How to Calculate the Enthalpy of Formation of a Molecule – Part 1

The standard enthalpy of formation of a compound (ΔfHѲ) is the change of enthalpy (heat) during the formation of one mole of the compound/substance from its constituent elements in their reference/standard states.


The bond dissociation energy (BDE, D0, or DH°) is the standard enthalpy change when a chemical bond A−B is cleaved by homolysis to produce radical fragments A & B. BDE is one measure of the bond strength & temperature-dependent.

The BDE differs from the bond energy (BE aka mean bond, bond enthalpy, average bond enthalpy, or bond strength) except for diatomic molecules. While the BDE is the energy of a single bond, the BE is the average of all the BDEs of the bonds of the same type for a given molecule. IUPAC (international union of pure and applied chemistry) defines BE in the gas-phase usually at a temperature of 298.15 K.

Mnemonic: B-FOR: Bonds Broken BeFORe (sounds almost like B-FOR) bonds FORmed

Question:

The bond dissociation energies of H2(g), Cl2(g) & HCl(g) are 104, 58 & 103 kcal/mol respectively. Calculate the enthalpy of formation of HCl(g).

Solution:


Click to enlarge the image

Are you energized now to calculate the BE of a molecule at the drop of a hat❓ Let Chemaficionado know at mychemistryhomework@gmail.com❗

References:

(1)    Wikipedia Contributors. Standard enthalpy of formation. Wikipedia. https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation.
(2)    Wikipedia Contributors. Bond dissociation energy. Wikipedia.
(3)    Bond energy. Wikipedia. https://en.wikipedia.org/wiki/Bond_energy.

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