Foods and Fuels

Energy Relations in Chemistry: Thermochemistry

Foods and Fuels

Foods and Fuels

The energy released when 1 gram of material is combusted is called its fuel value

Note: since all heats of combustion are exothermic it is customary to leave off the negative sign when reporting fuel values.

Foods

Most of the energy our body needs comes from fats and carbohydrates.

Carbohydrates are broken down in the intestines to glucose. Glucose is transported in the blood to cells where it is oxidized to produce CO₂, H₂O and energy:

C₆H₁₂O₆(s)+6O₂(g) -> 6CO₂(g)+6H₂O(l) DH°_rxn=-2816 kJ

The breakdown of fats also produces CO₂ and H₂O

Any excess energy in the body is stored as fats

Insoluble in water so they can be separated and stored in the body
They produce more energy per gram than carbohydrates or proteins

Compound	Fuel Value (kJ/gram)
Fats	38
Carbohydrates	17
Proteins	17

About 100 kJ per kilogram of body weight per day is required to keep the body functioning at a minimum level

Exercise

Light exercise burns up about 800 kJ/hour of energy
Heavy exercise burns up about 2000 kJ/hour

Fuels

Different types of fuels contain varying amounts of carbon, hydrogen and oxygen.

The greater the percentage of carbon and hydrogen in the fuel the higher the fuel value

Fuel	C (%)	H (%)	O (%)	Fuel Value (kJ/g)
Wood	50	6	44	18
Bituminous Coal	77	5	7	32
Gasoline	85	15	0	48
Hydrogen	0	100	0	142

The average daily consumption of energy per person in the U.S. is about 8.8 x 10⁵ kJ, or about 100 times greater than our food energy requirements

Our energy comes primarily from the combustion of fossil fuels (hydrocarbons derived from ancient plants and animals)

Coal represents 90% of the fossil fuels on earth. However, it typically contains sulfur, which when combusted can lead to environmental pollution (acid rain)

Solar energy: on a clear day the sun's energy which strikes the earth equals 1kJ per square meter per second.

If we could utilize the solar energy which strikes 0.1% of the U.S. land mass we would have enough power to run the country

Hydrogen: clean burning (produces only water) and high fuel value. Hydrogen can be made from coal as well as methane

C(coal) + H₂O(g) -> CO(g)+H₂(g)

CH₄(g) + H₂O(g) -> CO(g) + 3H₂(g)

1996 Michael Blaber