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Ladies and Gentlemen, I would like to welcome you to our course "Physical Chemistry". My name is
Dr. Lauth. Our first lesson is, "What the heck is Physical Chemistry?" or in other words
"How to express chemistry in numbers?" This is exactly what Physical Chemistry is
like, To express Chemistry - Molecules that react from an initial state to a final state
- in numbers. In physics, the concept of mass point is widely
used. the object whose motion is to be studied is "compressed" to one point and
The point´s motion is described with formulas. In chemistry, this is not a viable option:
Here we have a lot of mass points, namely our molecules, our atoms that carry out chemical
reactions. It is not useful to describe each mass point
individually. It´s a much better way to summarize all the
mass points we are interested in to an entity called SYSTEM. A SYSTEM is a part of the universe,
in which there are very many particles. The task of physical chemistry it is now to
describe this system with numbers. Outside of the system is the surroundings.
Between system and surroundings are real or imaginary boundaries.
Depending on whether these boundaries are permeable to energy or mass we speak of open
systems, closed systems and isolated systems. A very abstract formulation, but it is universally
applicable. The volume of air in this container is a closed
system, for example: Although it can exchange energy with the surroundings (heat and work)
there will be no mass transfer. The contents of a thermos is an isolated system. Exchange
of energy and matter are excluded. Every open vessel is an open system, the upper
system boundary being an imaginary border. To describe a system thermodynamically complete,
we need more figures. Let us first determine, if the system is homogeneous or whether several
PHASES exist. So the first number to thermodynamically describe
a system is the number of PHASES. In this system, we have a dense phase, a liquid phase
and a less dense phase, a gas phase. So this is a two-phase system.
This system is a single phase system. The second system again is a two-phase system
Don´t mix the number of phases with the number of COMPONENTs. The number of components indicates,
how many types of particles are present in our system. Although we have two phases in
this example, there´s only one component, namely, the white particles shown here. In
the second picture we have two phases and two components.
In this vessel we have only water, i.e. H2O molecules. This is also a one-component system.
Air is a mixture of nitrogen and oxygen, that is, we have at least a two component system
Components and PHASEs are two important variables to describe a system. But they are still not
sufficient to describe the system clearly. In fact, we need more variables -- more numbers
- to describe the system so that any expert
in the world can build a perfect copy of it. These variables - usually provided with units
-- are properties of the system such as mass or volume.
Many of these variables, we know from everyday life; they are characteristics that we associate
with the system. The mass as a characteristic, which has to do with the weight. The volume,
a characteristic which quantifies the extent of the system.
These state variables - as they are called -- make sense because you can quantitatively
characterize the system with it, and they are MEASURABLE. These are the only prerequisites
we impose on a state variable. Mass and volume are extensive state variables:
a doubling of the system leads to a doubling of these variables. Density - another property,
however, is an intensive state variable Doubling of the system this size remains unchanged.
The amount of substance - very commonly used in chemistry - is an extensive condition again.
The mole fraction (for multi-component systems) is an intensive state variable, a property
indicating concentration. Further, very important intensive state variables are temperature
and pressure. Thermodynamics - a main branch of physical
chemistry -- even invents new state variables - because these state variables are useful.
Because it is possible to describe certain natural laws easily with these new variables
One of these state variables is the enthalpy (abbreviated to H) The enthalpy is as a property
of a system such as the mass or volume of the system. We can say this certain volume
of air has got enthalpy. The enthalpy can be measured and it is useful
because it is a measure of the amount of energy in this system. It is an extensive state variable:
if I double the system, I'll double the enthalpy. I can make any extensive state variable an
intensive state variable, as I divide it by the mass or amount of substance. Then I get
the corresponding specific or molar quantity. H divided by m e.g. is the specific enthalpy
h; joules per kilogram is the unit. A state variable that is also very important in thermodynamics
is Gibbs free energy G. G is a measure of a system´s instability.
In fact Stability is an important issue in thermodynamics.
Often questions like the following arise: "How stable is a system? Is there a chemical
or physical process to make it more stable?" Is a change of the system -- a so called process
possible or are we already at equilibrium, the point of maximum stability?"
Gibbs free enthalpy provides the answer to all of these questions. It is also an extensive
quantity. The corresponding intensive quantity "partial
molar enthalpy" even has an own name: it is called "chemical potential μ". But more on
that later. You don´t need all of these state variables
to describe the system completely. In simple systems typically very few of them
suffice. Gibbs phase rule tells us, how many variables
we need. Gibbs was one of the greatest thermodynamicists
formulated at the end of the 19th Century the rule named after him:
Having a system with C components and P phases, I need exactly F-intensive state variables
to describe this system completely. F is called degrees of freedom.
Consider gaseous helium in this container. We'll have a one-component system with one
phase. According to GIBBS this system has two degrees of freedom.
I am free to choose two properties, for example temperature and molar volume. All other state
variables are thus defined. Consider a container with liquid water and
water vapor at equilibrium. So we'll have a one-component system with two phases. According
to Gibbs, this system has only one degree of freedom.
I can only pick one property freely -- for example temperature. All other state variables
are thus defined. If I choose a numerical value for temperature,
the density of the liquid phase, the density of the gas phase, the pressure of the gas,
the specific enthalpy and so on are completely specified.
One degree of freedom. Which property I choose is up to me
If I choose temperature, pressure and all other parameters are fixed.
I may alternatively choose pressure, but then the temperature is set
I even can choose specific enthalpy of me. ONE and only one variable of the large number
of state variables. (Summary :) Physical chemistry has the task
of describing many-body systems. For a full description we need the number of components,
number of phases, and a defined number of state variables which we can calculate as
F from Gibbs phase rule.