# Standard enthalpy of formation

The standard enthalpy of formation (ΔHf°) is the heat change when one mole of a compound forms from its elements in their standard states. Elements in their standard state have ΔHf° = 0. Then ΔH°rxn = ΣΔHf°(products) − ΣΔHf°(reactants).

Chemistry (university) · Part 05: Thermochemistry and thermodynamics · Card 5 of 9 · Last updated 2026-10-19

## Analogy

A price list: each compound has a 'cost' from its elements, and the reaction's total is products minus reactants.

## Example

**Methane burning**

ΔHf°: CH₄ −74.8, CO₂ −393.5, H₂O(l) −285.8
ΔH = (−393.5 + 2(−285.8)) − (−74.8)
ΔH = −890.3 kJ

## In practice

Tables of ΔHf° let you predict the heat of thousands of reactions without running them.

## Practice

### Flashcards

- **How do you get ΔH from formation enthalpies?** ΣΔHf°(products) − ΣΔHf°(reactants).

### Interview questions

**Use standard enthalpies of formation to calculate ΔH for burning methane, and say if it is endothermic or exothermic.**

- ΔH = ΣΔHf°(products) − ΣΔHf°(reactants)
- Use CO₂ −393.5, H₂O(l) −285.8, CH₄ −74.8
- Result about −890 kJ/mol
- Negative, so exothermic

## Questions people ask

### Why is ΔHf° of O₂ zero?

The enthalpy of formation is the energy change to make a compound from its elements. O₂ is already an element in its standard state, so nothing needs to be made, and the value is defined as zero. The same applies to H₂, N₂, solid carbon as graphite, and liquid bromine.

### Does a more negative ΔHf° mean a more stable compound?

A more negative ΔHf° means the compound is lower in enthalpy than its elements, so energy is released on forming it. That suggests stability against breaking into its elements. But whether a reaction actually happens also depends on entropy and on how fast it goes.

## Related concepts

- [Enthalpy](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-5/chemistry-university-part-5-concept-2)
- [Hess's law](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-5/chemistry-university-part-5-concept-4)
- [Bond enthalpy](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-5/chemistry-university-part-5-concept-6)

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