Consider the gas-phase chlorination of methane, CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)
CHEMISTRY · Multi-part · CBSE · Chemistry · Chemical Kinetics · Chemical Kinetics
Problem
Consider the gas-phase chlorination of methane, CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g), which is thought to proceed by a radical chain mechanism. A detailed potential-energy diagram for the process (relative to the reactants at 0 kJ mol⁻¹) contains four distinct transition states at +105, +60, +90, and +45 kJ mol⁻¹ and three intermediates at +15, +25, and +5 kJ mol⁻¹ before the products are reached at –80 kJ mol⁻¹.
(a) Propose a sequence of four elementary steps that is consistent with the energies given. Write balanced chemical equations for each elementary step, clearly identifying the radical species involved.
(b) For each proposed elementary step, indicate whether it is initiation, propagation, or termination. Justify your classification using features of the energy profile.
(c) Identify the rate-determining step of your mechanism and use the energy diagram to defend your choice.
(d) Suppose ultraviolet light of slightly lower intensity is used so that the first transition state increases to +120 kJ mol⁻¹ while the others remain the same. Predict qualitatively how this change would affect the overall reaction rate and which mechanistic step, if any, might shift to become rate determining.