`color{red}("Definition :")` It is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state.
● The unit of bond enthalpy is `kJ mol^(–1)`.
`color{red}("Example :")` The `H – H` bond enthalpy in hydrogen molecule is `435.8 kJ mol^(–1)`.
`H_2 (g) → H (g) + H (g) ; Delta_a H^v = 435.8 kJ mol^(-1)`
Similarly, the bond enthalpy for molecules containing multiple bonds, for example `O_2` and `N_2` will be as under :
`O_2 (O = O) (g) → O(g) +O(g) ; Delta_a H^v = 498 kJ mol^(-1) `
`N_2 (N = N ) (g) → N (g) +N(g) ; Delta_a H^v = 946.0 kJ mol^(-1)`
`=>` It is important that larger the bond dissociation enthalpy, stronger will be the bond in the molecule.
`=>` For a heteronuclear diatomic molecules like `HCl`, we have
`HCl (g) → H (g) +Cl (g) ; Delta_a H^v = 431.0 kJ mol^(-1)`
`=>` In case of polyatomic molecules, the measurement of bond strength is more complicated.
`color{red}("Example ")` In case of `H_2O` molecule, the enthalpy needed to break the two `O – H` bonds is not the same.
`H_2O(g) → H(g) + OH (g) ; Delta_a H_1^v = 502 kJ mol^(-1)`
`OH(g) → H (g) +O(g) ; Delta_a H_2^v = 427 kJ mol^(-1)`
● The difference in the `Δ_aH^v` value shows that the second `O – H` bond undergoes some change because of changed chemical environment.
● This is the reason for some difference in energy of the same `O – H` bond in different molecules like `C_2H_5OH` (ethanol) and water.
● Therefore in polyatomic molecules, the term mean or average bond enthalpy is used.
● It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken as explained below in case of water molecule,
`color{green}("Average bond enthalpy")` `= (502+427)/2`
` = 464.5 kJ mol^(-1)`
• `color{red}("Factors affecting bond enthalpy ")` :
`=>` Greater the size of atoms, greater is the bond length and less is bond dissociation enthalpy.
`=>` For the bond between the same two atoms,greater is multiplicity of bond,greater is the bond dissociation enthalpy. Eg. H-H
`=>` Greater the number of lone pairs of electrons present on the bonded atoms, greater is the repulsion between the atoms and hence less is BDE.
`color{red}("Definition :")` It is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state.
● The unit of bond enthalpy is `kJ mol^(–1)`.
`color{red}("Example :")` The `H – H` bond enthalpy in hydrogen molecule is `435.8 kJ mol^(–1)`.
`H_2 (g) → H (g) + H (g) ; Delta_a H^v = 435.8 kJ mol^(-1)`
Similarly, the bond enthalpy for molecules containing multiple bonds, for example `O_2` and `N_2` will be as under :
`O_2 (O = O) (g) → O(g) +O(g) ; Delta_a H^v = 498 kJ mol^(-1) `
`N_2 (N = N ) (g) → N (g) +N(g) ; Delta_a H^v = 946.0 kJ mol^(-1)`
`=>` It is important that larger the bond dissociation enthalpy, stronger will be the bond in the molecule.
`=>` For a heteronuclear diatomic molecules like `HCl`, we have
`HCl (g) → H (g) +Cl (g) ; Delta_a H^v = 431.0 kJ mol^(-1)`
`=>` In case of polyatomic molecules, the measurement of bond strength is more complicated.
`color{red}("Example ")` In case of `H_2O` molecule, the enthalpy needed to break the two `O – H` bonds is not the same.
`H_2O(g) → H(g) + OH (g) ; Delta_a H_1^v = 502 kJ mol^(-1)`
`OH(g) → H (g) +O(g) ; Delta_a H_2^v = 427 kJ mol^(-1)`
● The difference in the `Δ_aH^v` value shows that the second `O – H` bond undergoes some change because of changed chemical environment.
● This is the reason for some difference in energy of the same `O – H` bond in different molecules like `C_2H_5OH` (ethanol) and water.
● Therefore in polyatomic molecules, the term mean or average bond enthalpy is used.
● It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken as explained below in case of water molecule,
`color{green}("Average bond enthalpy")` `= (502+427)/2`
` = 464.5 kJ mol^(-1)`
• `color{red}("Factors affecting bond enthalpy ")` :
`=>` Greater the size of atoms, greater is the bond length and less is bond dissociation enthalpy.
`=>` For the bond between the same two atoms,greater is multiplicity of bond,greater is the bond dissociation enthalpy. Eg. H-H
`=>` Greater the number of lone pairs of electrons present on the bonded atoms, greater is the repulsion between the atoms and hence less is BDE.