How do I calculate bond energies?

Calculate the ΔH for the ionic dissociation of Cl2:... PLZ HELP! :)?

• Bond energies can be combined with values for other atomic properties to obtain ΔH values that cannot be measured directly. Use bond energy, ionization energy, and electron affinity values from your text to calculate the ΔH for the ionic dissociation of Cl2: Cl 2(g) => Cl+(g) + Cl –(g) Student Response Correct Answer c. 1150 kJ/mol

• Answer:

  You can split the reaction into three steps 1. Dissociation of Chlorine: Cl₂(g) → 2 Cl enthalpy of the step is equal to the bond energy of the split bond: ∆H₁ = D(Cl-Cl) = 240kJ/mol 2. Ionization of one chlorine atom: Cl(g) → Cl⁺(g) + e⁻ enthalpy of this equals the first ionization energy of chlorine ∆H₂ = 1256kJ/mol 3. capture of the electron from the other chlorine atom: Cl(g) + e⁻ → Cl⁻(g) enthalpy of this equals the electron affinity of chlorine: ∆H₃ = -349kJ/mol So for the over all reaction Cl₂(g) → Cl⁺(g) + Cl⁻(g) ∆H = ∆H₁ + ∆H₂ + ∆H₃ = 240kJ/mol + 1256kJ/mol - 349kJ/mol = 1147kJ/mol Small deviation to proposed correct answers may be caused by slightly different tabulated values of the properties used in calculation.