Use bond energies to calculate δhrxn for the reaction. n2(g)+3cl2(g)→2ncl3(g)

ΔHrxn = ΣδΗ(bond breaking) - ΣδΗ(bond making)

Bond enthalpies,
N ≡ N ⇒ 945 kJ mol⁻¹
N - Cl ⇒ 192 kJ mol⁻¹
Cl - Cl⇒ 242 kJ mol⁻¹

According to the balanced equation,
ΣδΗ(bond breaking)  = N ≡ N x 1 + Cl - Cl x 3
                                  = 945 + 3(242)
                                  = 1671 kJ mol⁻¹
ΣδΗ(bond making)    = N - Cl x 3 x 2
                                 = 192 x 6
                                 = 1152 kJ mol⁻¹

δHrxn = ΣδΗ(bond breaking) - ΣδΗ(bond making)
          = 1671 kJ mol⁻¹ - 1152 kJ mol⁻¹
          = 519 kJ mol⁻¹

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