System, Surroundings, and State
What is the System and Surroundings ?
** Imagine
you have a container holding some material of interest to you, as in
Figure blow.
** The
container does a good job of separating the material from everything else.
** Imagine,
too, that you want to make measurements of the properties of that material,
independent from the measurements of everything else around it.
** The
material of interest is defined as the system.
** The
“everything else” is defined as the surroundings.
** These
definitions have an important function because they specify what part of the
universe we are interested in: the system. Furthermore, using these
definitions, we can immediately ask other questions: What interactions are
there between the system and the surroundings? What is exchanged between the
system and the surroundings?
** For
now, we consider the system itself. How do we describe it? That depends on the
system. For example, a glass of milk is described differently from the interior
of a star. But for now, let us pick a simple system, chemically speaking.
What is the State ?
** Consider
a system that consists of a pure gas. How can we describe this system?
** Well,
the gas has a certain volume, a certain pressure, a certain temperature, a
certain chemical composition, a certain number of atoms or molecules, a certain
chemical reactivity, and so on.
** If
we can measure, or even dictate, the values of those descriptors, then we know
everything we need to know about the properties of our system. We say that we
know the state of our system.
Equilibrium between System and Surroundings
** If
the state of the system shows no tendency to change, we say that the system is
at equilibrium with the surroundings.
** The
equilibrium condition is a fundamental consideration of thermodynamics.
** Although
not all systems are at equilibrium, we almost always use equilibrium as a
reference point for understanding the thermodynamics of a system.
** Equilibrium
can be a difficult condition to define for a system. For example, a mixture of
H2 and O2 gases may show no noticeable tendency to
change, but it is not at equilibrium. It’s just that the reaction between these
two gases is so slow at normal temperatures and in the absence of a catalyst
that there is no perceptible change.
Energy of System
** There
is one other characteristic of our system that we ought to know: its energy.
** The
energy is related to all of the other measurables of our system (as the
measurables are related to each other, as we will see shortly).
** The
understanding of how the energy of a system relates to its other measurables is
called thermodynamics (literally, “heat movement’’).
** Although
thermodynamics (“thermo’’) ultimately deals with energy, it deals with other measurables
too, and so the understanding of how those measurables relate to each other is
an aspect of thermodynamics.
How do we define the state of the system?
** To
begin, we focus on its physical description, as opposed to the chemical
description.
** We
find that we are able to describe the macroscopic properties of our gaseous
system using only a few observables: they are the system’s pressure,
temperature, volume, and amount of matter (see Table blow).
** These
measurements are easily identifiable and have well-defined units.
(1) Volume (V)
** Volume has common units of liter, milliliter, or cubic
centimeter.
** The cubic meter is the Système International (SI) unit
of volume but these other units are commonly used as a matter of convenience.
(2) Pressure (P)
** Pressure has common units of atmosphere, torr, pascal (1
pascal = 1 N/m2 and is the SI unit for pressure), or bar.
** Volume and pressure also have obvious minimum values
against which a scale can be based. Zero volume and zero pressure are both
easily definable. Amount of material is similar.
** It is easy to specify an amount in a system, and having
nothing in the system corresponds to an amount of zero.
(3)The temperature of a system (T)
** The
temperature of a system has not always been an obvious measurable of a system,
and the concept of a “minimum temperature” is relatively recent.
** In
1603, Galileo was the first to try to quantify changes in temperature with a
water thermometer.
** Gabriel
Daniel Fahrenheit devised the first widely accepted numerical temperature scale
after developing a successful mercury thermometer in 1714, with zero set at the
lowest temperature he could generate in his lab.
** Anders
Celsius developed a different scale in 1742 in which the zero point was set at
the freezing point of water. These are relative, not absolute, temperatures.
**
Warmer and colder objects have a temperature value in these relative scales
that is decided with respect to these and other defined points in the scale. In
both cases, temperatures lower than zero are possible and so the temperature of
a system can sometimes be reported as a negative value.
Note: Volume, pressure, and
amount cannot have a negative value, and later we define a temperature scale
that cannot, either. Temperature is now considered a well-understood variable
of a system.
Reference:
physical Chemistry /David W. Ball / Cleveland State University /2011 .
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