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Bond energy
From Wikipedia, the free encyclopedia
In chemistry, bond energy (E) is a measure of bond strength in a chemical bond. For example the carbon-hydrogen bond energy in methane E(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and 4 hydrogen radicals divided by 4. Bond energy (E) should not be confused with bond dissociation energy.
Another example: an O–H bond of a water molecule (H–O–H) has 493.4 kJ mol-1 of bond dissociation energy, and 424.4 kJ mol-1 is needed to cleave the remaining O–H bond. The bond energy of the O–H bonds in water is 458.9 kJ mol-1, which is the average of the values.
Some bond energy trends (units are in kcal/mol and (kJ/mol)) :
| H | F | Cl | Br | I | OH | N | NH2 | |
|---|---|---|---|---|---|---|---|---|
| bond energies H–X | 104 (436) | 135 (570) | 103 (431) | 87 (366) | 71 (298) | 119 (498) | 110 ( 460) | — |
| bond energies CH3–X | 105 (440) | 109 (452) | 84 (352) | 70 (293) | 56 (236) | 91 (382) | — | 87 (365) |
Bond strength (energy) can be directly related to the bond distance. Therefore we can use the metallic, ionic, or covalent radii of each atom in the molecule to determine the bond strength. For example, the covalent radius of boron is estimated at 83.0 pm, but the bond length of B–B in B2Cl4 is 175 pm, a significantly larger value. This would indicate that the bond between the two boron atoms is a rather weak single bond. In another example, the metallic radius of rhenium is 137.5 pm, with a Re–Re bond length of 224 pm in the compound Re2Cl8. From this data, we can conclude that the bond is a very strong bond or a quadruple bond. This method of determination is most useful for covalently bonded compounds .
- ↑ Handbook of Chemistry & Physics 65th Edition CRC Press ISBN 0-8493-0465-2
- ↑ Alcock, N.W. Bonding and Structure: structural principles in inorganic and organic chemistry Ellis Horwood Ltd., New York. 1990 pp. 40-42.