# Introduction to Thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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1

Introduction to Thermodynamics

What is Thermodynamics?

-

Thermodynamics is the study of the properties of matter, especially energy.

-

Thermodynamics examines the macroscopic properties only

-

To examine thermodynamic properties, knowing the details of

composition is
completely unnecessary.

-

Thermodynamic properties found only by measuring bulk sample, not individual
molecules.

-

We shall see that from six relatively easy quantities to measure, we can find a
wealth of information.

-

Review of Classical Mechanics.

Kinetic Energy

-

energy of motion

Newton’s Second Law

F = ma

Work

Potential

Energy

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Thermodynamic Variables

-

Thermodynamic variables
are used to characterize the energy of a state.

Types of Thermodynamic Variables

Extensive

-

Quantities that depend on the extent (amount) of matter.

-

Exam
ples are volume, mass, internal energy, etc…

Intensive

-

Quantities that do not depend on the extent of matter.

-

Examples are temperature, density, pressure, etc…

Other

-

Some quantities are neither extensive nor intensive

-

Examples include surface

area, absorbance, etc…

-

For a particular system, the relationship between thermodynamic variables in called
an
equation of state
.

Thermodynamics versus Kinetics

-

Both subjects attempt to predict the results of chemical changes.

-

Thermodynamics exami
nes energy changes while kinetics examines reaction rates
and mechanisms.

-

Thermodynamics considers only the final and initial states of system (i.e.,
thermodynamics is outside of time) whereas kinetics is interested in the system at
all times.

-

Thermody
namics predicts if a reaction can happen whereas kinetics predicts how it
can happen.

Consider a metaphor

Game 1

Quarter

1

2

3

4

Final

UNO

14

14

0

0

28

UNK

0

0

7

14

21

Game 2

Quarter

1

2

3

4

Final

UNO

0

0

14

14

28

UNK

14

7

0

0

21

Game 3

Quarter

1

2

3

4

Final

UNO

7

7

7

7

28

UNK

7

7

7

0

21

All three games are thermodynamically the same; however, kinetically, they are
different.

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Types of Systems

Open

-

system is open to mass flux and energy flux

-

Examples: DSC, human body, internal combusti
on engine, etc…

Closed

-

system is open to energy flux but closed to mass flux.

-

Examples: refrigerator, light bulb, earth (approx.), etc…

Isolated

-

system is closed to mass flux and energy flux

-

Examples: calorimeter, cooler (approx.), etc…

A
diabatic

-

closed system that where there is no heat flow.

-

Example: balloon popping, explosions (initially), etc…

Zeroth Law of Thermodynamics

If body A is in thermal equilibrium with body B and body B is in thermal equilibrium
with body C, then bod
y A and body C are in thermal equilibrium.

The zeroth law of thermodynamics is also known as the law of thermometry.

-

If body B (a thermometer) is calibrated using body A at a specific temperature,
then when body B is also in equilibrium with body C,
body A and body C must
be the same temperature.

All thermometers correlate a physical property with a particular circumstance of
thermal equilibrium.

Examples of physical properties used: thermal expansion, resistivity, volume of ideal
gas, etc…

Measure
ment of Thermodynamic Variables

Temperature

Temperature Scales

All temperature scales have two features.

1. Reference

-

all useful temperature scales have a zero value.

-

thermodynamic temperatures scale make zero the lowest possible value.

-

absol
ute temperature scale (no negative temperature)

2. Interval

-

all useful temperature scales have even spaced intervals.

4

Common temperature scales.

Fahreinheit

-

Reference set at lowest temperature achievable with saturated salt water.

-

Interval se
t as 1/180 of temperature difference between the freezing point of
water and the boiling point of water.

Celsius

-

Reference set at freezing point of pure water.

-

Interval set as 1/100 of temperature difference between the freezing point of
water
and the boiling point of water.

-

Experimentally, the reference of the Celsius scale is defined by triple point
of water being exactly 0.01

C.

Réaumur

-

Reference set at freezing point of pure water.

-

Interval set as 1/80 of temperature differe
nce between the freezing point of
water and the boiling point of water.

Kelvin

-

Reference set at absolute zero.

-

Interval set as 1/100 of temperature difference between the freezing point of
water and the boiling point of water.

-

Officially,
it is improper to write “

K” or say “degrees Kelvin”: write “K”
and say “Kelvin”.

-

Kelvin is degree Celsius plus exactly 273.15. This relationship implies that
the reference for Kelvin can be defined by the triple point of water as
exactly 273.16 K.

Rankine

-

Reference set at absolute zero.

-

Interval set as 1/180 of temperature difference between the freezing point of
water and the boiling point of water.

Temperature measurements in thermodynamics require an absolute temperature scale
such a
s the Kelvin or Rankine scale. The reasons for this need are based upon the
third law of thermodynamics that we will consider later.

Scientists almost always use the Kelvin scale whereas sometimes engineers will use
the Rankine scale.

As we will see lat
er, temperature will help to evaluate the amount of energy within a
thermodynamic system. Much later (second semester), we will see how temperature
is related to the way in which energy is distributed among its different possible
modes.

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Thermometers

Liquid

-

Liquids where the thermal expansivity (how a liquid expands with
increasing temperature) is linear when the liquid is confined to a small tube
are commonly used for thermometers.

-

Examples of liquids commonly used are mercury and alcohol.

-

More specialized liquids include mineral spirits, kerosene, toluene,
mercury/thallium amalgam, mercury/gallium amalgam, etc…

Resistance

-

The electrical resistance of a material can be very sensitive to change is
temperature.

-

The correlation be
tween temperature and resistance is quite nonlinear.

-

Thermometer based on resistance is known as a
thermistor
.

-

Common materials used include platinum (often with rhodium), copper
(often with nickel), nickel (often with silicon and chromium), iron
, tungsten
(often with rhenium), etc…

Ideal Gas Thermometer

-

Based on Charles’ law, the volume change for an ideal gas should be linear.

-

where V
0

is a reference volume (to base

0

upon)

-

is the thermal expansivity o
f the gas

-

is the temperature.

-

For the Kelvin scale, we choose a reference volume as the volume of an
ideal gas at the temperature of the water triple point.

-

We perform a series of measurements of the ideal gas volume at various
temperatures
.

-

However, gases only become ideal in the limit of zero pressure.

-

Practically, we need to attempt to keep the pressure small.

-

At some low temperature the gas will condense so that we lose the ability to
gather points and lower temperatures.

-

But we consider Charles’ law to be true if the gas had not condensed, we
can extrapolate the volume to absolute zero.

-

Thus we have the volume at the reference temperature and absolute zero.

-

We divide the volume difference by the temperature

we define at the
reference point. (e.g. 273.15). (We’ve already defined the temperature at
the other point as zero.)

-

Thus the volume can be written as a ratio of volumes multiplied by
reference temperature.

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Pressure

Definition from classical mechanics:

Definition can be applied to gases as their mass exerts a force distributed over an area.

For example, A square meter column of air weights 101,325 N or 22,730 lbs.

Units of pressure

SI Unit of pressure is
Pascal

Pa

Old standard pressure is 1 atmosphere (atm).

By definition.

IUPAC Unit of pressure is the
bar
.

By definition,

Caution
:

Standard thermodynamic quantities, (G, H, S, etc…) are measured at
the present time with 1 bar as the standard pressure as opposed the old
standard of one atmosphere. (1% is enough of a difference about
which to be concerned.)

Also historically usefu
l (and commonly used) are the units of Torr or mmHg

-

see discussion of mercury barometers below.

By definition:

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Barometers

Pressure can be measured with a
barometer
.

-

Pressure of mercury column on surfac
e of Hg pool balances atmospheric pressure
pushing on Hg pool.

-

Height of column indicates atmospheric pressure.

-

At sea level, column of Hg is 760 mm (29.92 in.)

-

At sea level, column of H
2
O is 33.7 ft!!

(see water barom
eter in atrium of DSC)

-

specific gravity of Hg = 13.6

Manometers

Pressure can be measured also with a
manometer
.

-

manometers do not have a standing pool of fluid.

Two types of manometers exist:
open
-
end

and
closed
-
end

1. Open
-
end

Mercury (or anot
her dense fluid) is used to fill the tube. The end of the tube is
exposed to atmospheric (laboratory) pressure.

atmospheric pressure

pressure of Hg column

vacuum (no air
)

Hg pool

System pressure is less than
atmospheric pressure.

System pressure is greater
than atmospheric pressure.

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2. Closed
-
end

The end of the tube is sealed from the atmosphere and pump to a vacuum.

Amount

Standard amount is the mole.

1 mole = 6.022 141 99 x 10
23

items

-

6.022 141 99 x 10
23

known as

(best value, CODATA 1998)

-

also known as Loschmidt’s number

-

symbol is N
A
. (I may inadvertently use also

L)

By definition, a mole is the number of atoms in 0.012 kg of
12
C exactly.

1. Use x
-
ray diffraction of nearly perf
ect crystal (e.g. NaCl or Ti) to find spacing
between atoms.

2. Measure mass of atoms using mass spectrometry (using
12
C as reference)

3. From measuring density of crystal (unit cell), calculate Avogadro’s number.

System pressure cannot be
directly compared to
atmospheric pressure.