Enthalpy Change Calculator Formula

Understand the math behind the enthalpy change calculator. Each variable explained with a worked example.

Use the Enthalpy Change Calculator

Formulas Used

Enthalpy Change (ΔH°)

delta_h = sum_hf_products - sum_hf_reactants

Variables

VariableDescriptionDefault
sum_hf_productsSum of ΔHf° (Products)(kJ)-890
sum_hf_reactantsSum of ΔHf° (Reactants)(kJ)-75

How It Works

Enthalpy Change of Reaction

Formula

ΔH°rxn = Σ ΔHf°(products) − Σ ΔHf°(reactants)

Where:

  • ΔH°rxn = standard enthalpy of reaction
  • ΔHf° = standard enthalpy of formation

    • Negative ΔH means exothermic (releases heat). Positive ΔH means endothermic (absorbs heat).

    Worked Example

    Combustion of methane: products ΔHf° sum = -890 kJ, reactants ΔHf° sum = -75 kJ.

    sum_hf_products = -890sum_hf_reactants = -75
    1. 01ΔH°rxn = Σ ΔHf°(products) - Σ ΔHf°(reactants)
    2. 02ΔH°rxn = -890 - (-75)
    3. 03ΔH°rxn = -890 + 75
    4. 04ΔH°rxn = -815 kJ (exothermic)

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