Chemistry - A Molecular Science

energy can neither be created nor destroyed,

then the energy of the thermodynamic

universe must remain constant. Conseque

ntly, we can use Equation 9.1 as another

statement of the first law.

ΔE

univ

= 0 for all processes

Eq. 9.1

To better understand how energy is transferre

d between a system and its surroundings, we

substitute

E + Δ

EΔ

for sur

EΔ

univ

in Equation 9.1 to obtain following:

ΔE +

ΔE

sur

= 0

The above can be solved for the energy change of the system.

ΔE = -

ΔE

sur

Eq. 9.2

Equation 9.2 indicates that

the only way that the energy of a system can be changed is by

exchanging energy with its surroundings

.

Heat and work are the two most common ways of exchanging energy between a

system and its surroundings.

Heat

(q

) is that form of energy that is transferred as a result

of a temperature difference, and

work

(w

) is that form of energy that is transferred when

one object moves another.

q is defined as the heat absorbed by

the system

. When a

system absorbs heat it absorbs

energy, so its energy increases;

i.e.

, q

0 means that heat

flows into the system increasing the energy, but if

q < 0 heat flows out of the system to

decrease its energy.

w is defined as the work

done on

the system

. When work is done on

a system, the energy of the system increases;

i.e.

, w

0 means that work is done on the

system, which increases its energy, but

w < 0 means that work is done by the system,

which decreases its energy. Equation 9.3, which

is another statement of the first law of

thermodynamics, shows that the energy of a

system increases when it absorbs heat (

q)

from or has work (

w) done on it by its surroundings.

ΔE =

q +

†w

Eq.

9.3

The signs of

q and

w simply indicate the direction

of energy flow. However, the

direction of energy flow is frequently given verbally without using the sign explicitly. The common expressions used to state the direction are given in Table 9.1. For example,

w =

-10 J is usually read as 10 J of work was done

by

the system, and

q = -10 J would be read

10 J of heat was

given off by

the system. Thus, if a question asks for the amount of heat

that is given off, and

q = -10 J, the answer is +10 J. Processes for which

q > 0 are said to

be

endothermic

(heat into the system), and processes for which

q < 0 are said to be

exothermic

(heat exits the system).

†^

E depends only upon the initial and final states, so it is called a Δ

state

function

. However, q and w depend upon how the system goes from

one state to the other. Consider t

hat the energy of a system can be

increased by 10 J (

E = 10 J) in an infinite number of ways. For Δ

example, q = 10 J and w = 0, q = 5 J and w = 5 J, q = 0 and w = 10 J, or q = 15 J and w = -5 J, to name just a few. q and w depend on how the energy change is accomplished, not ju

st the final and initial states, so

they are not state functions. This is why no ‘

’ is used in front of q or w. Δ

Table 9.1

Expressions used for the direction of energy flow

> 0 q

heat is

absorbed

by the system

q < 0

heat is

given off

or

evolved

by the system

> 0 w

work is done

on

the system

w < 0

work is done

by

the system

Chapter 9 Reaction Energetics

185

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North

Carolina

State

University