2. THERMODYNAMICS OF LIVING SYSTEMS
27
By analogy with Q, AH is considered negative when heat is lost to the
surroundings and positive when heat is gained by the system. The mistaken idea that the sign of AH for a chemical reaction determines
whether or not the reaction proceeds spontaneously was once generally
accepted. As we will show below, the sign of another thermodynamic
parameter, AF (the change in free energy), is the determining factor
in ordinary chemical and biological processes.
Thermochemistry, or the conversion of chemical energy into heat
energy, is the direct application of the First Law to chemical reactions.
Animal calorimetry is also a direct application of the First Law. The
proof of the applicability of the First Law to animal metabolism consists in comparing the heat of oxidation of nutrients within and outside
the body. Outside the body, ΔΗ is determined by an ordinary calorimeter. For example, if we burn glucose in a calorimeter we have
C 6 H 12 0 6 + 60 2 -> 6C0 2 + 6H 2 0
AH = -673 kcal.
Within the body, the nature of the food oxidized is determined by
measuring the respiratory quotient and urinary nitrogen. Quantitatively
the amount of each substance (fat, carbohydrate, protein) is computed
from carbon dioxide production, oxygen consumption, and urinary
nitrogen excretion. Calorimetry is used to determine the heat dissipated
by the animal and is compared with the heat as computed from the
foods oxidized. Since the observed and computed heats usually agree
within experimental error, this is taken as proof that the First Law does
apply to animal processes.
It is not necessary to determine ΔΗ experimentally for many reactions since the data are available in the literature. It is also possible to
calculate AH for many other reactions from the published values for
the heats of formation of compounds from their elements, since for any
chemical reaction, AH equals the sum of heats of formation of products
minus the sum of heats of formation of reactants. Thus AH, as stressed
above, is independent of the path and depends only on the reactants
and their states and the products and their states. For example, let us
calculate AH for the oxidation of glucose at 25° according to the following equation
CeH 12 O e (solid) + 60 2 (gas) -* 6C0 2 (gas) + 6H 2 0 (liquid)
The heats of formation per mole are —301 kcal, for glucose, zero for
oxygen (heats of formation of all elements are arbitrarily taken as
zero), —94 kcal, for carbon dioxide and —68.3 kcal, for water. Thus
AH for the over-all reaction is
AH = [(6 X -94) + (6 X -68.3)] - [-301 + 0] - [-974] - [-301] = -673 kcal.
27
By analogy with Q, AH is considered negative when heat is lost to the
surroundings and positive when heat is gained by the system. The mistaken idea that the sign of AH for a chemical reaction determines
whether or not the reaction proceeds spontaneously was once generally
accepted. As we will show below, the sign of another thermodynamic
parameter, AF (the change in free energy), is the determining factor
in ordinary chemical and biological processes.
Thermochemistry, or the conversion of chemical energy into heat
energy, is the direct application of the First Law to chemical reactions.
Animal calorimetry is also a direct application of the First Law. The
proof of the applicability of the First Law to animal metabolism consists in comparing the heat of oxidation of nutrients within and outside
the body. Outside the body, ΔΗ is determined by an ordinary calorimeter. For example, if we burn glucose in a calorimeter we have
C 6 H 12 0 6 + 60 2 -> 6C0 2 + 6H 2 0
AH = -673 kcal.
Within the body, the nature of the food oxidized is determined by
measuring the respiratory quotient and urinary nitrogen. Quantitatively
the amount of each substance (fat, carbohydrate, protein) is computed
from carbon dioxide production, oxygen consumption, and urinary
nitrogen excretion. Calorimetry is used to determine the heat dissipated
by the animal and is compared with the heat as computed from the
foods oxidized. Since the observed and computed heats usually agree
within experimental error, this is taken as proof that the First Law does
apply to animal processes.
It is not necessary to determine ΔΗ experimentally for many reactions since the data are available in the literature. It is also possible to
calculate AH for many other reactions from the published values for
the heats of formation of compounds from their elements, since for any
chemical reaction, AH equals the sum of heats of formation of products
minus the sum of heats of formation of reactants. Thus AH, as stressed
above, is independent of the path and depends only on the reactants
and their states and the products and their states. For example, let us
calculate AH for the oxidation of glucose at 25° according to the following equation
CeH 12 O e (solid) + 60 2 (gas) -* 6C0 2 (gas) + 6H 2 0 (liquid)
The heats of formation per mole are —301 kcal, for glucose, zero for
oxygen (heats of formation of all elements are arbitrarily taken as
zero), —94 kcal, for carbon dioxide and —68.3 kcal, for water. Thus
AH for the over-all reaction is
AH = [(6 X -94) + (6 X -68.3)] - [-301 + 0] - [-974] - [-301] = -673 kcal.