Chlorination of Methane Energy Changes

We saw in Section 10.2A that we can calculate the overall heat of reaction from bond dissociation energies. We can also calculate the heat of reaction for each individual step of a mechanism  

In the chain-initiating step only one bond is broken—the bond between two chlorine atoms— and no bonds are formed. The heat of reaction for this step is simply the bond dissociation energy for a chlorine molecule, and it is highly endothermic. 

In the chain-terminating steps bonds are formed, but no bonds are broken. As a result, all of the chain-terminating steps are highly exothermic. 

Each of the chain-propagating steps, on the other hand, requires the breaking of one bond and the formation of another. The value of A/f for each of these steps is the difference between the bond dissociation energy of the bond that is broken and the bond dissociation energy for the bond that is formed. The first chain-propagating step is slightly endothermic (A.H° = +8 kJ mol ), but the second is exothermic by a large amount (A /° = -109 kJ moF ). 

Assuming the same mechanism applies, calculate AH° for the chain-initiating, chain-propagating, and chain-terminating steps involved in the fluorination of methane. 

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Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

In our consideration of the chlorination of methane, we have so far been concerned chiefly with the particles involved-molecules and atoms- and the changes that they undergo. As with any reaction, however, it is important to consider also the energy changes involved, since these changes determine to a large extent how fast the reaction will go, and, in fact, whether it will take place at all. 

Figure 2.8. Potential energy changes during progress of reaction chlorination of methane. Formation of radical is difficult step.

Energy change = D (Bonds broken) — D (Bonds formed) Use the data in Table 7.1 to calculate an energy change for the reaction of methane with chlorine. 

We can use average bond energies to estimate the enthalpy change of a reaction. For example, consider the reaction between methane and chlorine ... 

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