3.5.1 Thermodynamics - Bond enthalpy


Students should:
  • be able to use mean bond enthalpies to calculate an approximate value of ΔH for other reactions
  • be able to explain why values from mean bond enthalpy calculations differ from those determined from enthalpy cycles

Average bond enthalpy

The average bond enthalpy term is the average amount of energy needed to break a specific type of bond, measured over a wide variety of different molecules. It is essentially the average of all of the bond dissociation enthalpies for a specific type of bond.

Bond enthalpy data table

Table of bond enthalpies / kJ mol-1
C-C 348
C-H 412
C-O 360
C=C 612
C=O 743
Cl-Cl 242
O-H 463
Cl-H 431
C-Cl 338
O=O 496
Ref: CRC Handbook of chemistry and physics - Edition 44

Bond enthalpies are measured per mole of bonds. The units of the bond enthalpy term is kJ mol-1


Reaction enthalpy from bond energy terms

For any chemical reaction:

Reactants products

There is a rearrangement of the bonding between the component atoms of the reactants and products. Thanks to Hess' law, we can follow an alternative route from the reactants to the products by breaking all of the bonds and then reforming the new bonds.

Breaking the reactant's bonds is an endothermic process, it requires energy. Making the product's bonds is an exothermic process. Using the alternative route via the individual atoms we can state that he enthalpy change of the reaction is the SUM of these two processes.

Enthalpy change = step 1 (bond breaking) + step 2 (bond formation)

However, forming the products releases energy, i.e. delta H is negative, giving:

Σ the reactant bond enthalpies - Σ the product bond enthalpies.

ΔH(reaction)= ΣΔH(bond enthalpy reactants) - ΣΔH(bond enthalpy products)

Bond enthalpy

In reality, all of the bonds in the reactants are not broken before reforming them into the products, this is another example Hess' law; the route may be different, but the final answer must be the same.

Example: Use the bond enthalpy terms from the table above to find the enthalpy change of the following reaction:

CH4 + 2O2 CO2 + 2H2O

Sum of the bond enthalpy terms of the reactants:

  • 4 x C-H = 4 x 412 = 1648 kJ
  • 2 x O=O = 2 x 496 = 992 kJ

Total = 2640 kJ

Sum of the bond enthalpy terms of the products

  • 2 x C=O = 2 x 743 = 1486 kJ
  • 4 x H-O = 4 x 463 = 1852 kJ

Total = 3338 kJ

Reaction enthalpy change = 2640 - 3338 = -698 kJ