Enthalpy of Combustion per Mole Calculator
From a calorimetry experiment, find the molar enthalpy of combustion of a fuel using water mass, temperature rise and fuel burned.
How it works
q = m(water) × c × ΔT · moles = mass(fuel) ÷ MW · ΔHc = −q ÷ moles (per mole, kJ)
A simple combustion experiment heats a known mass of water with the flame of a burning fuel. The heat the water absorbs is its mass times its specific heat (4.18 J/g°C) times its temperature rise. Dividing that heat by the moles of fuel burned — fuel mass over molar mass — gives the enthalpy of combustion per mole, reported as negative because combustion releases heat. Lab values fall short of textbook figures because much heat escapes to the surroundings and the apparatus rather than reaching the water.
Worked example
Heating 200 g of water by 25 °C absorbs q = 200 × 4.18 × 25 = 20,900 J. Burning 0.5 g of ethanol (46.07 g/mol) is 0.5 ÷ 46.07 = 0.01085 mol, so ΔHc = −20,900 ÷ 0.01085 = −1.93 × 10⁶ J/mol ≈ −1,926 kJ/mol.
Frequently asked questions
Why is the answer negative?
Combustion is exothermic — it releases energy to the surroundings — and by convention released heat carries a negative enthalpy sign. The magnitude is the energy per mole; the minus sign marks the direction of energy flow.
Why is my value lower than the data-book figure?
Simple spirit-burner calorimetry loses a large fraction of the heat to the air, the container and incomplete combustion, so measured magnitudes are often only 40–70% of the true value. Insulating the vessel and shielding the flame improves the result.
Do I include the mass of the container?
This calculator uses only the water. For a more accurate result you would add the heat absorbed by the calorimeter itself (its heat capacity times ΔT), which raises the computed magnitude toward the true value.
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This calculator is for educational purposes. Double-check important results before acting on them.