CH4(g) + Cl2(g) → CH3 Cl(s) + HCl(g) The major factor that influence the rate of the reaction above is

1. Analyze the question

The reaction provided is: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

This is a classic example of a free-radical substitution reaction (specifically, the halogenation of an alkane). You are asked to identify the major factor that influences the rate of this specific reaction.

2. Explain the concept

In organic chemistry, alkanes like methane (CH₄) are generally unreactive under normal conditions. For methane to react with chlorine (Cl₂), the chlorine molecule must first undergo homolytic fission (splitting into two chlorine radicals, Cl•).

This process requires energy to break the Cl-Cl bond. In this specific reaction, this energy is provided by ultraviolet (UV) light or sunlight. Without this energy source, the reaction will not proceed at any significant rate at room temperature.

3. Show the solution

• Step 1: Identify the reaction type. This is a photochemical reaction.
• Step 2: Understand the mechanism. The reaction proceeds via a free-radical chain mechanism: • Initiation: Cl₂ + hv (light) → 2Cl• • Propagation: CH₄ + Cl• → CH₃• + HCl, then CH₃• + Cl₂ → CH₃Cl + Cl•
• Step 3: Evaluate the options. • Catalyst: While catalysts speed up reactions, they are not the primary requirement for initiating this specific photochemical process. • Temperature: While temperature affects reaction rates, it is not the major factor that triggers this specific substitution reaction. • Concentration: Changing concentration affects the rate of many reactions, but it won't initiate this one without the presence of light. • Light: The reaction is photochemical. The presence of UV light is essential to provide the activation energy needed to break the chlorine bond and start the chain reaction.

4. Identify the answer

The major factor that influences the rate of this reaction is light. Therefore, the correct option is D.

5. Tip for your exam

Whenever you see a reaction involving an alkane and a halogen (like Cl₂ or Br₂) in your JAMB or WASSCE questions, look for keywords like "UV light," "sunlight," or the symbol hv (which represents a photon of light). These reactions are almost always photochemical, and "light" is the key factor.