Two hydrogen-halogen reactions are shown below.
Reaction X: H2(g) + Cl2(g) → 2 HCl(g)
Reaction Y: H2(g) + Br2(g) → 2 HBr(g)
Average bond energies (in kJ mol⁻¹):
H–H 436 Cl–Cl 243 Br–Br 193 H–Cl 431 H–Br 366
Using the data provided, address the following.
(a) Estimate the enthalpy change, ΔHX, for Reaction X and the enthalpy change, ΔHY, for Reaction Y.
(b) Identify which reaction is more exothermic. Support your answer with calculated values from part (a).
(c) Describe briefly why substituting Br for Cl in the product weakens the overall exothermic character of the reaction, referring specifically to bond strengths.