When ethene, C\(_2\)H\(_4\) reacts with hydrogen bromide (HBr), the product is bromoethane (C\(_2\)H\(_5\)Br): 

 C\(_2\)H\(_4\) + HBr  → C\(_2\)H\(_5\)Br

This reaction is an addition reaction where HBr adds across the double bond of ethene: 

1. The double bond breaks
2. A hydrogen atom attaches to one of the carbons
3. A bromine atom attaches to the other