CHAPTER V
HEAT OF COMBUSTION (CALORIMETRY)
All chemical reactions involve either evolution or absorption
of heat energy. The measurement of the "heat of reaction"
has proved to be a valuable method for investigation, in both
pure and applied chemistry. We are here particularly concerned
with the amount of heat evolved by the combustion of fuels and
of foods. In the case of fuels this, of course, has a direct bearing
upon the value of the fuel when it is burned for the production
of useful heat, while the heat of combustion of foods and feeding
stuffs is of interest in connection with their relative values as
energy producers in the animal body. However, the student
is cautioned against the fallacy that the "calorific value7' (heat
of combustion) of a food is the only criterion as to its value.
Even an elementary study of physiology should convince one
that such matters as balancing of diet, proper proportioning of
rougher and more refined foods, stimulation of appetite, per cent
of contained nitrogen, etc., are of prime importance in this
connection.
Units of Measurement.—In scientific work the accepted unit
of heat is the calorie. As ordinarily used this is the heat that is
absorbed by 1 gm of pure water as its temperature rises 1° C.
(strictly, from 15° to 16°). The specific heat of water is not the
same for all temperatures but the variation is only 0.013 over
a range of 0° to 100°.
In engineering work, for expressing the value of fuels, the
British thermal unit (B.t.u.) is more often used. This is the
heat absorbed by 1 Ib of water as its temperature rises 1° F.
The "calorific value," "fuel value" or "heat units," as it is
variously expressed, is the number of calories per gram or of
B.t.u. per pound, made available by burning the material. The
following equations represent relative values:
cal per gm X 1.8 = B.t.u. per Ib;1 (1)
B.t.u. per Ib , /rtN
--------f-------- = cal. per gm. (2)
JL.o
1 For the derivation of these formulas see MAHIN, "Quantitative Analy-
sis," 2nd ed., p. 314.
103