What Is Enthalpy?
Enthalpy (H) measures the total energy of a thermodynamic system — both the internal energy and the energy associated with pressure and volume. It is a state function that depends only on the equilibrium state of a system, making it ideal for analyzing chemical reactions at constant pressure.
The enthalpy formula is:
H = U + p·V
where U is internal energy, p is pressure, and V is volume.
Change in Enthalpy (ΔH)
The more practical quantity is the change in enthalpy — the total energy exchanged in a process. At constant pressure, ΔH equals the heat absorbed or released:
ΔH = ΔU + p·ΔV
This calculator supports two methods:
- Hess's Law — calculate ΔHrxn from standard enthalpies of formation.
- ΔH = ΔU + p·ΔV — calculate ΔH directly from internal energy change and PV work.
Endothermic vs. Exothermic Reactions
The sign of ΔH immediately reveals the nature of a reaction:
- Endothermic (ΔH > 0): The system absorbs heat from the surroundings. Energy must be supplied for the reaction to proceed (e.g., melting ice, photosynthesis).
- Exothermic (ΔH < 0): The system releases heat to the surroundings. Energy is liberated (e.g., combustion of fuel, neutralization reactions).
Standard Enthalpy of Formation (ΔH°f)
The standard enthalpy of formation (ΔH°f) is the heat change when one mole of a compound is formed from its elements in their standard states at 298 K and 1 atm. By convention:
- ΔH°f of any element in its standard state = 0 (H₂, O₂, N₂, C(graphite), etc.)
- Negative ΔH°f — compound is more stable than its elements (energy released during formation).
- Positive ΔH°f — compound is less stable than its elements (energy absorbed during formation).
Common Standard Enthalpies of Formation (kJ/mol, 298 K)
| Substance | ΔH°f (kJ/mol) | ΔH°f (BTU/mol) |
|---|---|---|
| H₂O (l, liquid water) | −285.83 | −271.00 |
| H₂O (g, steam) | −241.82 | −229.13 |
| CO₂ (g) | −393.51 | −372.97 |
| CO (g) | −110.53 | −104.76 |
| CH₄ (g, methane) | −74.81 | −70.90 |
| C₂H₅OH (l, ethanol) | −277.69 | −263.23 |
| NH₃ (g) | −46.19 | −43.77 |
| HCl (g) | −92.31 | −87.49 |
| NO₂ (g) | +33.20 | +31.47 |
| SO₂ (g) | −296.84 | −281.32 |
| NaCl (s) | −411.15 | −389.68 |
How to Calculate the Enthalpy of a Reaction (Hess's Law)
Hess's Law states that the total enthalpy change of a reaction is the sum of enthalpies of formation of products minus the sum for reactants:
ΔH°rxn = Σ [n · ΔH°f(products)] − Σ [n · ΔH°f(reactants)]
where n is the stoichiometric coefficient of each substance.
Example: Combustion of Methane
CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l)
- Products: 1 × (−393.51) + 2 × (−285.83) = −965.17 kJ/mol
- Reactants: 1 × (−74.81) + 2 × 0 = −74.81 kJ/mol
- ΔH°rxn = −965.17 − (−74.81) = −890.36 kJ/mol (exothermic)
Unit Conversion
The calculator supports both metric (SI) and US customary (imperial) units:
- 1 kJ/mol = 0.9478 BTU/mol
- 1 BTU/mol = 1.0551 kJ/mol
- For the ΔH = ΔU + p·ΔV mode: energy in J, kJ, cal, kcal, BTU, or ft·lbf; pressure in Pa, kPa, MPa, bar, atm, or psi; volume in m³, L, mL, ft³, gal, or in³.
FAQs
- What units are used for enthalpy?
- Enthalpy is measured in joules (J) or kilojoules (kJ) in SI units, and in BTU (British Thermal Units) in the US customary system. For molar quantities, kJ/mol and BTU/mol are standard.
- Is enthalpy the same as heat?
- At constant pressure, the enthalpy change ΔH equals the heat qp absorbed or released by the system. At constant volume, internal energy (ΔU) equals heat instead.
- What is a negative enthalpy change?
- A negative ΔH means an exothermic process — the system releases energy. A positive ΔH indicates an endothermic process where energy is absorbed.
- How accurate is Hess's Law?
- Very accurate at standard conditions (298 K, 1 atm). Results may vary at different temperatures and pressures, requiring heat capacity corrections (Kirchhoff's Law).