The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride:
Reaction 1: CH4 (g) + C12 (g) → CH3 C1 (g) + HC1 (g)
This reaction is very slow in the absence of light. However, C12 (g) can absorb light to form C1 atoms:
Reaction 2: C12 (g) + hv → 2C1 (g)
Once the C1 atoms are generated, they can catalyze the reaction of CH4 and C12, according to the following proposed mechanism:
Reaction 3: CH4 (g) + C1 (g) → CH3 C1 (g) + HC1 (g)
Reaction 1: CH3 (g) + C12 (g) → CH3 C1 (g) + C1 (g)
The enthalpy changes and activation energies for these two reactions are tabulated as follows:
Reaction | ΔH0(KJ/mol) | Ea (KJ/mol) |
3 | +4 | 17 |
4 | --109 | 4 |
By using the bond enthalpy for C12 (Table 8.4), determine the longest
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