The reaction that is occuring is a substitution reaction as ethan is already a saturated molecule. The ultraviolet light provides the energy needed to break the bonds between hydrogen and carbon in ethane so that the chlorine atoms can replace the hydrogen ones.
Ethane's chemical formula is CH3CH3 and since chlorine is a diatomic gas, its formula is Cl2
The equation representing this reaction is therefore:
CH3CH3 +Cl2 ->CH2ClCH2Cl+H2
So the products of the reation are 1,2-dichloro ethane and hydrogen gas