Bomb Calorimeter
Definition and meaning of Bomb Calorimeter in chemistry.
A bomb calorimeter is a device that measures the heat released or absorbed by a combustion reaction by burning a sample inside a sealed, constant-volume steel vessel submerged in a known mass of water.
In more detail
Because the "bomb" is rigid and sealed, the reaction occurs at constant volume, so no pressure-volume work is done (w = 0) and the measured heat equals the change in internal energy, qv = ΔU, rather than enthalpy directly. The sample is ignited electrically in an excess of pressurized oxygen to ensure complete combustion, and the resulting temperature rise of the surrounding water bath, combined with the calorimeter's known heat capacity, gives the heat released. Enthalpy of combustion can then be obtained from ΔH = ΔU + Δn(gas)RT. Bomb calorimeters are essential for determining fuel energy content and the caloric values of foods.
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 |
A 1.000 g pellet of benzoic acid is burned in a bomb calorimeter's oxygen-filled chamber; the temperature of the surrounding water rises by a measured amount, allowing calculation of the calorimeter's heat capacity, which is then used to determine the heat of combustion of an unknown fuel sample.
Frequently asked questions
Does a bomb calorimeter measure ΔH or ΔU?
It directly measures ΔU, since the constant-volume vessel does no PV work; ΔH must be calculated separately using 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.