Enthalpy Calculator

Calculate enthalpy change (delta H) for chemical reactions using standard enthalpies of formation. Find heat released or absorbed for any reaction at standard conditions.

🔥 Thermochemistry📐 ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants)🧪 Chemistry

Sum of product formation enthalpies ΔH°f (kJ)

Sum of reactant formation enthalpies ΔH°f (kJ)

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Formula & Reference

Variable	Symbol	Formula	Units
Enthalpy Calculator	See formula	ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants)	kJ/mol

Step-by-Step Examples

Example 1

Combustion of Methane

CH₄ + 2O₂ → CO₂ + 2H₂O. ΔH°f: CO₂=-393.5, H₂O=-285.8, CH₄=-74.8, O₂=0.

Products: -393.5 + 2(-285.8) = -965.1 kJ
Reactants: -74.8 + 2(0) = -74.8 kJ
ΔH = -965.1 - (-74.8) = -890.3 kJ/mol

✓ Combustion of methane releases 890.3 kJ/mol

Example 2

Decomposition of H₂O

2H₂O → 2H₂ + O₂. Products: 0. Reactants: 2(-285.8)=-571.6.

ΔH = 0 - (-571.6) = +571.6 kJ/mol
Endothermic: requires 571.6 kJ

✓ Endothermic: 571.6 kJ/mol required

Example 3

Formation of NH₃

N₂ + 3H₂ → 2NH₃. ΔH°f(NH₃) = -46.1 kJ/mol.

Products: 2(-46.1) = -92.2 kJ
Reactants: 0 + 0 = 0
ΔH = -92.2 kJ/mol

✓ Exothermic: 92.2 kJ/mol released

Real-World Applications

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Fuel Energy

Combustion enthalpies determine energy content of fuels for engineering and environmental calculations.

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Reaction Safety

Highly exothermic reactions (ΔH ≪ 0) need cooling to prevent runaway. Important for lab safety.

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Chemical Plants

Reactor heat management uses enthalpy to design cooling/heating systems.

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Metabolism

Standard enthalpies calculate caloric content: glucose combustion ΔH = -2803 kJ/mol.

Common Mistakes to Avoid

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Sign of ΔH

Negative ΔH = exothermic (releases heat). Positive ΔH = endothermic (absorbs heat).

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Standard state elements have ΔH°f = 0

O₂(g), H₂(g), C(graphite), Na(s) etc. all have ΔH°f = 0 kJ/mol by definition.

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Must multiply by coefficients

If 2 mol CO₂ produced: use 2 × (-393.5) = -787.0 kJ. The coefficient is essential.

Frequently Asked Questions

What is standard enthalpy of formation? ▾

Energy change when 1 mole of compound forms from elements in standard states. Elements in standard states have ΔH°f = 0.

What is Hess's Law? ▾

ΔH for a reaction is the same regardless of path. You can add enthalpies of intermediate reactions to find ΔH for the target reaction.

How do exothermic and endothermic differ? ▾

Exothermic (ΔH < 0): products lower energy than reactants; heat released. Endothermic (ΔH > 0): products higher energy; heat absorbed.

What is the relationship between ΔH and ΔU? ▾

ΔH = ΔU + PΔV = ΔU + ΔnRT for ideal gases. At constant pressure, ΔH is the heat exchanged.

What are typical enthalpy values? ▾

Strong acid-base: -57 kJ/mol. C-H combustion: -800 to -3000 kJ/mol. Ionic dissolution: -3 to +30 kJ/mol.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship tracks energy transfer, state-function change or the balance between enthalpy and entropy in a chemical process. Assumption: Keep energy units compatible, use kelvin for absolute temperature, and match standard states and reaction stoichiometry. Thermodynamic favorability does not determine reaction speed.

Enthalpy of Reaction Calculator →Entropy Calculator →Gibbs Free Energy Calculator →Chemistry Formula Explorer →