ 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
  • $\begingroup$ 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
  • $\begingroup$ 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
  • $\begingroup$ 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.)

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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)

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