Bomb Calorimeter
Definition and meaning of Bomb Calorimeter in chemistry.
A bomb calorimeter is a device that measures the heat released or absorbed during a combustion reaction. A sample burns inside a sealed, constant-volume steel vessel that sits inside a known mass of water.
In more detail
Because the sealed vessel, often called the bomb, is rigid, the reaction inside cannot expand or push against anything. No pressure-volume work happens, so the heat measured equals the change in internal energy of the system, written qv = ΔU, rather than enthalpy directly. To make sure the sample burns completely, chemists ignite it electrically inside an excess of pressurized oxygen gas.
Heat released by combustion flows into the surrounding water bath and raises its temperature. Combining that temperature rise with the calorimeter's known heat capacity gives the total heat released. Converting this internal energy value to enthalpy of combustion requires one more step, using the relationship ΔH = ΔU + Δn(gas)RT, where Δn(gas) is the change in moles of gas during the reaction.
Bomb calorimeters are essential tools for measuring the energy content of fuels and the caloric value of foods, since both rely on accurately measuring heat from combustion. The Calorie values printed on nutrition labels ultimately trace back to bomb calorimeter measurements of how much heat food samples release when fully burned.
Key facts
| Field | Physical Chemistry |
|---|---|
| Quantity measured | qv = ΔU (constant volume) |
| Key relation | ΔH = ΔU + Δn(gas)RT |
| Common use | Heat of combustion of fuels and foods |
| Calibration standard | Benzoic acid |
A chemist burns a 1.000 gram pellet of benzoic acid inside a bomb calorimeter's oxygen-filled chamber. The surrounding water rises by a measured temperature, which lets the chemist calculate the calorimeter's exact heat capacity. That calibrated heat capacity is then used to find the heat of combustion of an unknown fuel sample tested in the same instrument. Food scientists run this same procedure on dried food samples to determine the Calorie content listed on packaging.
Frequently asked questions
Does a bomb calorimeter measure ΔH or ΔU?
It directly measures ΔU, since the constant-volume vessel does no PV work. Chemists then calculate ΔH separately using the relationship ΔH = ΔU + Δn(gas)RT.
Why is the sample burned in excess oxygen?
Excess oxygen ensures complete combustion, so all the heat released corresponds accurately to the reaction's true heat of combustion.
Why must the calorimeter be calibrated before use?
Every calorimeter absorbs a slightly different amount of heat for each degree its temperature rises. Burning a standard substance with a known heat of combustion, like benzoic acid, lets chemists calculate this heat capacity precisely before testing an unknown sample.