Question

Using the standard enthalpy of formation data in Appendix G, show how the standard enthalpy of formation of HCl(g) can be used to determine the bond energy.

HCl(g)⟶1/2H₂(g)+1/2Cl₂(g)ΔH∘1=−ΔH∘f[HCl(g)

Answer 1

The bond energy is the energy needed to break a bond, while the standard enthalpy of formation measures the change in enthalpy during the formation of a compound from its elements. The standard enthalpy of formation of HCl(g) can be used to approximate the bond energy of HCl by using the enthalpy change during the formation of the molecule.

Step-by-step explanation:

The bond energy of a molecule like HCl(g) refers to the energy required to break one mole of the bonds in the gas phase, under standard conditions, to form the individual atoms, such as H(g) and Cl(g). Conversely, the standard enthalpy of formation (ΔH°f) of a substance like HCl(g) is the change in enthalpy when one mole of the substance forms from its constituent elements in their standard states. For diatomic elements like H₂ and Cl₂, the standard enthalpies of formation are zero.

Calculating Bond Energy Using Standard Enthalpy of Formation

To determine the bond energy from the standard enthalpy of formation of HCl(g), we must consider the enthalpy changes involved in breaking the bonds of the diatomic molecules (H-H and Cl-Cl) and the formation of HCl. If the standard enthalpy of formation for HCl(g) is given as -92.307 kJ/mol, this indicates the release of energy when HCl forms from its elements.

The bond energy for one mole of HCl bonds can be approximated as twice the magnitude of the standard enthalpy of formation as the reaction forming two moles of HCl involves the breaking of one mole each of H-H and Cl-Cl bonds and the formation of two moles of HCl bonds.