Question

Use bond energy values to estimate Δh for each of the following reactions in the gas phase. (a) H₂ + Cl₂ → 2 HCl (b) N₂ + 3 H₂ → 2 NH₃

Answer 1

Using bond energy values, the estimate for the enthalpy change (ΔH) for the reaction H₂ + Cl₂ → 2HCl is calculated to be -185 kJ/mol, accounting for both bond breaking and bond forming processes.

Step-by-step explanation:

To estimate the ΔH (enthalpy change) for the reaction H₂(g) + Cl₂(g) → 2HCl(g), we start by considering the energies required to break the H-H and Cl-Cl bonds and the energy released when forming H-Cl bonds. We use the bond energy values given: the bond energy of H-H is 436 kJ/mol, Cl-Cl is 243 kJ/mol, and each H-Cl bond formed has a bond energy of 432 kJ/mol. Since we form two H-Cl bonds, we need to double this value.

To break the H-H and Cl-Cl bonds requires 436 kJ/mol + 243 kJ/mol, which sums to 679 kJ/mol. The formation of two moles of H-Cl bonds releases 2 × 432 kJ, which is 864 kJ. Therefore, the overall enthalpy change for the reaction is the energy released minus the energy required: ΔH = (864 kJ) - (679 kJ) = -185 kJ/mol.