BioPHYSICAL chemistry

different for each type of transition for a given molecule. For water, the
transition from water to vapor – that is, the boiling transition – has a much
larger enthalpy of 40.7 kJ mol−^1 than found for the freezingtransition.
The enthalpy change that is often encountered in biological reactions
is a chemical change. For example, an electron may be removed from a
molecule, causing a change in the ionization state of a molecule. Bonds can
also be broken or formed, resulting in corresponding enthalpy changes that
are termed bond enthalpies. In any given reaction, there may be several
types of change occurring simultaneously. The usefulness of the enthalpy
concept is that the overall enthalpy change for the reaction is additive
and is given simply by the sum of the changes of the products minus the
changes of the reactants. By convention, the enthalpy of elements is equal
to zero when they are in a stable state under standard conditions.
As an example, the thermodynamic properties of foods
can be discussed in terms of the enthalpy of combustion per
gram of food. Humans need about 10^7 J of food every day. If
the energy is provided entirely by glucose (Figure 2.8), a total
of about 600 g per day is required because glucose has a specific
enthalpy of 1.7 × 104 Jg−^1. By comparison, digestible carbo-
hydrates have a slightly higher enthalpy of 1.7 × 104 Jg−^1 and
fats, which are commonly used as an energy-storage material,
have values of 3.8 × 104 Jg−^1.
These enthalpies of combustion can be calculated by summing the con-
tributions from the combustion process (Table 2.1). The oxidation of
glucose results in the production of carbon dioxide and water:

C 6 H 12 O 6 (solid) +6O 2 (gas) →6CO 2 (gas) +6H 2 O (liquid) (2.30)

The enthalpy contribution from the products is calculated by using the
enthalpies of formation for carbon dioxide multiplied by the relative num-
bers from the balanced equation above:

ΔH°products=6(−393.5 kJ mol−^1 ) +6(−285.8 kJ mol−^1 )
=−4076 kJ mol−^1 (2.31)

There are two reactants that could contribute to the enthalpy. The enthalpy
of formation of glucose is −1273.2 kJ mol−^1 (Table 2.1). Since molecular
oxygen by convention is considered to be stable it has zero contribution.
The subtraction of the reactants from the products yields:

(ΔH°)oxidation=−4076 kJ mol−^1 −(−1273.2 kJ mol−^1 )
=−2802 kJ mol−^1 (2.32)

Thus 2.8 × 106 J mol−^1 of heat is released upon oxidation of glucose during
metabolic pathways, or equivalently 1.6 × 104 Jg−^1 as used above. Due to

CHAPTER 2 FIRST LAW OF THERMODYNAMICS 39

H

HO

CH 2 OH

HO

H H

OH

HO

H

H

O

Figure 2.8The

structure of glucose.