Enthalpy Changes for Halogenation Reactions

In Section 3.7, we discussed the chlorination of methane. Now we ll return to that reaction and consider it in a more general way and from a different perspective. We will consider the enthalpy changes that accompany the various propagation steps in the reaction. We will examine the enthalpy change for each step, and see how it contributes to the overall enthalpy change. 

we ll consider the fluorination of methane and the enthalpy changes associated with the two propagation steps. 

The propagation steps for fluorination are highly exothermic, and the reaction is difficult to control. The large quantity of heat released during the reaction causes the reaction temperature to rise, which in turn increases the rate of the reaction. As a consequence, the reaction may lead to an explosion. 

When we turn to chlorination, we find that the first propagation step is very slightly endothermic. The second step is exothermic, but less so than the second step in fluorination. 

The net reaction is exothermic, but the enthalpy change is less than for fluorination. Chlorination reactions can be more easily controlled than fluorination, so free radical chlorination is an important industtial process. 

---