Hess's Law Calculator
Calculate the standard enthalpy of reaction using Hess's Law. Enter the stoichiometric coefficients and standard formation enthalpies for up to four products and four reactants. The calculator applies the formula: delta-H(rxn) = sum of (delta-Hf products) minus sum of (delta-Hf reactants).
Hess's Law states that the total enthalpy change for a reaction depends only on the initial and final states, not on the path taken. This lets you calculate reaction enthalpies from tabulated formation enthalpies without running the reaction.
The Formula
ΔH°rxn = Σ[n × ΔHf°(products)] - Σ[n × ΔHf°(reactants)]
Where n is the stoichiometric coefficient of each species.
Worked Example: Combustion of Methane
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
Formation enthalpies:
- CH4(g): -74.8 kJ/mol
- O2(g): 0 kJ/mol (element in standard state)
- CO2(g): -393.5 kJ/mol
- H2O(l): -285.8 kJ/mol
Calculation:
- Products: 1(-393.5) + 2(-285.8) = -393.5 + (-571.6) = -965.1 kJ/mol
- Reactants: 1(-74.8) + 2(0) = -74.8 kJ/mol
- ΔH°rxn = -965.1 - (-74.8) = -890.3 kJ/mol (exothermic)
Important Rules
- Elements in their standard state (O2 gas, Fe solid, C graphite) have ΔHf° = 0 by definition
- A negative ΔH°rxn means the reaction releases heat (exothermic)
- A positive ΔH°rxn means the reaction absorbs heat (endothermic)
- Formation enthalpies are always for forming 1 mole of compound from its elements
Common Formation Enthalpies (kJ/mol)
| Compound | ΔHf° |
|---|---|
| H2O(l) | -285.8 |
| H2O(g) | -241.8 |
| CO2(g) | -393.5 |
| CH4(g) | -74.8 |
| NH3(g) | -45.9 |
| C2H5OH(l) | -277.7 |
| NaCl(s) | -411.2 |