Page 29 - Color Atlas of Biochemistry

20 Basics
Enthalpy and entropy
The change in the free enthalpy of a chemical
reaction (i. e., its ∆G) depends on a number of of energy. Both of these statements are
factors—e. g., the concentrations of the reac-
tants and the temperature (see p.18). Two
further factors associated with molecular
changes occurring during the reaction are dis-
cussed here.
A. Heat of reaction and calorimetry
All chemical reactions involve heat exchange.
Reactions that release heat are called
exothermic, and those that consume heat
are called endothermic.Heatexchangeis
measured as the enthalpy change ∆H(the
heat of reaction). This corresponds to the
heat exchange at constant pressure. In exo-
thermic reactions, the system loses heat, and
∆H is negative. When the reaction is endo-
thermic, the system gains heat, and ∆Hbe-
comes positive.
In many reactions, ∆Hand ∆Gare similarin
magnitude (see B1, for example). This fact is
used to estimate the caloric content of foods.
In living organisms, nutrients are usually oxi- thermic (∆H > 0)—i. e., the liquid cools. Never-
dized by oxygen to CO 2 and H 2 O(see p.112).
The maximum amount of chemical work sup-
plied by a particular foodstuff (i. e., the ∆Gfor system decreases.The Na and Cl ions are
the oxidation of the utilizable constituents)
can be estimated by burning a weighed
amount in a calorimeter in an oxygen atmo-
sphere. The heat of the reaction increases the
water temperature in the calorimeter. The
reaction heat canthenbe calculated from
the temperature difference ∆T.
B. Enthalpy and entropy
The reaction enthalpy ∆H and the change in
free enthalpy ∆Gare not always of the same
magnitude. There are even reactions that oc-
cur spontaneously (∆G < 0) even though they
are endothermic (∆H > 0). The reason for this
is that changes in the degree of order of the
system also strongly affect the progress of a
reaction. This change is measured as the en-
tropy change ('S).
Entropy is a physical value that describes
the degree of order of a system.The lower the
degree of order, the larger the entropy. Thus,
when a process leads to increase in disor-
der—and everyday experience shows that
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
this is the normal state of affairs—∆Sis pos-
itive for this process. An increase in the order
in a system (∆S < 0) always requires an input
consequences of an important natural law,
the Second Law of Thermodynamics. The
connection between changes in enthalpy
and entropy is described quantitatively by
the Gibbs–Helmholtz equation (∆G= ∆H–
T ∆S). The following examples will help
explain these relationships.
In the knall-gas (oxyhydrogen) reaction
(1), gaseous oxygen and gaseous hydrogen
react to form liquid water. Like many redox
reactions, this reaction is strongly exothermic
(i. e., ∆H < 0). However, during the reaction,
the degree of order increases. The total num-
ber of molecules is reduced by one-third, and
a more highly ordered liquid is formed from
freely moving gas molecules. As a result of the
increase in the degree of order (∆S< 0), the
term –T ∆S becomes positive. However, this
is more than compensated for by the decrease
in enthalpy, and the reaction is still strongly
exergonic (∆G<0).
The dissolutionofsaltinwater (2)is endo-
theless, the process still occurs spontane-
ously, since the degree of order in the
–
+
initially rigidly fixed in a crystal lattice. In
solution, they move about independently
and in random directions through the fluid.
The decrease in order (∆S> 0) leads to a
negative –T ∆Sterm, which compensates
for the positive ∆Hterm and results ina
negative ∆G term overall. Processes of this
type are described as being entropy-driven.
The folding of proteins (see p. 74) and the
formation of ordered lipid structures in water
(see p. 28) are also mainly entropy-driven.