Not all alkenes were created equal. Like carbocations, alkenes are stabilized through a phenomenon called hyperconjugation.
Hyperconjugation allows adjacent -R groups (mostly C-C and C-H σ-bonds) to create shared molecular orbitals with π-bonds, stabilizing the bond.
Understanding trends of alkene stability.
So basically, the more –R groups attached to the double bond, the more stable the double bond will be.
Specifically, when it comes to di-substituted bonds, the order os stability is gem > trans > cis.
Rank the following alkenes in order of lowest to highest heat of combustion.
Heat of Combustion