Hess's Law Calculator

Apply Hess's Law to calculate reaction enthalpy by summing intermediate reaction enthalpies. Reverse and multiply steps as needed. Path-independent enthalpy addition.

🔥 Thermochemistry📐 ΔH = ΔH1 + ΔH2 + ΔH3🧪 Chemistry

ΔH1 (kJ/mol)

Multiplier 1

ΔH2 (kJ/mol)

Multiplier 2

ΔH3 optional (kJ/mol)

Multiplier 3

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Hess's Law Calculator	See formula	ΔH = ΔH1 + ΔH2 + ΔH3	kJ/mol

Step-by-Step Examples

Example 1

CO Formation

Find dH for C+0.5O2 to CO. Given: (1) C+O2 to CO2 dH=-393.5; (2) CO+0.5O2 to CO2 dH=-283.0.

Reverse step 2: CO2 to CO+0.5O2, dH=+283.0
Step 1 x1: dH=-393.5, Step 2 reversed x-1: dH=+283.0
Net: -393.5+283.0 = -110.5 kJ/mol

✓ delta-H(CO formation) = -110.5 kJ/mol

Example 2

Two-Step Path

A to C via B. A to B dH1=-200 kJ, B to C dH2=+50 kJ.

dH_total = -200 + 50 = -150 kJ/mol

✓ Net delta-H = -150 kJ/mol

Example 3

Reversed Reaction

Forward: CO2 formation dH=-393.5 kJ/mol. Reverse?

Multiply by -1: CO2 to C+O2
dH = +393.5 kJ/mol

✓ Reversed reaction: +393.5 kJ/mol

Real-World Applications

🔥

Thermochemical Cycles

Calculate enthalpy for reactions that cannot be measured directly.

💎

Born-Haber Cycle

Lattice energy of ionic compounds calculated using Hess Law cycles.

🧪

Reaction Planning

Predict net thermochemistry for multi-step reaction sequences.

📚

Physical Chemistry

Hess Law shows enthalpy is a state function, not path-dependent.

Common Mistakes to Avoid

⚠️

Reversing flips the sign

If you reverse a reaction, multiply dH by -1. If you scale by factor n, multiply by n.

⚠️

Intermediate species must cancel

Substances that appear on both sides of summed equations must cancel exactly.

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State of matter matters

dHf(H2O(g))=-241.8 vs dHf(H2O(l))=-285.8 kJ/mol. 44 kJ difference is heat of vaporization.

Frequently Asked Questions

What is Hess Law? ▾

Because enthalpy is a state function, delta-H depends only on initial and final states. You can add any series of reactions to find net delta-H.

How do I apply Hess Law? ▾

Write target reaction. Manipulate given reactions (reverse and/or scale) so intermediates cancel. Sum the delta-H values.

What is a state function? ▾

A property depending only on state (T, P, composition), not path. Enthalpy, entropy, and Gibbs energy are all state functions.

Why can some reactions not be measured directly? ▾

Some are too slow, produce unstable intermediates, have side reactions, or require extreme conditions. Hess Law allows indirect calculation.

What is the Born-Haber cycle? ▾

A Hess Law cycle to find lattice energy of ionic solids, using formation, ionization, electron affinity, and sublimation enthalpies.

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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.

Born-Haber Cycle Calculator →Lattice Energy Calculator →Reaction Calorimetry Calculator →Chemistry Formula Explorer →