The Foundations of Chemistry

a state function. The difference between the internal energy of the products and the
internal energy of the reactants of a chemical reaction or physical change,
E,is given by
the equation

EEfinalEinitialEproductsEreactantsqw

The terms qand wrepresent heat and work, respectively. These are two ways in which
energy can flow into or out of a system. Workinvolves a change of energy in which a
body is moved through a distance, d, against some force, f; that is, wfd.

E(amount of heat absorbed by system)(amount of work done on system)

The following conventions apply to the signs of qand w.

qis positive: Heat is absorbedby the system from the surroundings.

qis negative: Heat is releasedby the system to the surroundings.

wis positive: Work is done onthe system by the surroundings.

wis negative: Work is done bythe system on the surroundings.

Whenever a given amount of energy is added to or removed from a system, either as
heat or as work, the energy of the system changes by that same amount. Thus the equa-
tion
Eqwis another way of expressing the First Law of Thermodynamics (see
Section 15-1).
The only type of work involved in most chemical and physical changes is pressure–
volume work. From dimensional analysis we can see that the product of pressure and
volume is work. Pressure is the force exerted per unit area, where area is distance squared,
d^2 ; volume is distance cubed, d^3. Thus, the product of pressure and volume is force times
distance, which is work. An example of a physical change (a phase change) in which the
system expands and thus does work as it absorbs heat is shown in Figure 15-6. Even if
the weight of the book had not been present, the expanding system pushing against the
atmosphere would have done work for the expansion.
When energy is released by a reacting system,
Eis negative; energy can be written
as a product in the equation for the reaction. When the system absorbs energy from the
surroundings,
Eis positive; energy can be written as a reactant in the equation.
For example, the complete combustion of CH 4 at constant volume at 25°C releases
energy.

Figure 15-6 A system that absorbs

heat and does work. (a) Some

powdered dry ice (solid CO 2 ) is

placed into a flexible bag, which is

then sealed. (b) As the dry ice

absorbs heat from the surroundings,

some solid CO 2 sublimes to form

gaseous CO 2. The larger volume of

the gas causes the bag to expand.

The expanding gas does the work of

raising a book that has been placed

on the bag. Work would be done by

the same amount of expansion, even

if the book were not present, as the

bag pushes against the surrounding

atmosphere. The heat absorbed by

such a process at constant pressure,

qp, is equal to 
Hfor the process.

15-10 Changes in Internal Energy, E 613

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 6.13, The First Law of

Thermodynamics.

Sign conventions for qand w.

SYSTEM

q > 0

q < 0

w > 0

w < 0

SURROUNDINGS

Heat

released

by system

Work done

by system

Heat

absorbed

by system

Work done

on system

(a) (b)



d

F

2 d

(^3) Fdw
h
h
6
(^66)
6
PV