Why are trans alkenes more stable than cis alkenes?
Trans alkenes are more stable than cis alkenes because they have fewer steric interactions.
In cis-but-2-ene, there is steric hindrance, because the methyl groups are on the same side of the double bond.
Because of this steric crowding, there are van der Waals repulsive forces between the electron clouds of the groups.
This decreases the stability of the cis alkene.
In trans-but-2-ene, the methyl groups are on opposite sides of the double bond, and the repulsion does not occur.