Why are the physical properties of alkenes are similar to alkanes, but the chemical properties are different?

Question

Can somebody give me a example and explain to me why the physical properties of alkenes are similar to alkanes, but the chemical properties are different?

Answer 1

The physical properties are similar because the molecules weigh about the same and experience the same intermolecular forces.  For example, propane has a molar mass of 44.1g/mol, while that of propene is 42.1g/mol.  Both molecules only undergo dispersion forces, as there are no ions, dipoles, or opportunities for hydrogen bonding.

The chemical properties are different because the bonding is different.  In alkanes, all of the electron clouds from the carbons are located along the axes of the bonds with other atoms.  As such, alkanes are pretty unreactive.  In alkenes, there are p electrons "sticking out" perpendicular to the (double) bond axis - other atoms can grab these and react with the alkene more easily.