The chlorination of vinyl chloride, C2h3CL + Cl2 -> C2H3Cl3, is believed to proceed by the following mechanism (* implies radical):
Cl2 --(k1)--> 2Cl*
Cl* + C2H3Cl --k2--> C2H3Cl2*
C2H3Cl2* + Cl2 --k3--> C2H3Cl3 + Cl*
C2H3Cl2* + C2H3Cl* --k4--> stable species
Derive the rate law expression for the chlorination of vinyl chloride based on this mechanism.