A chemist is studying the gas-phase reaction
H₂(g) + Cl₂(g) → 2 HCl(g)
Average bond enthalpies are provided below (values in kJ mol⁻¹):
• H–H 436
• Cl–Cl 243
• H–Cl 431
Assume the reaction occurs in a single elementary step involving simultaneous cleavage of the H–H and Cl–Cl bonds and formation of two H–Cl bonds.
(a) Calculate the enthalpy change ΔH_rxn (in kJ mol⁻¹) for the reaction using the given average bond enthalpies.
(b) Based on your result in part (a), describe the sign of the overall energy change (ΔE) on a reaction energy profile and relate that sign to the relative numbers and strengths of bonds broken versus formed.
(c) Sketch a qualitative reaction energy profile for this elementary process. In your sketch label the following features: reactants, transition state, products, activation energy (E_a), and overall energy change (ΔE).
(d) Explain why the activation energy you labeled in part (c) is positive even though the reaction is overall exothermic.