Reaction enthalpy (or heat of reaction, ΔH) measures the total energy released or absorbed during a chemical reaction. Exothermic reactions release heat (ΔH < 0), like combustion or rusting. Endothermic reactions absorb heat (ΔH > 0), like melting ice or photosynthesis. Enthalpy is central to understanding reaction energetics and predicting whether reactions will be spontaneous.
The Formula
Using Hess's Law:
ΔH_reaction = Σ ΔH_f(products) - Σ ΔH_f(reactants)
Where ΔH_f is the standard enthalpy of formation (energy to form 1 mole of substance from elements in standard state).
Or from experimental data:
ΔH = q / n
Where q is heat absorbed/released (in joules) and n is moles of limiting reagent.
Worked Example
Calculate enthalpy for: 2H₂ + O₂ → 2H₂O
Using standard enthalpies of formation (at 25°C):
- ΔH_f(H₂) = 0 kJ/mol
- ΔH_f(O₂) = 0 kJ/mol
- ΔH_f(H₂O) = -285.8 kJ/mol
ΔH_reaction = [2 × (-285.8)] - [2 × 0 + 1 × 0]
= -571.6 - 0
= -571.6 kJ/mol
This is a highly exothermic reaction — burning hydrogen releases 571.6 kJ per mole of oxygen consumed.
Types of Reactions
| Type | ΔH | Example |
|---|---|---|
| Exothermic | < 0 | Combustion, neutralization, freezing |
| Endothermic | > 0 | Melting, dissolution, photosynthesis |
| Thermoneutral | ≈ 0 | Phase changes at equilibrium |
Spontaneity and ΔG
Enthalpy alone doesn't determine if a reaction is spontaneous. The Gibbs free energy combines enthalpy and entropy:
ΔG = ΔH - TΔS
A reaction is spontaneous when ΔG < 0. An exothermic reaction with negative ΔH is more likely to be spontaneous, but entropy (ΔS) also matters.
Calorimetry
Experimentally, enthalpy is found by measuring heat release in a calorimeter:
q = m × c × ΔT
Where m is mass, c is specific heat capacity, and ΔT is temperature change. The reaction enthalpy is -q (negative because heat released by reaction is absorbed by the calorimeter).
Tips
Remember that formation enthalpies of elements in their standard state are zero by definition. When looking up values, note the state (solid, liquid, gas) — different states have different enthalpies. Also be careful with sign conventions: negative ΔH means heat released to surroundings.
Use our Reaction Enthalpy Calculator to compute reaction enthalpy from formation enthalpies instantly.