Some important ideas about energy are summarized in the First Law of Thermody-
namics.The total amount of energy in the universe is constant.The Law of Conservation of Energyis just another statement of the First Law of Ther-
modynamics.Energy is neither created nor destroyed in ordinary chemical reactions and phys-
ical changes.SOME THERMODYNAMIC TERMS
The substances involved in the chemical and physical changes that we are studying are
called the system.Everything in the system’s environment constitutes its surroundings.
The universeis the system plus its surroundings. The system may be thought of as the
part of the universe under investigation. The First Law of Thermodynamics tells us that
energy is neither created nor destroyed; it is only transferred between the system and its
surroundings.15-2
In Chapter 1 we pointed out the
equivalence of matter and energy.
The word “energy” is understood to
include the energy equivalent of
all matter in the universe. Stated
differently, the total amount of mass
and energy in the universe is constant.
594 CHAPTER 15: Chemical Thermodynamics
See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.13, The First Law of
Thermodynamics.
Figure 15-2 An endothermic process. (a) When solid hydrated barium hydroxide,
Ba(OH) 2 8H 2 O, and excesssolid ammonium nitrate, NH 4 NO 3 , are mixed, a reaction occurs.Ba(OH) 2 8H 2 O(s)2NH 4 NO 3 (s)88nBa(NO 3 ) 2 (s)2NH 3 (g)10H 2 O()The excess ammonium nitrate dissolves in the water produced in the reaction. (b) The
dissolution process is very endothermic. If the flask is placed on a wet wooden block, the
water freezes and attaches the block to the flask.(a) ( b)