We end this section with a warning, however. Many biochemistry texts sim-
plify the reaction in equation 2.61 as
ATP ADP + phosphate rxnH°24.3 kJ (2.62)
(It is not uncommon in organic or biological chemistry for complex chemical
processes to be written using only the important chemical species.) For the
uninitiated, the reaction written in equation 2.62 suggests that an ATP mole-
cule is breaking apart into ADP and phosphate molecules, and 24.3 kJ of en-
ergy is given off. However, in basic chemistry we should learn that it always re-
quiresenergy to break a chemical bond; this reaction should be endothermic,
not exothermic. How can a chemical bond be broken and energy be given off?
The reason for the confusion is the absence of the H 2 O molecule. More
bonds are being broken and formed than equation 2.62 implies, and with the
inclusion of water (as in equation 2.61), the overall enthalpy change of the
ATP →ADP conversion is negative. Confusion arises when complex reactions
are simplified and an unsuspecting reader does not recognize the implications
of the simplification.
The lesson? Even complex biochemical processes are governed by the con-
cepts of thermodynamics.2.13 Summary
The first law of thermodynamics concerns energy.The total energy of an iso-
lated system is constant. If the total energy of a closed system changes, it can
manifest itself as either work or heat, nothing else. Because the internal energy
Uis not always the best way to keep track of the energy of a system, we define
the enthalpy,H, which can be a more convenient state function. Because many
chemical processes occur under constant-pressure conditions, enthalpy is of-
ten more convenient than internal energy.
There are many mathematical ways of keeping track of the energy changes
of a system. The examples we have presented in this chapter are all based on
the first law of thermodynamics. Many of them demand a certain condition,
like constant pressure, constant volume, or constant temperature. Although
this might seem inconvenient, by defining the changes in a system in these
ways, we can calculate the change in energy of our system. This is an impor-
tant goal of thermodynamics. As we will see in the next chapter, it is not the
only important goal.
The other task in thermodynamics is embodied in the question “What
processes tend to occur by themselves, without any effort (that is, work) on our
part?” In other words, what processes are spontaneous? Nothing about the first
law of thermodynamics helps us answer that question unequivocally. That’s be-
cause it can’t.A lot of exploration and experimentation showed that energy is
not the only concern of thermodynamics. Other concerns are also important,
and it turns out that those concerns play major roles in how we view our
universe.JQPJ
62 CHAPTER 2 The First Law of Thermodynamics