Equation and purpose
Sum stepwise enthalpies to find overall reaction enthalpy.
\[\Delta H_{rxn}=\sum \Delta H_{steps}\]
Sum stepwise enthalpies to find overall reaction enthalpy.
REARRANGEMENTS
$ΔH = ΣΔH_f(products)-ΣΔH_f(reactants)$
Dimensional check: kJ/mol ✓
Variables and units
| Symbol | Quantity | Unit | Notes |
|---|---|---|---|
| ΔH_rxn | ΔH_rxn | — | — |
| ΔH_steps | ΔH_steps | — | — |
Conditions and limitations
H is state function; reverse sign when reversing step.
Worked example
Combine combustions to find formation ΔH.
Hess’s Law
Sum stepwise enthalpies to find overall reaction enthalpy.
Equation
Inline: $\Delta H_{rxn}=\sum \Delta H_{steps}$$
Variables
| Symbol | Quantity | Unit | Notes |
|---|---|---|---|
| reaction enthalpy | kJ·mol⁻¹ | — | |
| step enthalpies | kJ·mol⁻¹ | — |
Conditions and limitations
H is state function; reverse sign when reversing step.
Rearrangements
Dimensional check: kJ/mol ✓
Worked example
Combine combustions to find formation ΔH.
Steps: write equation → substitute with units → check dimensional consistency → report with correct significant figures.
Common errors
- Forgetting stoichiometric coefficients
Related resources
- thermochemistry-thermodynamics topic
- Calculators — linked only when a real calculator implements this formula.
- Constants — sourced constants with version when values are required.
Sources
- NIST Physical Constants 2022 — CODATA.
Reviewed 2026-08-21 by Editorial Team; due 2027-08-21.
Common errors
- Forgetting stoichiometric coefficients
RELATED FORMULAS
C
Chemistry Fundamentals Editorial TeamAuthor
E
Editorial Review BoardReviewer
Editorial Team — transparent review placeholder
Reviewed August 21, 2026
SOURCES
- [nist-2022]NIST Physical Constants 2022 (2022). NISThttps://physics.nist.gov/constants