Reaction Enthalpy Calculator (Hess's Law)

Reaction enthalpy (ΔH_rxn) is the heat released or absorbed when a chemical reaction occurs at constant pressure. Negative ΔH means heat is released (exothermic); positive ΔH means heat is absorbed (endothermic).

Hess’s Law Formula:

ΔH_rxn = Σ ΔH_f (products) − Σ ΔH_f (reactants)

Where ΔH_f = standard enthalpy of formation (kJ/mol), defined as the heat to form 1 mole of a compound from its elements in standard states.

Bond Enthalpy Method:

ΔH_rxn ≈ Σ (bonds broken, reactants) − Σ (bonds formed, products)

Breaking bonds requires energy (+); forming bonds releases energy (−).

Common Standard Enthalpies of Formation (kJ/mol at 25 °C):

Worked Example:

Combustion of methane: CH4 + 2O2 → CO2 + 2H2O

ΔH_rxn = [ΔH_f(CO2) + 2 × ΔH_f(H2O)] − [ΔH_f(CH4) + 2 × ΔH_f(O2)]

ΔH_rxn = [−393.5 + 2 × (−285.8)] − [−74.8 + 2 × 0] = [−393.5 − 571.6] − [−74.8] = −965.1 − (−74.8) = −890.3 kJ/mol

Combustion of methane releases 890.3 kJ per mole, which is strongly exothermic.
That is 212.8 kcal, roughly the food energy in three slices of bread, out of 16 grams of gas.

Calorimetry Connection:

q = m × c × ΔT

Where q is the heat measured in a calorimeter, m = mass, c = specific heat capacity, ΔT = temperature change.

Watch the sign convention in bond-energy mode

The two methods on this page want their numbers in opposite arrangements, and mixing them up is the single most common way to get a right-looking answer with the wrong sign.

In formation mode you enter ΔH°f values, which are mostly negative for stable compounds, and the page does products minus reactants. In bond-energy mode you enter bond dissociation energies, which are always positive, and the page does reactants minus products. Bond energies have no negative entries at all, so if you have typed a minus sign into bond mode, something has gone wrong.

A quick sanity check: burning anything in oxygen has to come out negative. If it does not, the two sides are swapped.

Practical Tips:

• Standard state for elements (O2, N2, C as graphite) is always ΔH_f = 0 by definition
• Hess’s Law is exact for enthalpy because enthalpy is a state function: path doesn’t matter, only start and end states
• Thermite reaction (iron oxide + aluminum): ΔH = −852 kJ/mol, extremely exothermic
• Bond energies give an approximation, not an exact answer. They are averages taken across many different molecules, so a bond-energy result typically lands within about 10% of the formation-enthalpy one rather than matching it