Heat of Reaction Calculator

The Heat of Reaction Calculator finds the enthalpy change (ΔH) of a chemical reaction — the amount of heat released or absorbed when reactants transform into products. Select your calculation method (Standard Formation Enthalpies or Hess's Law), then choose your reactants and products with their stoichiometric coefficients to get the ΔH in kJ/mol, reaction type (exothermic or endothermic), and a breakdown of total reactant and product enthalpies.

kJ/mol

Only needed if Custom is selected

kJ/mol

Only needed if Custom is selected

Results

Enthalpy Change (ΔH)

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Reaction Type

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Energy Description

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Total Reactant Enthalpy

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Total Product Enthalpy

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Frequently Asked Questions

What is enthalpy and how is it measured?

Enthalpy (H) is a thermodynamic property that measures the total heat content of a system. It represents the sum of internal energy and the product of pressure and volume. Enthalpy change (ΔH) indicates the heat absorbed or released during a chemical reaction at constant pressure.

How do I know if a reaction is endothermic or exothermic?

A reaction is endothermic if ΔH is positive (heat is absorbed from surroundings) and exothermic if ΔH is negative (heat is released to surroundings). Endothermic reactions feel cold while exothermic reactions feel warm or hot.

What is the difference between standard formation enthalpy and heat of reaction?

Standard formation enthalpy (ΔHf°) is the enthalpy change when one mole of a compound forms from its elements in standard states. Heat of reaction (ΔHrxn) is the enthalpy change for any chemical reaction, calculated using formation enthalpies of reactants and products.

How do I calculate enthalpy change using formation enthalpies?

Use the formula: ΔHrxn = Σ(ΔHf° products) - Σ(ΔHf° reactants). Multiply each formation enthalpy by its stoichiometric coefficient, sum all products, sum all reactants, then subtract reactants from products.

What are some common standard formation enthalpy values?

Key values include: H2O(l) = -285.8 kJ/mol, CO2(g) = -393.5 kJ/mol, NH3(g) = -46.1 kJ/mol, and CH4(g) = -74.8 kJ/mol. Elements in their standard states have ΔHf° = 0 kJ/mol.

Why is the enthalpy of formation of elements zero?

By definition, elements in their most stable form under standard conditions (25°C, 1 atm) have zero formation enthalpy because they are already in their reference state and require no energy to 'form' from themselves.

Can I use this calculator for combustion reactions?

Yes, this calculator works for all types of reactions including combustion. For combustion reactions, you'll typically see large negative ΔH values because combustion reactions are highly exothermic, releasing significant amounts of energy.