Estimate ΔH for H2(g) + F2(g) → 2HF(g) using average bond enthalpies: H-H = 436 kJ/mol, F-F = 159 kJ/mol, H-F = 567 kJ/mol.

Core Concept

Bonds broken are one H-H and one F-F, absorbing 436 + 159 = 595 kJ. Bonds formed are two H-F bonds, releasing 2 × 567 = 1134 kJ. ΔH = 595 - 1134 = -539 kJ. The strongly negative value reflects how much stronger the new H-F bonds are than the bonds they replace, so net energy floods out as heat.