Writing the overall chemical equation from a mechanism: Carbon tetrachloride, `C CI_(4)`, is obtained by chlorinating methane or an incompletely chlorinated methane such as chloroform, `CHCI_(3)`. The mechanism for the gas phase chlorination of `CHCI_(3)` is
`CI_(2)hArr2overset(.)(C )I` , Elementary reaction
`overset(.)(C )I+CHCI_(3)rarrHCI+overset(.)(C )CI_(3)` , Elementary reaction
`overset(.)(C )I+overset(.)(C )CI_(3)rarrC CI_(4)` , Elementary reaction
Obtain the net, or overall, chemical equation from this mechanism
Straregy : The overall chemical equation, which represents the net result of the elementary reaction in the mechanism, is obtined by adding the steps together and cancelling species that occur on borth sides.

Explanation

The first step produces two CI atoms (a reaction intermediate). One CI tom is used in the second step and another is used in the third step. Thus, all CI atoms cancel. Similary, the intermediate `C CI_(3)`, produced in the second step, is used up in the third step. We can cancle all CI and `C CI_(3)` species to get overall chemical equation.
`CI_(2)hArr2cancel(CI)`
`cancel(CI)+CHCI_(3)rarrHCI+C cancel(CI_(3))`
`cancel(CI)+C cancel(CI_(3))rarrC CI_(4)`
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`CI_(2)+CHCI_(3)rarrHCI+C CI_(4)` Overall equaion