Based on bond energies, the reaction is endothermic, because stronger bonds are broken than formed.
The total change in bond energy of a reaction is calculated using the formula below:
A reaction is exothermic if the total change in bond energy is negative meaning that stronger bonds are formed than broken.
A reaction is endothermic if the total change in bond energy is positive meaning that stronger bonds are broken than formed.
Given a reaction such as: C2H6 → C2H4 + H2
Sum of energies of bonds broken:
C-C + 6 × H-C
347 + 6 × 413
Sum of energy of bonds broken = 2825 kJ
Sum of energy of bonds formed:
C=C + 4 × H-C + H-H
614 + 4 × 413 + 432
Sum of energy of bonds formed = 2725 kJ
Total change in bond energy = 2825 - 2725
Total change in bond energy = 100 kJ
Since the value of total change in bond energy is positive, the reaction is endothermic.
Therefore, the reaction is endothermic, because stronger bonds are broken than formed.
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