Reaction Enthalpy Calculator (Hess's Law)

Enthalpy of reaction (ΔH°rxn) tells you how much heat is released or absorbed during a chemical reaction.

Method 1: Standard Enthalpies of Formation (Hess’s Law)

ΔH°rxn = Σ n × ΔH°f(products) − Σ m × ΔH°f(reactants)

where n and m are stoichiometric coefficients.

The standard enthalpy of formation (ΔH°f) of any element in its standard state is zero.

Common ΔH°f values (kJ/mol) at 25°C:

• H₂O(l): −285.8 | H₂O(g): −241.8
• CO₂(g): −393.5 | CO(g): −110.5
• NH₃(g): −46.0 | HCl(g): −92.3
• CH₄(g): −74.8 | C₂H₅OH(l): −277.7
• NaCl(s): −411.2 | CaO(s): −635.1

Method 2: Bond Energies

ΔH°rxn = Σ D(bonds broken) − Σ D(bonds formed)

Breaking bonds requires energy (+); forming bonds releases energy (−).

Common bond dissociation energies (kJ/mol):

• H−H: 436 | O=O: 498 | N≡N: 945
• C−H: 413 | C−C: 347 | C=C: 614 | C≡C: 839
• O−H: 463 | C−O: 360 | C=O: 799

Sign convention:

• ΔH < 0: exothermic (heat released to surroundings)
• ΔH > 0: endothermic (heat absorbed from surroundings)

Note: Bond energy method gives approximate results. The formation enthalpy method gives exact standard values.