Free radical substitution - bromination of ethane
If there is an excess of ethane, you would be getting the monosubstituted bromoethane, $\ce{CH3CH2Br}$, as the main product. 

When ethane, $\ce{CH3CH3}$, is in excess, for any bromine radical formed, most of the reacting particles around it would be an unreacted ethane molecule (high concentration of $\ce{CH3CH3}$), this means that there is a greater chance of a bromine radical colliding with and reacting with $\ce{CH3CH3}$ to form the monosubstituted bromoethane. 

This is in contrast with when bromine is in excess, where the excess of highly reactive bromine radicals mean that most of them react fully with the available ethane molecules to form fully substituted $\ce{CBr3CBr3}$.