Question

define the term bond enthalpy? What an average value is taken in the polyatomic molecules

Answer 1

bond enthalpy also known as bond energy is defined as a energy required to break one molecule of a stated bond. A formalism is developed for the convenient calculation of first order anharmonic  vibrational averages of molecular property which depends on nuclear position coordinates. This formalism is used to calculate the average geometry of CF4 in vibrational excited states

Answer 2

Explanation:

Bond enthalpy is defined as energy require to 1 break or cleave 1 mole of a given chemical bond at standard conditions.

An average bond enthalpy is defined as energy require to 1 break or cleave 1 mole of a bond in a given molecule in gaseous state under standard conditions.

Average bond enthalpy:

$\Delta H_{avg,X-Y} =\frac{\sum \Delta H_{i,X-Y}}{i}$

i  =1,2,3,4......