Hess Law

Total enthalpy change is independent of pathway; sums stepwise ΔH.

Short definition

Total enthalpy change is independent of pathway; sums stepwise ΔH.

Full explanation

Hess Law in the context of thermochemistry thermodynamics: Total enthalpy change is independent of pathway; sums stepwise ΔH. In practice this means relating macroscopic observation (mass, pressure, color), particle-level picture (atoms, electrons, ions moving and rearranging), and symbolic representation (formulas, equations, units). For Hess Law, students connect the definition to linked lessons: do not just memorize the phrase — predict what changes when conditions change and check with dimensional analysis.

Notation and units

• Notation: ΔH_rxn=ΣΔH_steps
• Common units: SI or derived, temperature in kelvins when in gas/energy contexts.
• Symbolic hint: See related formula if applicable.

Example

Example for hess law: In a worked problem where this term appears, substitute measured values with correct units and check that the dimensional result matches the intended quantity. For instance, calculating a hess law often uses the form $n=m/M$ or $c=n/V$ or $PV=nRT$ with $T$ in kelvins — illustrating how hess law leads to a numerically different but dimensionally correct result when units are handled.

Confused with

• Bond Energy — distinction: Hess Law is total enthalpy change is independent of pathway; sums stepwise δh. while Bond Energy has a different defining property. See cross-link in glossary.

Related resources

• Topic: thermochemistry-thermodynamics
• Formula library — when a specific equation involves this term it is linked.
• Constants — if a constant is required, it is sourced and versioned (NIST 2022).

Sources

• NIST Physical Constants 2022 (nist-2022, CODATA 2022). Accessed 2026-08-21.

Last reviewed: 2026-08-21 — Review due: 2027-08-21 — Author: Chemistry Fundamentals Editorial Team — Reviewer: Editorial Review Board (transparent placeholder).

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