Calculating the enthalpy change

Enthalpy change is the  difference between the energy taken to break the bonds and energy released by making the bonds. These bond values are available in chemical data books and can be used to calculate the overall energy change.

e.g: Enthalpy change for the burning of methane

CH₄  + 2O₂ --->  CO₂  +2 H₂O

Bond energies should be given as

C– H   =  99 KJ/mol
O=O   = 119 KJ/mol
C=O   = 192 KJ/mol
O– H  = 111 KJ/mol

By looking at the structures of the molecules involving in the reaction for breaking bonds

• 4 C-H bonds
• 2 O=O bonds has to be broken

Total energy for breaking the bonds 

99*4 +119*2 = 

396+238=

634 KJ/mol

For making bonds from the reaction

• 2 C=O bonds
• 4 O– H bonds has to be made

Total energy for making the bonds

192* 2+ 111*4= 

384 +444= 

828  KJ/mol

Enthalpy change= energy needed to break the bonds- energy taken to make the bonds

         ΔH°        =634-828

                        = - 194  KJ/mol

If the enthalpy change come as a negative value it is an exothermic reaction. If it was positive it is endothermic. According to this example this reaction is exothermic.

To refer the standard bond energy values click this link

Next about hydrogen as a fuel

            

Arunigd 1:36 AM   bond energies   burning of methane   calculating enthalpy change   enthalpy change   exothermic reaction example

Author : Arunigd

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