The reaction of HBr with ethene is an example of reaction

The given reaction is an example of an electrophilic addition reaction, where first proton is attached to alkene result in the formation of a carbocation. Later, bromide ion attack to the carbocation resulting in the formation of an alkyl halide.

${\mathrm{CH}}_2={\mathrm{CH}}_2+\mathrm{H}-\mathrm{Br}\to {\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{Br}$