Chemistry Practice Problems Bond Energy Practice Problems Solution: The rates of many atmospheric reactions are accele...

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Solution: 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:ReactionΔH˚(kJ/mol) Ea (kJ/mol)3+4174-1094By using the bond enthalpy for Cl2 (242 kJ/mol), determine the longest wavelength of light that is energetic enough to cause reaction 2 to occur. In which portion of the electromagnetic spectrum is this light found?

Problem
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:
ReactionΔH˚(kJ/molEa (kJ/mol)
3+417
4-1094


By using the bond enthalpy for Cl2 (242 kJ/mol), determine the longest wavelength of light that is energetic enough to cause reaction 2 to occur. In which portion of the electromagnetic spectrum is this light found?