Enthalpy Change Using Bond Energies

Steps:

• Identify bonds broken: one H-H (436 kJ/mol) and one Cl-Cl (242 kJ/mol); total energy input = 436 + 242 = 678 kJ/mol.
• Identify bonds formed: two H-Cl (431 kJ/mol each); total energy released = 2 × 431 = 862 kJ/mol.
• Calculate ΔH = energy to break bonds - energy from forming bonds = 678 - 862 = -184 kJ/mol.
• The negative value indicates an exothermic reaction.

Why B is correct:

• ΔH = -184 kJ/mol matches the formula for bond enthalpy change, where breaking bonds requires energy and forming bonds releases more energy.

Why the others are wrong:

• A: Incorrect sign; -247 reverses the exothermic nature.
• C: Positive value (+184) ignores the net energy release from bond formation.
• D: +247 miscalculates the difference and sign.

Final answer: B