Enthalpy Change Calculator Formula
Understand the math behind the enthalpy change calculator. Each variable explained with a worked example.
Formulas Used
Enthalpy Change (ΔH°)
delta_h = sum_hf_products - sum_hf_reactantsVariables
| Variable | Description | Default |
|---|---|---|
sum_hf_products | Sum of ΔHf° (Products)(kJ) | -890 |
sum_hf_reactants | Sum of ΔHf° (Reactants)(kJ) | -75 |
How It Works
Enthalpy Change of Reaction
Formula
ΔH°rxn = Σ ΔHf°(products) − Σ ΔHf°(reactants)
Where:
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.
- 01ΔH°rxn = Σ ΔHf°(products) - Σ ΔHf°(reactants)
- 02ΔH°rxn = -890 - (-75)
- 03ΔH°rxn = -890 + 75
- 04ΔH°rxn = -815 kJ (exothermic)
Frequently Asked Questions
What is enthalpy of formation?
The standard enthalpy of formation (ΔHf°) is the heat change when 1 mole of a compound forms from its elements in their standard states.
What is the ΔHf° of elements in their standard state?
By definition, ΔHf° = 0 for elements in their most stable form at standard conditions (e.g., O₂(g), C(graphite), Fe(s)).
How do I know if a reaction is exothermic?
If ΔH < 0, the reaction releases heat to the surroundings and is exothermic. If ΔH > 0, it absorbs heat and is endothermic.
Learn More
Guide
How to Calculate Enthalpy Change
Learn how to calculate enthalpy change (ΔH) using standard enthalpies of formation, Hess's Law, and calorimetry. Includes worked examples for exothermic and endothermic reactions.
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Open Enthalpy Change Calculator