Bond Enthalpies
The energy required to break a covalent bond in a gaseous substance is called the bond energy for the chemical bond.
For example, for the diatomic molecule HCl is defined as the standard enthalpy change for the reaction:
HCl (g)

H (g) + Cl (g)
H° = 431 kJ mol-1
The enthalpy change for the equation
H2O (g)  O (g) + 2 H (g)
is Hreaction = + 925 kJ. Since water is made up of two O-H covalent bonds, the value of Hreaction is the energy
required to break these bonds. Since Hreaction is positive, at constant pressure 925 kJ of energy is required to break
two moles H-O covalent bonds in 1 mole of H2O. The average molar bond enthalpy for a hydrogen to oxygen bond is
equal to one half of the total energy input as heat required to break two moles of hydrogen to oxygen bonds or about
463 kJ mol-1.
It has been found that the hydrogen to oxygen molar bond enthalpy in a variety of compounds is about the same as the
H-O bond enthalpy in water. Bond enthalpies are a function of the particular atoms forming the bond rather than the
particular substance in which they are incorporated.
Some typical molar bond enthalpies are listed below:
Bond
O-H
O-O
C-O
O=O
C=O
C-C
C=C
C= C
C-H
Bond
enthalpy
464
142
351
502
730
347
615
811
414
Bond
C-F
C-Cl
C-Br
C-N
C=N
C= N
N-H
N-N
N=N
Bond
enthalpy
439
439
276
293
615
890
390
159
418
Bond
N=N
F-F
Cl-Cl
Br-Br
H-H
H-F
H-Cl
H-Br
H-S
Bond
enthalpy
945
155
243
192
435
565
431
368
364
In general it is helpful to picture a chemical reaction as taking place in two steps. First the chemical bonds in the
reactants are broken down into their constituent atoms and then the atoms are rejoined to form the product molecules.
The first step requires an input of energy to break the chemical bonds of the reactants. The second step releases energy
as new chemical bonds are formed. The enthalpy change for the reaction, Hrxn, is the difference between the energy
required to break the chemical bonds of the reactants and the energy released in the formation of the chemical bonds for
the products.
Example 1: Calculate the enthalpy of reaction for the following reaction:
CH4 (g) + Cl2 (g)
 CH3Cl (g) + HCl(g)
In the above reaction it is necessary to break one C-H bond and one Cl-Cl bond and to form one C-Cl bond and one HCl bond.
Reactants: (Bonds broken)
4
C-H
414 kJ x 4
= 1656 kJ
1
Cl-Cl 243 kJ x 1
= 243 kJ
Total = 1899 kJ (absorbed H is +)
Products (Bonds formed)
3
1
1
C-H 414 kJ x 3
C-Cl 439 kJ x 1
H-Cl 431 kJ x 1
= 1242 kJ
= 439
= 431
Total = 2112 kJ (Released H is -)
Change in enthalpy = - 213 kJ

Exercises:
1.
The enthalpy change for the reaction:
ClF3 (g)  Cl (g) + 3 F (g) is 514 kJ.
Using this information and the bond enthalpies on the previous page, calculate the average Cl-F bond energy in
ClF3.
2.
Use the bond enthalpies on the previous page to calculate Hreaction for the following reaction:
CH4 (g) + 2 Cl2 (g)  CH2Cl2 (g) + 2 HCl (g)
3.
Hydrazine, N2H4 and its derivatives are used as rocket fuels. Use the molar bond enthalpies on the previous
page to calculate the molar enthalpy of formation of N2H4 (g).
4.
The formation of water from oxygen and hydrogen involves the reaction:
2 H2 (g) + O2 (g)  2 H2O (g)
Use the bond enthalpies on the previous page to calculate the enthalpy of formation of H 2O (g).
5.
The formation of ammonia from nitrogen and hydrogen involves the reaction:
3 H2 (g) + N2 (g)  2 NH3 (g)
Use the bond enthalpies on the previous page to calculate the enthalpy of formation of NH 3 (g) .