طاقة رابطة

فى الكيمياء, طاقة الرابطة (E) هى مقياس لقوة الرابطة الكيميائية. فمثلا طاقة الرابطة C-H في جزيء الميثان (E(C-H هى التغير في الإنثالبي الحادث عند تكسر جزيء واحد من الميثان إلى ذرة كربون وأربعة ذرات هيدروجين مقسوما على أر بعة. ويجب عدم الخلط بين طاقة الرابطة (E) وبين طاقة تفكك رابطة.

ومثال أخر: هو الرابطة O-H في جزيء الماء (H-O-H) لها 493.4 kJ mol^-1 من طاقة تفكك الرابطة، وتحتاج 424.4 kJ mol^-1 لكسر الرابطة O-H المتبقية. بينما طاقة الرابطة O-H في جزيء الماء 458.9 kJ mol^-1, وهو متوسط القيمتين.

The bond dissociation energy (enthalpy) is also referred to as bond disruption energy, bond energy, bond strength, or binding energy (abbreviation: BDE, BE, or D). It is defined as the standard enthalpy change of the following fission: R - X → R + X. The BDE, denoted by Dº(R - X), is usually derived by the thermochemical equation,

${\begin{array}{lcl}\mathrm {D^{\circ }(R-} X)\ =\Delta H_{f}^{\circ }\mathrm {(R)} +\Delta H_{f}^{\circ }(X)-\Delta H_{f}^{\circ }(\mathrm {R} X)\end{array}}$

The enthalpy of formation ΔH_fº of a large number of atoms, free radicals, ions, clusters and compounds is available from the websites of NIST, NASA, CODATA, and IUPAC. Most authors prefer to use the BDE values at 298.15 K.

For example, the carbon–hydrogen bond energy in methane BE_(C–H) is the enthalpy change (∆H) of breaking one molecule of methane into a carbon atom and four hydrogen radicals, divided by four. The exact value for a certain pair of bonded elements varies somewhat depending on the specific molecule, so tabulated bond energies are generally averages from a number of selected typical chemical species containing that type of bond.

Bond energy (BE) is the average of all bond-dissociation energies of a single type of bond in a given molecule. The bond-dissociation energies of several different bonds of the same type can vary even within a single molecule. For example, a water molecule is composed of two O–H bonds bonded as H–O–H. The bond energy for H₂O is the average of energy required to break each of the two O–H bonds in sequence:

${\begin{array}{lcl}\mathrm {H-O-H} &\rightarrow &\mathrm {H\cdot +\cdot O-H} &,D_{1}\\\mathrm {\cdot O-H} &\rightarrow &\mathrm {\cdot O\cdot +\cdot H} &,D_{2}\\\mathrm {H-O-H} &\rightarrow &\mathrm {H\cdot +\cdot O\cdot +\cdot H} &,D=(D_{1}+D_{2})/2\\\end{array}}$

Although the two bonds are the equivalent in the original symmetric molecule, the bond-dissociation energy of an oxygen–hydrogen bond varies slightly depending on whether or not there is another hydrogen atom bonded to the oxygen atom.

When the bond is broken, the bonding electron pair will split equally to the products. This process is called homolytic bond cleavage (homolytic cleavage; homolysis) and results in the formation of radicals.

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 توقع قوة الرابط حسب نصف القطر

Metallic radius, ionic radius, and covalent radius of each atom in a molecule can be used to estimate 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.

 العوامل المؤثرة على طاقة الروابط الأيونية

The electronegativity of the two atoms bonding together affects ionic bond energy. Greater differences in electronegativity correspond to stronger ionic bonds.

 انظر أيضاً

• طاقة ترابط
• طاقة التأين
• Isodesmic reaction
• Lattice energy