BioPHYSICAL chemistry

• Closed systems: a system is described as closed when energy can be
  exchanged between the system and surroundings but not matter. An
  example of this is a cup of hot water that is sealed but not insulated.
  The water vapor cannot escape but heat can still leave the cup.


• Isolated systems: a system can be considered to be isolated when nei-
  ther matter nor energy can leave the system for the surroundings. An
  example of this is a cup of hot water that is not only sealed to prevent
  water vapor from escaping but is also insulated, so preventing heat from
  leaving the cup.

In considering energy flow, two terms must be
defined that are commonly used in thermody-
namics, adiabatic and diathermic (Figure 2.2).
An isolatedsystem where energy cannot flow
from the system to the surroundings is called
an adiabaticsystem, as occurs when the walls
of the cup containing hot water are insulated.
When a system is either open or closed, energy
can exchange with the surroundings and such
a system is called a diathermicsystem. Such a
case corresponds to a container of hot water
with the walls of the container being thin and
allowing heat to pass through the wall. A dia-
thermic process that results in the release of heat
into the surroundings is called exothermicwhereas a process that absorbs
heat in called endothermic. Of these two types of process, exothermic reac-
tions are much more common with combustible reactions; for example,
an organiccompound being oxidized by molecular oxygen.

State functions

The power of thermodynamics is that relationships can be established
among the different properties of the system. An important aspect is that
the process by which a system was established does not determine the
properties. For example, the heat output of a system due to a temperature
change is only dependent upon the initial and final temperature values
and not the rate at which the temperature changed. These types of prop-
erties are called state functions; that is, they are dependent only upon
the state of the system and not the path that was used to prepare the
system. With state functions, conclusions can be made about reactions
that are very general and dependent only upon a few specific parameters.
Changes in a state function between any two states of a system are not
dependent upon the path; changes are dependent only upon the initial
and final conditions. In addition to the state functions already introduced,

CHAPTER 2 FIRST LAW OF THERMODYNAMICS 25

HEAT

Adiabatic

Cold Hot Cold

HEAT

Diathermic

Hot Cold

Figure 2.2The

passage of heat from

a system is allowed

only when the walls

are diathermic.

No heat transfer is

allowed for adiabatic

walls even when

there is a difference

in temperature.