How a Bomb Calorimeter Works
A bomb calorimeter measures the energy released during combustion. A known mass of sample is burned in oxygen inside a sealed metal vessel called the bomb. The heat released is absorbed by the surrounding water and calorimeter body, producing a measurable temperature rise.
The heat absorbed by the calorimeter system is used to estimate the energy released by the sample. This is useful for fuel testing, food energy analysis, thermochemistry, and combustion experiments.
Bomb Calorimeter Formula
The heat absorbed by the water is:
The heat absorbed by the calorimeter body is:
The corrected heat released by the sample is estimated as:
The energy released per gram is:
Sign Convention
The calorimeter absorbs heat, so qcalorimeter is positive. The combustion reaction releases heat, so by thermodynamic convention:
Many fuel and food calculations report the released energy as a positive value, such as kJ/g or MJ/kg.
How to Use the Calculator
- Enter the mass of water in grams.
- Enter the measured temperature rise ΔT in °C.
- Use 4.184 J/g°C for water unless a different value is required.
- Enter the calorimeter constant in J/°C. Use 0 if you only want the water heat estimate.
- Enter correction energy if fuse wire, acid formation, or other corrections are known.
- Enter the mass of sample burned in grams.
- Optionally enter molar mass to calculate kJ/mol.
Example Calculation
Suppose a 2 g sample is burned in a bomb calorimeter with:
- Mass of water = 1000 g
- Specific heat of water = 4.184 J/g°C
- Calorimeter constant = 500 J/°C
- Temperature rise = 5°C
- Correction energy = 0 J
Heat absorbed is:
Energy released per gram is:
Common Heat of Combustion Values
These are approximate energy values. Units are kJ/g, not J/g.
| Substance | Approximate Heat of Combustion |
|---|---|
| Glucose | ≈ 15.6 kJ/g |
| Methane | ≈ 55.5 kJ/g |
| Octane | ≈ 47.7 kJ/g |
| Hydrogen | ≈ 120–142 kJ/g depending on basis |
Important Assumptions
- The sample burns completely.
- The calorimeter is well insulated.
- The temperature rise is measured accurately.
- The calorimeter constant is known or can be estimated.
- Correction energy is subtracted when additional heat sources, such as fuse wire, contribute to the measured temperature rise.
- The calculation reports constant-volume combustion energy. It is often close to, but not always exactly equal to, constant-pressure enthalpy of combustion.
Frequently Asked Questions
What does a bomb calorimeter measure?
It measures the heat released by combustion through the temperature rise of the water and calorimeter system.
Why is the calorimeter constant important?
The calorimeter body absorbs heat too. The calorimeter constant accounts for the heat capacity of the calorimeter hardware.
Is heat of combustion positive or negative?
The released energy is commonly reported as positive. By reaction sign convention, combustion heat is negative because the reaction releases heat.
What is calorific value?
Calorific value is the energy released per unit mass of fuel, often reported in kJ/g, MJ/kg, or kJ/mol.
Can this calculator be used for food energy?
Yes, the same principle is used for estimating energy released from food samples, although laboratory corrections and experimental conditions matter.