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.

Verified step by step guidance

1. Identify the enthalpy changes (ΔH) and activation energies (Ea) for reactions 3 and 4 from the table provided.
2. For Reaction 3: Note that ΔH = +4 kJ/mol and Ea = 17 kJ/mol.
3. For Reaction 4: Note that ΔH = -109 kJ/mol and Ea = 4 kJ/mol.
4. Sketch the energy profile diagram: Start with the energy level of the reactants (CH4 and Cl2).
5. Draw the energy peaks for each reaction step, indicating the activation energies and the enthalpy changes, and finally show the energy level of the products (CH3Cl and HCl).

Key Concepts

Activation Energy (Ea)

Activation energy is the minimum energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to form products. In the context of the given reactions, lower activation energy indicates that the reaction can proceed more quickly, which is crucial for understanding how light absorption by Cl2 facilitates the reaction with methane.

Enthalpy Change (ΔH)

Enthalpy change refers to the heat content change during a chemical reaction at constant pressure. A negative ΔH indicates that the reaction is exothermic, releasing energy. In the provided reactions, the enthalpy changes help determine the overall energy profile and stability of the products compared to the reactants, which is essential for sketching the energy diagram.

Catalysis

Catalysis is the process by which a substance (the catalyst) increases the rate of a chemical reaction without being consumed in the process. In this case, the Cl atoms generated from the light-absorbing Cl2 act as a catalyst, facilitating the reaction between methane and chlorine. Understanding catalysis is key to analyzing how the reaction mechanism is altered and how energy profiles are affected.