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: CH41g2 + Cl21g2 ¡ CH3Cl1g2 + HCl1g2
This reaction is very slow in the absence of light. However,
Cl21g2 can absorb light to form Cl atoms:
Reaction 2: Cl21g2 + hv ¡ 2 Cl1g2
Once the Cl atoms are generated, they can catalyze the
reaction of CH4 and Cl2, according to the following proposed
Reaction 3: CH41g2 + Cl1g2 ¡ CH31g2 + HCl1g2
Reaction 4: CH31g2 + Cl21g2 ¡ CH3Cl1g2 + Cl1g2
The enthalpy changes and activation energies for these
two reactions are tabulated as follows:
Reaction H 1kJ ,mol 2 Ea 1kJ ,mol 2
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.
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Master Energy Diagrams Concept 1 with a bite sized video explanation from Jules Bruno