Chemical Potential Energy
Discussion
introduction
Calorimetry and so on.
Q = mc∆T
Burn a sample of the substance in oxygen, use the heat given off to warm a bit of water and watch the temperature rise.
When comparing substances, it's often more instructive to speak of their specific energy or specific work or gravimetric energy density or volumetric energy density (energy per mass) symbol e or w.
| e = | E |
| m |
or energy density (energy per volume) symbol u or η (eta) or ε (epsilon).
| u = | E |
| V |
rather than their energy.
| simple compounds | specific energy (MJ/kg) |
|---|---|
| carbon (to CO2) | 32.8 |
| carbon (to CO) | 22.7 |
| ethanol | 29.7 |
| hydrogen | 142 |
| methane | 55.5 |
| methanol | 22.7 |
| nitromethane | 11.6 |
| octane | 47.9 |
| propane | 50.3 |
| sulfur | 9.16 |
| foods | specific energy (MJ/kg) |
|---|---|
| carbohydrates | 17.2 |
| fats | 38.9 |
| proteins | 17.2 |
| explosives* | specific energy (MJ/kg) |
|---|---|
| ANFO | 6.3 |
| BTTN | |
| C4 | 6.3 |
| composition b | |
| composition c | |
| dynamite | 7.5 |
| gunpowder | 3.0 |
| HMX | |
| lead azide | |
| nitrocellulose | |
| nitroglycerine | 6.4 |
| PETN | 5.8 |
| RDX | 6.3 |
| semtex | |
| silver azide | |
| silver nitride | 0.93 |
| sodium azide | |
| TATP | |
| torpex | 6.3 |
| TNT | 4.184 |
| fossil fuels | specific energy (MJ/kg) |
|---|---|
| coal, anthracite | >32.6 |
| coal, bituminous | 24.4–32.6 |
| coal, sub-bituminous | 19.3–24.4 |
| coal, lignite | 14.7–19.3 |
| diesel | 45.3 |
| gasoline, automotive | 45.8 |
| gasoline, aviation | 43.1 |
| jet a | 43.3 |
| jet b | 43.5 |
| kerosene | 46.3 |
| oil, crude (petroleum) | 41.9 |
| oil, heating | 42.5 |
| biomass fuels | energy density (MJ/kg) |
|---|---|
| alfalfa straw | 18.4 |
| charcoal | 29.0 |
| coconut husks | 9.8 |
| coconut shells | 17.9 |
| coffee husks | 16.0 |
| cotton hulls | 19.4 |
| cotton stalks | 17.4 |
| dung, air dry | 12.0 |
| groundnut shells | 19.7 |
| maize cobs | 18.9 |
| maize stalks | 18.2 |
| peat | 14.6 |
| rice hulls | 15.5 |
| rice straw | 15.2 |
| sugar cane bagasse | 19.0 |
| sorghum bagasse | 18.9 |
| soybean stalks | 19.4 |
| wheat straw | 18.9 |
| wood, green | 10.9 |
| wood, air dry | 15.5 |
| wood, oven dry | 20.0 |
fuel
A word about fuel based units.
| fuel unit | energy (MJ) | source |
|---|---|---|
| cubic meter natural gas (15 °C, 1 atm) |
37.1–38.7 | Energy Information Administration |
| 37.3 | Carbon Dioxide Information Analysis Center | |
| 38.3 | ConEdison (New York City) | |
| 37.9 | Niagara Mohawk (New York State) | |
| 38.4 | PSE&G (New Jersey) | |
| toe ton of oil equivalent (1,000 kg) |
41,868 | International Energy Agency |
| 42,622 | Statistics Norway | |
| 44,760 | Carbon Dioxide Information Analysis Center | |
| tce ton of coal equivalent (1,000 kg) |
29,308 | International Energy Agency |
| 29,310 | Center for Energy Efficiency | |
| 29,290 | Carbon Dioxide Information Analysis Center | |
| ton of tnt | 4,184 | exact value by definition |
food
Respiration (cellular respiration, that is, not pulmonary respiration or breathing as it is better known) is chemically identical to combustion, but it takes place at a much slower rate.
C6H12O6 + O2 → CO2 + H2O + energy
or in English…
sugar combined with oxygen turns into carbon dioxide and water and releases energy
The energy content of foods is no longer measured directly much anymore. Instead, the food item in question is analyzed to determine the amount of protein, fat, and carbohydrate present in one serving. These numbers are then multiplied by the respective energy densities. In the US, where kilocalories are used instead of megajoules, the rule is 4-9-4: 4 kcal/g for protein, 9 kcal/g for fat, and 4 kcal/g for carbohydrate.
| foods | kcal/kg | MJ/kg |
|---|---|---|
| carbohydrates | 4 | 17.2 |
| fats | 9 | 38.9 |
| proteins | 4 | 17.2 |
Adjustments are needed for indigestible fiber and specific energy factors/coefficient of digestibility.
Atwater system. Nutrition Labeling and Education Act of 1990.
Basal metabolic rate. Revised Harris-Benedict equations.
| Qmen[kJ/day] = | 0370 + 56.05 m[kg] |
| Qwomen[kJ/day] = | 1872 + 38.69 m[kg] |