Enthalpy of Reaction Calculator

Calculate heat of reaction using formation enthalpies or bond energies

Enthalpy of Reaction Calculator

Enthalpy of Reaction (ΔH°rxn): Heat change during a chemical reaction at constant pressure.
Negative ΔH = exothermic (heat released), Positive ΔH = endothermic (heat absorbed).

ΔH°rxn = Σn·ΔH°f(products) - Σm·ΔH°f(reactants)

Reactants

Products

Notes:

  • • ΔH°f for elements in standard state = 0 kJ/mol
  • • Exothermic: ΔH < 0 (negative), Endothermic: ΔH > 0 (positive)
  • • Standard conditions: 25°C (298 K) and 1 atm
  • • Bond breaking requires energy, bond forming releases energy

Understanding Enthalpy of Reaction

Enthalpy of reaction (ΔH°rxn) is the heat absorbed or released during a chemical reaction at constant pressure. It's a key thermodynamic property that determines whether reactions release or absorb energy.

Methods to Calculate ΔH°rxn

Method 1: Formation Enthalpies

ΔH°rxn = Σn·ΔH°f(products) - Σm·ΔH°f(reactants)

Sum of formation enthalpies of products minus sum of formation enthalpies of reactants, each multiplied by stoichiometric coefficients.

Method 2: Bond Energies

ΔH°rxn = Σ(Bonds Broken) - Σ(Bonds Formed)

Energy required to break bonds minus energy released when forming bonds.

Example: Combustion of Methane

Reaction: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Given ΔH°f values (kJ/mol):

  • CH₄(g): -74.8
  • O₂(g): 0 (element in standard state)
  • CO₂(g): -393.5
  • H₂O(l): -285.8

Calculation:

ΔH°rxn = [1(-393.5) + 2(-285.8)] - [1(-74.8) + 2(0)]
ΔH°rxn = [-393.5 - 571.6] - [-74.8]
ΔH°rxn = -965.1 + 74.8
ΔH°rxn = -890.3 kJ/mol

Exothermic: 890.3 kJ of heat released per mole of CH₄ burned

Common Standard Formation Enthalpies

SubstanceΔH°f (kJ/mol)State
H₂O-285.8liquid
H₂O-241.8gas
CO₂-393.5gas
NH₃-45.9gas
CH₄-74.8gas
NaCl-411.2solid

Applications

  • 🔥
    Energy Production: Calculating heat output from fuels and combustion
  • 🏭
    Industrial Processes: Designing energy-efficient chemical plants
  • 🌡️
    Thermal Management: Predicting temperature changes in reactors
  • ♻️
    Environmental Science: Understanding energy flows in ecosystems

🌡️Quick Reference

Exothermic:

ΔH < 0 (heat released)

Endothermic:

ΔH > 0 (heat absorbed)

Elements:

ΔH°f = 0 in standard state

🔗Related Calculators

📐Related Formulas

🎯Where It's Used

  • 🔥

    Combustion

    Fuel energy calculations

  • 🏭

    Industry

    Process optimization