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 — strongly exothermic.

Calorimetry Connection:

q = m × c × ΔT

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

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