Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

9.1

THE FIRST LAW OF THERMODYNAMICS

Thermodynamics is similar to bookkeeping. In

thermodynamics the flow of energy is

monitored, while in bookkeeping, the flow of m

oney is monitored. The direction of flow is

given by the sign of the change. By convention, the change in a variable is denoted by placing a delta (

) in front of the variable. In addition, Δ

change is always defined as the

variable's final value minus its initial value

: Δ

X = X

final

• X

initial

. If

X is positive, XΔ

final

Xinitial

, and X increases. If

X is negative, XΔ

final

< X

initial

and X decreases.

Consider an example of cash flow in wh

ich you write a check for $50 to a friend who

uses the same bank. The first step in "se

tting up the books" is to define the reference

account, which has a balance of B. The sign of the balance change,

B, gives the direction Δ

of money flow in that account. For example, if your account is the reference account then B = -$50. The minus sign signifies that money flowed out of your account. If your Δ friend's account is the reference,

B = +$50 as money flowed into that account. Finally, if Δ

the bank is the reference,

B = 0 as no money entered or left the bank; the money you Δ

exchanged with your friend stayed in the bank.

A thermodynamic problem would be set up the same way. Suppose that 50 J of energy

is transferred from

A to

B. The first step is to define the thermodynamic

system

, which is

that portion of the universe under i

nvestigation (the reference). If

A is the system, then the

energy change is

E = -50 J because the energy flows out of Δ

A. That portion of the

universe that exchanges energy

with the system is called the

surroundings

. System

B^

would be the surroundings in this example. No

subscript is used for system quantities, but

the subscript '

sur’

is used to denote surroundings quantities, so

EΔ

sur

= +50 J because 50 J

of energy flows into the surroundings

B. The system and its surroundings combine to form

the thermodynamic

universe

, which is denoted with a subscript '

univ

.' Thus, we would

write the

EΔ

univ

=

E + Δ

EΔ

sur

= -50 J + 50 J = 0. If we had chosen

B as the system, then

A^

would have been the surroundings, then

E = +50 J and Δ

EΔ

sur

= -50 J. Note that

EΔ

univ

= 0

either way. If your account had been defined as the system in the accounting example above, your friend's account would be the su

rroundings and the bank would have been the

universe. The money flow would then be

BΔ

univ

=

B + Δ

BΔ

= -$50 + $50 = 0. sur

The flow of energy is predicted by the

first law of thermodynamics

, which states that

Energy is neither created nor destroyed in any process.*

By definition, all energy change in a th

ermodynamic problem must remain within the

thermodynamic universe. If no energy can enter of leave the thermodynamic universe, and

* Actually, the term

mass-energy

should be used instead of energy

because mass and energy can be converted into one another by E = mc

2. In nuclear reactions, where large amounts of energy are

involved, there are substantial

changes in mass. However, the

mass changes in chemical processes are undetectable because the ener

gy

chan

ges are so small.

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State

University