Given: N2 + 3H2 → 2NH3 Bond Bond Energy (kJ/mol) N≡N 942 H-H 432 N-H 386 Use the bond energies to calculate the change in enthalpy for the reaction. The enthalpy change for the reaction is kilojoules.

Answer: - 78 KJ.

Explanation:

The reaction is between gaseous molecules, so we can use the bond energy to calculate the change in enthalpy. The change in enthalpy for a reaction can be calculated by the sum of all of the bond energies of reactants and products. For reactants, the bonds are broken and the energy is required for this. While, for products, the bonds are formed and the energy released from this. Thus, bond breaking is endothermic and the bond energy is positive. Also, bond formation is exothermic and the bond energy is negative. For N₂; there is only one triple bond and the bond energy (N≡N) = 942 KJ/mol. For H₂, there is only one single bond and the bond energy (H₋H) = 432 KJ/mol and there are 3 molecules are included in this reaction. So, the bond energy will by multiplied by 3. For NH₃, there are 3 single bonds between N and H and the bond energy (N₋H) = - 386 KJ/mol (negative sign because the bond here is formed) and there are 2 molecules will be formed from this reaction. So, the bond energy will be multiplied by 6. ΔH = [bond energy (N≡N) ] + 3 [bond energy (H₋H) ] + 6 [bond energy (N₋H) ] ΔH = [942] + 3 [432] + 6 [-386] = - 78 KJ. This means that the reaction 78 KJ of energy will be released from this reaction (Exothermic reaction).