Enthalpy Calculator
Calculate reaction enthalpy (delta H) from bond energies: bonds broken minus bonds formed.
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Sum all bond energies for bonds broken in the reactants, and sum all bond energies for bonds formed in the products. Enter both totals. delta H = broken - formed. A negative result means the reaction is exothermic.
About the Enthalpy Calculator
Enthalpy change (delta H) measures the heat absorbed or released during a chemical reaction at constant pressure. The bond energy method estimates delta H by comparing the energy required to break all bonds in reactants versus the energy released when forming all bonds in products. This approach provides reasonable estimates for gas-phase reactions.
Frequently Asked Questions
Why is delta H = bonds broken minus bonds formed?
Breaking bonds requires energy input (endothermic), while forming bonds releases energy (exothermic). The net enthalpy change is the difference. If more energy is released in forming bonds than consumed in breaking them, the reaction is exothermic (negative delta H).
Where do I find bond energy values?
Bond energy tables are found in chemistry textbooks and reference databases. Common values: C-H = 413 kJ/mol, O-H = 463 kJ/mol, C=O = 799 kJ/mol, O=O = 498 kJ/mol, C-C = 347 kJ/mol.
How accurate are bond energy calculations?
Bond energy estimates of delta H are approximate because they use average values. Actual bond energies depend on the molecular environment. For precise values, use standard enthalpies of formation (Hess's law) instead.