You can measure the output of energy from metabolising
energy rich foods via some simple laboratory experiments.
The most energy rich foods are vegetable oils and
animal fats, more so than carbohydrates.
You need to burn a known mass of material in a controlled
way that involves trapping the heat energy and gaining a measurement
of the chemical potential energy released as thermal energy
('heat').
Example 1. Very simple
method using a boiling tube
You can ignite a peanut or small lump bread of known mass and
hold it in a mounted needle under a boiling tube of cold water,
knowing the mass and initial temperature of the water.
After burning so much of it, the flame is extinguished, and,
when cool, reweigh the peanut, not forgetting to measure the
final temperature.
Suppose, after burning a 2.50 g lump of bread, 1.75 g remains
and the temperature of the water rose from 22.0o to
39.5oC.
Assume there is 30.0 g (~30 cm3) of water in the
boiling tube and the heat capacity of water is 4.20 J per g per
degree rise in temperature.
Calculate the energy in kJ released per gram of bread.
Mass of bread burned = 2.50 - 1.75 = 0.75 g.
Temperature rise = 39.5 - 22.0 = 19.5oC
Heat energy released = mass of water x heat capacity x
temperature rise
Chemical potential energy released from the bread = 30 x
4.2 x 19.5 = 2457 J
Energy released from the bread = 2457 / 0.75 = 3276 J/g,
3.28 kJ/g
(3 sf)
It
is a crude method with heat loss by convection and glass is a
poor thermal conductor.
Example 2. Using a
more accurate calorimeter (see
energy changes in chemistry)
You can use a more sophisticated
calorimeter system (illustrated on the right).
You can use this system employing a copper can (good heat
conductor) to hold the water and employ a draught shield to minimise
heat losses by convection..
You fill the little wick burner with vegetable oil and weigh it.
Pour in 100 g (~100 cm3) of water into the copper
calorimeter and measure its temperature.
Light the burner and place it carefully under the suspended
calorimeter (clamp not shown).
Burn for 5 minutes, extinguish the flame and take the final
increased temperature of the water after stirring it to get the
average temperature of the bulk liquid.
Suppose the burner containing the vegetable oil weighed 20.55 g.
After burning it weighed 20.05 g and the temperature rose from
24.5oC to 40.0 oC.
Mass of oil burned = 20.55 - 20.05 = 0.50 g
Temperature rise = 40.0 - 24.5 = 15.5oC.
Calculate the heat released in kg/g of vegetable oil.
Heat energy released = mass of water x heat capacity x
temperature rise
Chemical potential energy released from the oil = 100 x
4.2 x 15.5 = 6510 J
Energy released from the oil = 6510 / 0.50 = 13020 J/g,
13.0 kJ/g (3
sf)
Although it is a better method than the boiling tube, the flame
may be smoky and therefore incomplete combustion, so the calorific
value of J/kg will be less than the theoretical 100% conversion of
chemical potential energy.
See also page on
respiration.
