# The source for the N-N bond dissociation energy of 240 kJ/mol in the table

Authoring an exam question, I made the mistake of taking a bond dissociation energy from a table I found on the internet. It comes in different styles, but is always labeled Table 7.1. It gives a bond dissociation energy of $$\pu{240 kJ/mol}$$ for the - admittedly uncommon - nitrogen:nitrogen single bond. A student was puzzled that the enthalpy of formation for hydrazine, estimated based on this value, was so far off from the experimental value; the student then found a value for the bond dissociation energy of $$\pu{159 kJ/mol}$$ in a more rigorous search.

The General Chemistry textbooks available to me from home have values close to $$\pu{159 kJ/mol}$$ (OpenStax Chemistry: $$\pu{160 kJ/mol}$$; Kotz 6th ed (2006): $$\pu{163 kJ/mol}$$; Radel (1994): $$\pu{170 kJ/mol}$$). I am not worried about the small differences - it depends which set of molecules containing $$\ce{N-N}$$ bonds was used, and a substructure search of commercially available compounds shows that there is quite a variety, with many having partial double-bond character (the following sample are 3 from the 100 or so just from Sigma Aldrich):

My question is: Where did Table 7.1 originate, and where did the value for $$\ce{N-N}$$ originate?

Here is a selection of links to variations of table 7.1, with the first example as picture:

https://wps.prenhall.com/wps/media/objects/4678/4790506/ch07_02.htm

https://www.chegg.com/homework-help/questions-and-answers/use-bond-dissociation-energies-table-calculate-approximate-h-kilojoules-reaction-acethylen-q27780023

https://www.chegg.com/homework-help/questions-and-answers/average-bond-dissociation-energies-d-kj-mol-h-h-4362-h-c-410-h-f-570a-h-ci-432a-h-br-3662--q36015212

https://cdn.clutchprep.com/problem_images/65691-36fd337a4e3e837c.png

• The table is from McMurry's Chemistry textbook. I cannot tell the exact edition; for instance, your table appears as Table 7.2 in 7th edition from 2015 (ISBN 978-0-321-94317-0), also listing $D(\ce{N-N}) = \pu{240 kJ/mol}.$ – andselisk May 4 at 13:40
• @andselisk Strangely, the 8th edition has a value of 140 kJ/mol. (overshooting in the other direction to compensate, or fixing a single-digit typo?). – Karsten Theis May 4 at 14:19
• Strange indeed. For the record, Table 7.1 in 6th edition from 2012 also has $D(\ce{N-N}) = \pu{240 kJ/mol}.$ – andselisk May 4 at 14:24

## 1 Answer

Your question about the source of Table 7.1 is correctly answered by the andselisk's comment. The value of $$\pu{240 kJ\:mol-1}$$ for $$\ce{N-N}$$ bond is probably initiated from Encyclopedia of Inorganic Chemistry, which has listed bond energies of wide variety of bonds and their respective bond lengths. It has given two values for $$D_\circ$$ of $$\ce{N-N}$$ bond: ~$$\pu{167 kJ\:mol-1}$$ for $$\ce{N2H4}$$ in general and $$\pu{247\pm 13 kJ\:mol-1}$$ for $$\ce{H2N-NH2}$$ in particular.

The reason is given in the very first paragraph of the article. In their words:

There are two different ways to define the bond energy even for the simplest diatomic molecule (see the diagram below). $$\Delta D_{\circ'}$$ is the dissociation energy measured from the very bottom of the potential energy well. However, the molecule possesses zero-point energy and thus, the experimentally measured dissociation energy $$\Delta D_{\circ}$$ is somewhat less:

In poly atomic compound there are other considerations. For example, the mean bond energy of $$\ce{N-H}$$ is the average of three bond energies associated with sequential fission of the three $$\ce{N-H}$$ bonds in ammonia ($$\ce{NH3}$$). These three values are different from each other and also different from their average, the mean bond energy of $$\ce{N-H}$$. Using this mean bond energy of $$\ce{N-H}$$ in $$\ce{NH3}$$ and the heat of atomization of $$\ce{H2N-NH2}$$, a value of $$\pu{159 kJ\:mol-1}$$ has been found for the $$\ce{N-N}$$ single bond energy. Then again, using the mean bond energy of $$\ce{N-F}$$ in $$\ce{NF3}$$ and the heat of atomization of $$\ce{F2N-NF2}$$, a value of $$\pu{172 kJ\:mol-1}$$ has been found for the $$\ce{N-N}$$ single bond energy. These two values are in somewhat good agreement. However, the dissociation of the two molecules are quite different:

$$\ce{H2N-NH2 -> 2 H2N^.} \qquad \pu{247 kJ\:mol-1}$$ $$\ce{F2N-NF2 -> 2 F2N^.} \qquad \pu{88 kJ\:mol-1}$$

Therefore, it is warned that care should be exercised in using any values of the bond energies.

Source:

R. Bruce King (Editor in chief), "Bond Energies," Encyclopedia of Inorganic Chemistry; 2nd Edition, John Wiley & Sons, Ltd.: New York, NY, 2005 (ISBN: 978-0-470-86078-6). Bond Energies is available online: https://doi.org/10.1002/0470862106.id098

• So once the N-N bond in hydrazine is broken, the N-H bonds are much weaker than they would be in ammonia? I like talking about bond dissociation energies in a General Chemistry setting, but always say it is a crude estimate. Even so, I think it is helpful in e.g. understanding how elemental oxygen is a reagent in many exothermic reactions while elemental nitrogen is not (at least not with the reaction partners that are sitting out on the shelf - I guess if they reacted with elemental nitrogen, they would not be sitting there anymore). – Karsten Theis May 5 at 13:42