Bond Enthalpy Lab 2D: Reaction Heat from Bond Energies

Reaction

H₂ + Cl₂ → 2 HCl

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Energy Balance

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Absorbed breaking bonds0 kJ/mol

Released forming bonds0 kJ/mol

Σ D(bonds broken)

— kJ/mol

Σ D(bonds formed)

— kJ/mol

ΔH reaction

— kJ/mol

How it works

Bond dissociation energy D is the energy needed to break one mole of a specific covalent bond in the gas phase. Because breaking bonds always costs energy and forming bonds always releases it, the enthalpy change of a reaction can be estimated directly from a table of average bond energies:

ΔH(reaction) = ΣD(bonds broken in reactants) − ΣD(bonds formed in products)

This simulator breaks every bond in the reactant molecules first (an energy input, tallied in the red meter and traced as a rise on the energy diagram below the scene — the peak represents the fully atomized intermediate), then forms every bond in the product molecules (an energy output, tallied in the green meter as the diagram descends to the final ΔH level). The two totals are real average bond enthalpies in kJ/mol:

H–H 436   Cl–Cl 243   H–Cl 431
C–H  413   O=O  498   C=O 799   O–H 463
N≡N  945   N–H  391

If more energy is released forming bonds than was absorbed breaking them, ΔH < 0 — the reaction is exothermic (products are more stable, energy escapes as heat).
If breaking bonds costs more than forming them returns, ΔH > 0 — the reaction is endothermic.
This bond-energy method uses average tabulated values, so it approximates ΔH — a true value comes from measured standard enthalpies of formation via Hess's law, which is exact for the specific compounds involved.

H Cl C O N

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