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Thermochemistry Calculator

Compute ΔH° from formation enthalpies, apply Hess’s law, run calorimetry q = mcΔT, and scale heat with moles.

Thermochemistry calculator

ΔHf° · Hess’s law · calorimetry · scale heat

Enter coefficients and ΔHf° (kJ/mol), including the correct phase. Example: methane combustion to liquid water.

ΔH° = -890.3 kJ/mol-rxn

ΔH° = Σ nΔHf°(products) − Σ nΔHf°(reactants).

What is the Thermochemistry Calculator?

Thermochemistry relates heat to chemical change. The standard enthalpy of reaction can be estimated from tabulated standard enthalpies of formation: ΔH° = Σ nΔHf°(products) − Σ nΔHf°(reactants). Hess’s law says the net ΔH for a path equals the sum of ΔH for the steps, so you can reverse or scale tabulated reactions. Calorimetry measures heat via q = mcΔT for a substance that changes temperature.

The Thermochemistry Calculator covers formation-based ΔH°, Hess sums, calorimetry, and scaling heat with moles of reaction. Use consistent units (kJ for enthalpies; match mass and c for calorimetry).

  • ΔH° from formation enthalpies
  • Hess: reverse a step with a negative coefficient
  • q = m c ΔT for temperature changes

Mathematical / chemical formulas

Key relations:

ΔH° = Σ n ΔHf°(products) − Σ n ΔHf°(reactants)
ΔH_net = Σ (coeff_i × ΔH_i)
q = m c ΔT
q_rxn = n × ΔH

Step-by-step example: ΔH° for methane combustion

CH₄ + 2 O₂ → CO₂ + 2 H₂O(g) with ΔHf°: CH₄ −74.8, O₂ 0, CO₂ −393.5, H₂O(g) −241.8 kJ/mol.

  1. Products: (−393.5) + 2(−241.8) = −877.1 kJ.
  2. Reactants: (−74.8) + 2(0) = −74.8 kJ.
  3. ΔH° = −877.1 − (−74.8) = −802.3 kJ/mol-rxn.

Use From ΔHf° with the methane combustion preset values and confirm ΔH° ≈ −802 kJ.

Frequently asked questions

What is ΔHf° for an element in its standard state?

Zero by definition (for example O₂(g), C(graphite), Fe(s) at 1 bar and the reference temperature).

Is q for the system or surroundings?

In calorimetry, m c ΔT is usually the heat absorbed by the calorimeter contents. The reaction heat is often the negative of that if the reaction is the system heating the water.

References & further reading

Standards bodies, university open courseware, and peer-reviewed references that align with the methods used on this page.

Keep learning with more calculators and study guides on Online Science Tools.

Practice problems & worked examples

Practice alongside the thermochemistry above. Each problem includes a full worked solution so you can check your reasoning step by step.

Practice problem 1

ΔH° from formation data

For CH₄ + 2 O₂ → CO₂ + 2 H₂O(g), ΔHf°: CH₄ −74.8, CO₂ −393.5, H₂O(g) −241.8, O₂ 0. Find ΔH°.

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Worked solution

  1. Σ products = −393.5 + 2(−241.8) = −877.1.
  2. Σ reactants = −74.8.
  3. ΔH° = −877.1 − (−74.8) = −802.3 kJ.

Answer: ΔH° ≈ −802 kJ/mol-rxn

Practice problem 2

Calorimetry

100 g water, c = 4.184 J/g·°C, ΔT = +5.0 °C. Find q.

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Worked solution

  1. q = m c ΔT = 100 × 4.184 × 5.0 = 2092 J.

Answer: q = 2.09×10³ J

Practice problem 3

Hess reverse step

If step A has ΔH = +50 kJ and you reverse it, what ΔH do you use in the Hess sum?

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Worked solution

  1. Reversing flips the sign → −50 kJ (coefficient −1 × +50).

Answer: −50 kJ

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