Physical Chemistry , 1st ed.

H 2 (393.5 + 0) (110.5 + 241.8) kJ

H 2 41.2 kJ

Finally, the products of the reaction need to be brought to 500 K; the heat in-

volved in that step,H 3 ,is

H 3 q

(1 mol)37.11 

mo

J

lK

( 202 K) + (1 mol)29.89 

mo

J

lK

( 202 K)

where now Tis positive202 K:

H 3 +13,534 J 13.534 kJ

The overall rxnHis the sum of the three parts:

rxnHH 1 + H 2 + H 3

12.665 + (41.2) + 13.534 kJ

rxnH40.3 kJ

Figure 2.12 shows a diagram of the processes used to estimate H 500.

The answer in the above example is not much different from the rxnH°,

but it is different. It is also an approximation, since we are assuming that the

heat capacities do not vary with temperature. If one compares this to the ex-

perimental value ofH 500 of39.84 kJ, one sees that we are not far off. It is,

then, a good approximation. To be more accurate, an expression for Cpis nec-

essary instead of a constant, and an integral between 500 K and 298 K must be

evaluated for the H 1 and H 3 steps, as illustrated in Example 2.10.

Conceptually, however, this is no different than the above example.

2.12 Biochemical Reactions

Biology, the study of living things, is based on chemistry. Although biological

systems are very complex systems, their chemical reactions are still governed

by the basic concepts of thermodynamics. In this section, we review the thermo-

dynamics of some important biochemical processes.

In Example 2.17, we considered the oxidation of glucose:

C 6 H 12 O 6 (s) + 6O 2 (g) →6CO 2 (g) + 6H 2 O ()

The change in enthalpy of this reaction is 2799 kJ per mole of glucose oxi-

dized. The first point to make is that it doesn’t matter if the glucose is burned

in air or metabolized in our cells: for every 180.15 g (1 mol) of glucose that

reacts with oxygen, 2799 kJ of energy are given off. The second point is to rec-

ognize that this is a lot of energy! It’s enough to raise the temperature of

80.0 kg of water (the approximate mass of a human body) by over 8°C! The

molar volume of glucose is about 115 mL, illustrating that our cells are using

a very compact form of energy.

Photosynthesis is the process by which plants make glucose from carbon

dioxide and water. The overall reaction is

6CO 2 (g) + 6H 2 O () →C 6 H 12 O 6 (s) + 6O 2 (g)

60 CHAPTER 2 The First Law of Thermodynamics

Cool to

298 K

298 K

Warm to

q = H (^1) 500 K q = H 3
rxnH = H 2
H 500 = H 1 + H 2 + H 3
CO  H 2 O CO 2  H 2
500 K
CO  H 2 O H 500 CO 2  H 2
Figure 2.12 A graphical representation of how
one determines the rxnHat nonstandard tem-
peratures. The total change in enthalpy is the sum
of the enthalpy changes for the three steps.