Bond Energy Calculation from Reaction Enthalpy

Steps:

• Bonds broken: N≡N (941 kJ mol⁻¹) and O=O (498 kJ mol⁻¹), total 1439 kJ mol⁻¹.
• Bonds formed: 2 N=O bonds, releasing 2 × BE(NO).
• ΔH = energy absorbed to break bonds - energy released to form bonds = +180 kJ mol⁻¹.
• 1439 - 2 × BE(NO) = 180, so 2 × BE(NO) = 1259 kJ mol⁻¹.
• BE(NO) = 1259 / 2 = 630 kJ mol⁻¹.

Why A is correct:

• Matches the value from the bond energy equation using standard N≡N and O=O values with given ΔH.

Why the others are wrong:

• B: Gives ΔH ≈ -181 kJ mol⁻¹ (exothermic), contradicting +180 kJ mol⁻¹.
• C: Equals total energy for 2 N=O bonds, not single bond energy.
• D: Gives ΔH ≈ -181 kJ mol⁻¹ (exothermic), contradicting +180 kJ mol⁻¹.

Final answer: A