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Name: D. J.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Friday, April 26, 2002


Question:
If bond energy indicates the energy required to break a chemical bond hence indicating stability? How does a h-cl bond with a bond energy of 431kj/mol have a greater stability than a h-h bond with a bond energy of 436kj/mol according to the text we are using.


Replies:
This is a very interesting question. I haven't thought about this stuff for awhile, but I'll give it a try. I think there may be a couple of possible things to consider:

1. I'm not sure exactly what type of bond energy is reported in your table. Bond dissociation energies are given for specific bonds, while average bond energies are given for the type of bond over many compounds (I think this is the one that may show up in textbooks. If this is the case, the bond dissociation energy for the given bond by itself may be a little different).

2. The thermodynamic stability (related to the potential energy) may be different from the chemical stability (the resistance of the compound to a chemical reaction--related to something called activation energy, which is a little extra bit of energy required to get a particular reaction going). In other words, assuming the numbers can be compared (see #1 above), they do not tell the whole story about stability in terms of actual chemical reactions. I think there may be other factors here, as well, such as the larger size and larger number of electrons in Cl, bond lengths, electronegativity, etc. This is a little like theoretical and actual gas mileage on a car--manufacturers can predict it, but there may be several factors that affect the actual outcome.

I hope this helps. This was a fun question to think about.

Pat Rowe



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