Double bond formation in alkenes during cracking

In thermal or catalytic cracking, energy is used to break a long chain alkane into smaller alkanes/alkenes. Since the energy goes into breaking the bonds, how is the C=C bond in alkenes formed?

• Bond formation is exothermic and so favourable. Can you explain exactly what is confusing you? – bon Sep 30 '15 at 14:25
• Bond formation occurs as a result of the cracking - which is endothermic and requires heat. I understand that the input heat energy goes to breaking the bonds, but how is the double bond then formed? – pkmob Sep 30 '15 at 14:31
• Do you want the mechanism for cracking? Bond breaking is endothermic, bond formation is exothermic. – bon Sep 30 '15 at 14:34
• Alright, thank you, not fully clear, but I get the general drift. :) – pkmob Sep 30 '15 at 15:05

$$\ce{^{\cdot}CH2-CH3 <=> CH2=CH2 + H^{\cdot}}$$