Heat of Combustion Calculator

Heat of combustion is the amount of heat energy released when a specific quantity of a substance, usually one gram or one mole, burns completely in oxygen. It is measured experimentally using a bomb calorimeter, a sealed, insulated container in which the sample is ignited and the resulting temperature rise of the surrounding water bath is recorded.

Total heat released is q = Ccal x deltaT, where Ccal is the calorimeter's heat capacity in kJ per degree Celsius and deltaT is the measured temperature rise. Dividing q by the sample mass gives the specific heat of combustion in kJ per gram; multiplying that by the sample's molar mass gives the molar heat of combustion in kJ per mole.

Ccal, sometimes called the calorimeter constant, is how many kilojoules of heat are needed to raise the entire calorimeter assembly, including its water bath, by one degree Celsius. It is not a universal constant, every physical calorimeter has its own Ccal, determined by burning a reference substance with a precisely known heat of combustion, typically benzoic acid, and measuring the resulting temperature rise.

Thermodynamic sign convention defines enthalpy change from the perspective of the reacting system. Since combustion releases energy to the surroundings, the system loses enthalpy, so delta H combustion is negative, for example minus 2803 kJ per mole for glucose. This calculator reports the heat released as a positive magnitude, then notes the corresponding negative delta H combustion value.

Multiply the specific heat of combustion, in kJ per gram, by the substance's molar mass, in grams per mole. The gram units cancel, leaving kJ per mole. For example, a specific heat of combustion of 15.7 kJ per gram for glucose (molar mass 180.16 g/mol) gives a molar heat of combustion of about 2828.5 kJ per mole.

Heat of combustion is measured directly by burning one mole of a substance completely and recording the heat released. Enthalpy of formation is instead usually calculated indirectly, often by applying Hess's law to combustion data for the compound and its constituent elements, since many compounds cannot be formed cleanly and directly from their elements in a single measurable step.

No, in principle. Specific heat of combustion is an intensive property, it should be the same value regardless of how much sample was burned, since both the total heat released and the sample mass scale together. In practice, larger or smaller samples can shift the measured result slightly due to incomplete combustion or measurement error, which is why chemists average several trials.

Combustion is an exothermic reaction, it releases heat into the surrounding calorimeter and water bath, which must warm up as a direct physical consequence. A zero or negative deltaT would mean no heat was absorbed by the calorimeter, which is not physically possible for a genuine combustion reaction and signals a measurement or setup error.

A well-calibrated bomb calorimeter, run carefully with a known-purity sample, typically agrees with published standard heats of combustion within about 1 to 2 percent. Sources of error include incomplete combustion, heat loss not captured by the calorimeter constant, impurities in the sample, and fuse wire ignition energy not properly subtracted.

Yes. Enter the mass of fuel burned, the calorimeter's heat capacity, and the observed temperature rise, along with the fuel's molar mass if you want the molar heat of combustion. This applies equally to simple fuels, foodstuffs like glucose, and any other combustible sample tested in a bomb calorimeter.

Most educational bomb calorimeters have a heat capacity in the range of roughly 8 to 12 kJ per degree Celsius, though this varies by manufacturer and by how much water fills the surrounding bath. Always use the Ccal value determined for your own specific instrument through calibration, never a generic textbook figure.