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) + Cl₂(g) → CH₃Cl(g) + HCl(g)
This reaction is very slow in the absence of light. However, Cl₂(g) can absorb light to form Cl atoms:
Reaction 2: Cl₂(g) + hv → 2 Cl(g)
Once the Cl atoms are generated, they can catalyze the reaction of CH₄ and Cl₂, according to the following proposed mechanism:
Reaction 3: CH₄(g) + Cl(g) → CH₃(g) + HCl(g)
Reaction 4: CH₃(g) + Cl₂(g) → CH₃Cl(g) + Cl(g)
The enthalpy changes and activation energies for these two reactions are tabulated as follows:
Reaction ΔH° (kJ/mol) E_a (kJ/mol)
3 +4 17
4 -109 4 
(b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4.