Bomb Calorimeter Calculator

Calculate heat of combustion from bomb calorimeter data and convert to standard enthalpy.

For delta-H = delta-U + delta(n)RT
Please check your inputs and try again.

Why a Bomb Measures ΔU, Not ΔH

A bomb calorimeter is a sealed steel vessel of fixed volume. Because the volume cannot change, no expansion work is done, so all the released energy appears as heat. That makes the measured quantity the internal energy change, not enthalpy.

ΔU = −qV/n     ΔH = ΔU + ΔngasRT

The correction term involves only the change in moles of gas, since solids and liquids have negligible volume. For glucose combustion, six moles of oxygen become six of carbon dioxide, so Δngas = 0 and ΔH equals ΔU exactly.

ReactionΔngasΔH vs ΔU at 298 K
Glucose combustion0Identical
CH4 + 2O2 → CO2 + 2H2O(l)−2ΔH is 5.0 kJ/mol lower
C(s) + O2 → CO20Identical
2C8H18 + 25O2 → 16CO2 + 18H2O(l)−9 per 2 molCorrection is small but non-zero

Calibration Comes First

The calorimeter's heat capacity cannot be calculated — it depends on the steel, water and fittings. It is measured by burning benzoic acid, whose combustion enthalpy is known to high precision. Every subsequent measurement depends on that calibration being correct.

Food energy labelling derives from this technique. A dietary Calorie is a kilocalorie, or 4.184 kJ, and bomb values are adjusted downward because humans do not fully digest everything — dietary fibre in particular burns in a bomb but yields little metabolisable energy.

Worked Examples

Example 1: Glucose C6H12O6: dT=2.35C, Cv=10.32, 0.500g, MW=180
q=10.32x2.35=24.25kJ, n=0.00278mol
Result: delta-U=-8730 kJ/mol, delta-H=-2803 kJ/mol
Literature: -2803 kJ/mol check
Example 2: Benzoic acid (calibration): dT=3.10C, Cv=?
Known dH=-3227 kJ/mol calibrates Cv
Result: Cv=q/dT -> calibrate calorimeter
Always calibrate with known standard
Example 3: When the correction matters
Methane combustion, Δngas = −2 at 298 K
Result: ΔH = ΔU − 5.0 kJ/mol
ΔngasRT = −2 × 8.314 × 298 = −4.96 kJ/mol. Small relative to −890 kJ/mol but not negligible in precise work.
Example 4: Why glucose needs no correction
C6H12O6 + 6O2 → 6CO2 + 6H2O(l)
Result: Δngas = 0
Six moles of gas in, six out. The bomb value equals the enthalpy directly, which is convenient for food energy work.

Common Mistakes

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Assuming the bomb measures ΔH directly

Constant volume means no expansion work, so it measures ΔU. Converting requires adding ΔngasRT.

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Counting solids and liquids in Δngas

Only gaseous species contribute meaningfully to volume change. Water produced as liquid does not count; as vapour it would.

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Skipping or reusing an old calibration

Calorimeter heat capacity changes if water volume or components change. Benzoic acid calibration should be repeated regularly.

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Equating bomb energy with dietary energy

Humans do not extract all combustion energy. Fibre burns completely in a bomb but provides little metabolisable energy.

Frequently Asked Questions

Bomb calorimeter measures delta-U not delta-H?
Bomb is constant volume, so q_v = delta-U. Convert to delta-H using delta-H = delta-U + delta-n*RT where delta-n = change in moles of gas. For most combustions, difference is small (<1%).
Food calorie connection?
Food Calorie (kcal) is measured by bomb calorimetry. Carbohydrates: ~4 kcal/g (17 kJ/g). Fats: ~9 kcal/g (37 kJ/g). Proteins: ~4 kcal/g. Food labels use these Atwater factors.
Why does a bomb calorimeter measure ΔU rather than ΔH?
Because it operates at constant volume, so no expansion work occurs. All energy appears as heat, which corresponds to internal energy change.
How do I convert ΔU to ΔH?
Add ΔngasRT, where Δngas is the change in moles of gaseous species. Solids and liquids are excluded.
Why is benzoic acid used for calibration?
Its combustion enthalpy is known to high precision and it burns cleanly and completely. It is the international reference standard for bomb calorimetry.
Do food labels use bomb calorimeter values?
They derive from them, but adjusted downward. Humans do not fully metabolise all components — fibre burns in a bomb but yields little usable energy.

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.

Exothermic vs Endothermic Predictor →Enthalpy Calculator →Enthalpy of Reaction Calculator →Chemistry Formula Explorer →