Reaction Enthalpy Calculator (Hess's Law)
Calculate standard enthalpy of reaction using formation enthalpies (Hess's law) or bond energies.
Determine if a reaction is exothermic or endothermic.
Reaction enthalpy (ΔH_rxn) is the heat released or absorbed when a chemical reaction occurs at constant pressure. Negative ΔH means heat is released (exothermic); positive ΔH means heat is absorbed (endothermic).
Hess’s Law Formula:
ΔH_rxn = Σ ΔH_f (products) − Σ ΔH_f (reactants)
Where ΔH_f = standard enthalpy of formation (kJ/mol), defined as the heat to form 1 mole of a compound from its elements in standard states.
Bond Enthalpy Method:
ΔH_rxn ≈ Σ (bonds broken, reactants) − Σ (bonds formed, products)
Breaking bonds requires energy (+); forming bonds releases energy (−).
Common Standard Enthalpies of Formation (kJ/mol at 25 °C):
| Compound | ΔH_f (kJ/mol) |
|---|---|
| H2O (liquid) | −285.8 |
| CO2 (gas) | −393.5 |
| CH4 (methane gas) | −74.8 |
| NH3 (ammonia gas) | −46.1 |
| C6H12O6 (glucose) | −1,274 |
| N2O4 (gas) | +9.7 |
Worked Example:
Combustion of methane: CH4 + 2O2 → CO2 + 2H2O
ΔH_rxn = [ΔH_f(CO2) + 2 × ΔH_f(H2O)] − [ΔH_f(CH4) + 2 × ΔH_f(O2)]
ΔH_rxn = [−393.5 + 2 × (−285.8)] − [−74.8 + 2 × 0] = [−393.5 − 571.6] − [−74.8] = −965.1 − (−74.8) = −890.3 kJ/mol
Combustion of methane releases 890.3 kJ per mole — strongly exothermic.
Calorimetry Connection:
q = m × c × ΔT
Where q is the heat measured in a calorimeter, m = mass, c = specific heat capacity, ΔT = temperature change.
Practical Tips:
- Standard state for elements (O2, N2, C as graphite) is always ΔH_f = 0 by definition
- Hess’s Law is exact for enthalpy because enthalpy is a state function: path doesn’t matter, only start and end states
- Thermite reaction (iron oxide + aluminum): ΔH = −852 kJ/mol: extremely exothermic