Hess's Law states that the total enthalpy change for a reaction is the same regardless of the pathway. ΔH_rxn = Σ[ΔHf°(products)] − Σ[ΔHf°(reactants)].

What is standard enthalpy of formation?

ΔHf° is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 25°C and 1 bar. ΔHf° of all elements in standard state = 0 by definition.

How do bond energies give ΔH?

ΔH ≈ Σ(energies of bonds broken) − Σ(energies of bonds formed). Breaking bonds absorbs energy; forming bonds releases energy. This gives an approximate ΔH because average bond energies are used.

What is Kirchhoff's Law?

Kirchhoff's Law corrects ΔH for temperatures other than 298 K: ΔH(T) = ΔH(298) + ΔCp × (T − 298), where ΔCp is the difference in heat capacities of products and reactants.

What is the difference between ΔH and ΔU?

ΔH = ΔU + Δn_gas × RT, where Δn_gas is the change in moles of gas. For reactions without a gas phase change, ΔH ≈ ΔU. Calorimetry at constant volume measures ΔU; at constant pressure it measures ΔH.

Enthalpy Calculator

Calculate the enthalpy of reaction (ΔH_rxn) using Hess's Law from standard enthalpies of formation, from bond energies, or from calorimetry data.

ΔHf° Product 1 (kJ/mol)

kJ/mol

Coefficient of Product 1

ΔHf° Product 2 (kJ/mol)

kJ/mol

Coefficient of Product 2

ΔHf° Reactant 1 (kJ/mol)

kJ/mol

Coefficient of Reactant 1

ΔHf° Reactant 2 (kJ/mol)

kJ/mol

Coefficient of Reactant 2

ΔH_rxn (kJ/mol)

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Reaction Type —

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ΔHf° Product 1 (kJ/mol)

kJ/mol

Coeff Product 1

ΔHf° Product 2 (kJ/mol)

kJ/mol

Coeff Product 2

ΔHf° Reactant 1 (kJ/mol)

kJ/mol

Coeff Reactant 1

ΔHf° Reactant 2 (kJ/mol)

kJ/mol

Coeff Reactant 2

ΔH_rxn (kJ/mol)

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Reaction Type —

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ΔHf° Product 1 (kJ/mol)

kJ/mol

Coeff Product 1

ΔHf° Product 2 (kJ/mol)

kJ/mol

Coeff Product 2

ΔHf° Reactant 1 (kJ/mol)

kJ/mol

Coeff Reactant 1

ΔHf° Reactant 2 (kJ/mol)

kJ/mol

Coeff Reactant 2

ΔCp (J/(mol·K), Kirchhoff) ?

J/(mol·K)

Temperature T (K)

Δn_gas (moles gas change) ?

ΔH at 298 K (kJ/mol)

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ΔH at T (Kirchhoff, kJ/mol) —

ΔU (kJ/mol, const vol) —

Reaction Type —

How to Use This Calculator

Enter ΔHf° values for products (with their stoichiometric coefficients) and for reactants.
The calculator applies Hess's Law: ΔH_rxn = Σ(products) − Σ(reactants).
Use tabs for bond energy method or calorimetry.

Formula

ΔH_rxn = Σ n·ΔHf°(products) − Σ m·ΔHf°(reactants)

Bond method: ΔH = Σ(broken) − Σ(formed)

Calorimetry: q = m·c·ΔT

Example

Example: H₂(g) + ½O₂(g) → H₂O(l). ΔHf°[H₂O(l)] = −285.8. ΔHf°[H₂] = 0, ΔHf°[O₂] = 0. ΔH_rxn = −285.8 − 0 = −285.8 kJ/mol.

Frequently Asked Questions

Hess's Law states that the total enthalpy change for a reaction is the same regardless of the pathway. ΔH_rxn = Σ[ΔHf°(products)] − Σ[ΔHf°(reactants)].
ΔHf° is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 25°C and 1 bar. ΔHf° of all elements in standard state = 0 by definition.
ΔH ≈ Σ(energies of bonds broken) − Σ(energies of bonds formed). Breaking bonds absorbs energy; forming bonds releases energy. This gives an approximate ΔH because average bond energies are used.
Kirchhoff's Law corrects ΔH for temperatures other than 298 K: ΔH(T) = ΔH(298) + ΔCp × (T − 298), where ΔCp is the difference in heat capacities of products and reactants.
ΔH = ΔU + Δn_gas × RT, where Δn_gas is the change in moles of gas. For reactions without a gas phase change, ΔH ≈ ΔU. Calorimetry at constant volume measures ΔU; at constant pressure it measures ΔH.

Related Calculators

Sources & References (5) ▾

NIST Chemistry WebBook — Standard Enthalpies of Formation — NIST
ACS Education — Thermochemistry — American Chemical Society
OpenStax Chemistry 2e, Chapter 5 — Thermochemistry — OpenStax
IUPAC Quantities Units and Symbols in Physical Chemistry — Enthalpy — IUPAC
Atkins, P. & de Paula, J. — Physical Chemistry, 10th Ed., Ch. 2 — Internal Energy — Oxford University Press