Foods such as animal fats, vegetable
oils and carbohydrates like glucose and starch are concentrated chemical
energy stores.
They are metabolised in the body
to power all of a cell's chemistry and provide thermal energy to
warm blooded creatures like ourselves!
See also GCSE biology notes on
food tests.
You do a controlled burning of a
lump of food to get an idea of how much chemical energy it contains
by converting .
A
bit of burning food calorimetry (experimental set-up illustrated on the
right).
A very simple
investigation method to measure the calorific value of food
Wear eye protection and take care
near flames.
Add exactly 20 cm3 of
water to a pyrex boiling tube (better than narrow test tube).
The boiling tube is clamped above the
lab bench in an inclined position.
A 0-100oC thermometer is
carefully placed in the water.
You can use any dried food e.g.
beans, bread, nuts or pasta and a weighed lump of it is skewered
onto the end of a mounted needle.
The water is gently stirred with the
thermometer and the initial temperature recorded.
The food lump is ignited with a
bunsen burner flame and then held with a steady had under the bottom of
the boiling tube.
If the flame goes out, relight the
food, repeating this until it has all burned away (or the residue no
longer burns).
When all the food has burned away,
re-stir the water gently with the thermometer and note the final higher
temperature.
Calculation of energy content
The heat released = mass of water (g)
x temperature change (oC) x specific heat capacity of water.
e.g. 0.75 g nut was burned as much as
possible.
The initial and final
temperatures were 21oC and 43oC. Temperature
rise = 43 - 21 = 22oC.
20 cm3 of water is 20 g (density = 1.0 g/cm3).
Specific heat capacity of water
is 4.18 J/kgoC. (see GCSE physics notes for more on
specific heat capacity)
Therefore heat released = 20 x 22
x 4.18 = 1839 J
The result can then be expressed
in terms of energy content per unit mass of food e.g. J/g.
2500 J/g (2 sf) or 2.5 kJ/g or 2500 kJ/kg
You can think of this
calorific value
as a measure of energy density.
Experimental
errors
This experiment is NOT very accurate
at all, but it gives a rough estimate of the energy content of a food.
(i)
Heat is being continuously lost from the boiling tube which isn't
insulated.
(ii) Even more heat is lost by the
warm convecting flame gases rising beyond the boiling tube,
because you cannot collect all the thermal energy from the hot
flame gases.
(iii) You cannot burn every bit of
food, there is always a burnt out carbonised residue.
(iv) The yellowish flame tends to be
sooty, so all the carbon is not oxidised to carbon dioxide.
For commercial calorific values
scientists use a sealed and well insulated calorimeter, with no heat
lost, you get very accurate values of energy content. The instrument is
called a bomb calorimeter and pure oxygen is used - kapow!.
Further experiments
Repeat with different foods, but keep
the volume of cool start water the same.
Repeat the calculation and compare
the calorific values and energy densities of different foods.
You can also look up the molecular
structure of some of the molecules in the food e.g.
the above diagram shows a section of
a vegetable oil molecule (a long chain fatty acid), and as you can see,
there are lots of carbon and hydrogen atoms to be oxidised to carbon
dioxide and water respectively.
Fat molecules have a higher energy
density than carbohydrates because the latter contains a greater
proportion of oxygen atoms - so a greater proportion of the carbon atoms
are partially oxidised, so less energy can be released on burning or by
metabolic chemistry in the body.
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.
There are still heat losses by convection up the
side of the copper calorimeter.
