What is the standard enthalpy of formation (ΔH°f)?

ΔH°f is the heat absorbed or released when exactly one mole of a compound is formed from its constituent elements in their standard states at 298 K and 1 atm pressure. By definition, ΔH°f = 0 kJ/mol for pure elements in their most stable form (e.g., O₂(g), H₂(g), C(graphite), Na(s)).

How does Hess's Law apply here?

Hess's Law states that the total enthalpy change of a reaction is the same regardless of the pathway taken. The formation method exploits this: ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants), where n is the stoichiometric coefficient. This works because enthalpy is a state function.

Why is ΔH°f = 0 for elements in their standard state?

Formation enthalpy is defined relative to elements in their most stable standard form. Forming an element from itself requires no energy change, so the value is zero by convention. Common examples: H₂(g), O₂(g), N₂(g), C(graphite), Fe(s), Na(s).

What do negative and positive ΔH°rxn values mean?

A negative ΔH°rxn means the reaction is exothermic — it releases heat to the surroundings. A positive ΔH°rxn means it is endothermic — it absorbs heat. A value near zero indicates a thermoneutral reaction.

Where do the ΔH°f values in the database come from?

The values are standard thermodynamic data at 298 K and 1 atm, drawn from widely used sources such as NIST WebBook and standard general chemistry textbooks (Atkins, Zumdahl, Chang). They are approximate to within ±1 kJ/mol of reference values.

Can I enter a compound not in the database?

Yes. Use the "Add custom" option to type any ΔH°f value manually. This lets you work with any compound even if it is not in the built-in list.

Enthalpy of Formation Calculator

Look up standard enthalpies of formation from a built-in database and calculate ΔH°rxn using Hess's Law instantly.

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Summary

The Enthalpy of Formation Calculator computes the standard enthalpy of reaction (ΔH°rxn) using the formation method of Hess's Law: ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants). A searchable database of 60+ common compounds provides tabulated ΔH°f values at 298 K and 1 atm, so you can build a reaction equation by name rather than hunting through a textbook. All calculation is performed client-side with no data sent to any server.

How it works

Search the built-in compound database by name or formula to find a substance.
Click a compound to add it as a product or reactant and enter its stoichiometric coefficient.
The tool applies ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants) in real time.
The result is labeled exothermic (ΔH < 0) or endothermic (ΔH > 0) with a full algebraic breakdown.
Use the preset buttons to load classic reactions (methane combustion, water formation, etc.) instantly.
Copy the result to clipboard with one click.

Use cases

Calculate ΔH for combustion reactions without manually looking up formation enthalpies.
Verify thermochemistry homework answers step by step.
Prepare for AP Chemistry or university general chemistry exams.
Determine whether an industrial synthesis reaction is exothermic or endothermic.
Explore how changing stoichiometry affects the overall enthalpy change.
Look up standard enthalpies of formation for 60+ common compounds.
Teach or learn Hess's Law with instant visual feedback.
Cross-check ΔH°rxn values from published thermodynamic tables.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu