CHE THERMO REVIEWER
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THERMODYNAMICS
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was developed in the 19th century as a result of
the need to describe the basic operating
principles of the newly invented steam engine
and to provide a basis for relating the work
produced to the heat supplied
the name itself denotes power generated from
heat
First Law of Thermodynamics
o says that energy is conserved, meaning
that it is neither created nor destroyed.
o in scientific and engineering contexts,
energy is recognized as appearing in
various forms, useful because each form
has mathematical definition as a
function of some recognizable and
measurable characteristics of the real
world. Thus kinetic energy is defined as
a function of velocity, and gravitational
potential energy as a function of
elevation.
o Conservation implies the transformation
of one form of energy into another.
Second Law of Thermodynamics
o more difficult to comprehend because it
depends on entropy
The application of thermodynamics to any real
problem starts with the specification of a
particular region of space or body of matter
designated as the system.
Everything outside the system is called the
surroundings
Once a system has been selected, we must
describe its state. There are two possible points
of view, the macroscopic and the microscopic.
Macroscopic: composition, density,
temperature, and pressure
macroscopic
coordinates
require
no
assumptions regarding the structure of matter,
few in number, are suggested by our sense
perceptions, and are measured with relative
ease
A macroscopic description thus requires
specification of a few fundamental measurable
properties
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A microscopic description depends on the
existence and behavior of molecules, is not
directly related to our sense perceptions, and
treats quantities that cannot routinely be
directly measured
Bridging the length and time scales between the
microscopic behavior of molecules and the
macroscopic world is the subject of statistical
mechanics or statistical thermodynamics, which
applies the laws of quantum mechanics and
classical mechanics to large ensembles of atoms,
molecules, or other elementary objects to
predict and interpret macroscopic behavior
SI UNITS
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second, symbol s, the SI unit of time, is the
duration of 9,192,631,770 cycles of radiation
associated with a specified transition of the
cesium atom
meter, symbol m, is the fundamental unit of
length, defined as the distance light travels in a
vacuum during 1/299,792,458 of a second
kilogram, symbol kg, is the basic unit of mass,
defined as the mass of a platinum/iridium
cylinder kept at the International Bureau of
Weights and Measures at Sèvres, France
Temperature is a characteristic dimension of
thermodynamics, and is measured on the Kelvin
scale
The mole, symbol mol, is defined as the amount
of a substance represented by as many
elementary entities (e.g., molecules) as there are
atoms in 0.012 kg of carbon-12
The SI unit of force is the newton, symbol N,
derived from Newton’s second law, which
expresses force F as the product of mass m and
acceleration a: F = ma
a newton is the force that, when applied to a
mass of 1 kg, produces an acceleration of 1
m·s^−2
Essential to thermodynamics is the derived unit
for energy, the joule, symbol J, defined as 1 N·m
or 1 kg·m^2·s^−2
Kelvin
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Weight properly refers to the force of gravity on
a body, expressed in newtons, and not to its
mass, expressed in kilograms.
An absolute scale, it is based on the concept of a
lower limit of temperature, called absolute zero.
Its unit is the kelvin, symbol K. Celsius
temperatures, with symbol t, are defined in
relation to Kelvin temperatures
TK = TC – 273.15
Scientific and industrial practice depends on the
International Temperature Scale of 1990
(ITS−90)
states of pure substances like the ice and steam
points, and on standard instruments calibrated
at these temperatures
The platinum-resistance thermometer is an
example of a standard instrument; it is used for
temperatures from −259.35°C (the triple point of
hydrogen) to 961.78°C (the freezing point of
silver).
MEASURES OF AMOUNT OR SIZE
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Specific or molar density is defined as the
reciprocal of specific or molar volume: 𝜌 ≡ 𝑉 −1
intensive
thermodynamic
variables
independent of the size of a system
TEMPERATURE
Celsius
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defined by fixing zero as the ice point (freezing
point of water saturated with air at standard
atmospheric pressure) and 100 as the steam
point (boiling point of pure water at standard
atmospheric pressure)
Celsius temperatures can only be used in
thermodynamic
calculations
involving
temperature differences, which are of course the
same in both degrees Celsius and kelvins.
PRESSURE
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primary standard for pressure measurement is
the dead-weight gauge in which a known force is
balanced by fluid pressure acting on a piston of
known area: P ≡ F/A
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Gauge Pressures
o difference between the pressure of
interest and the pressure of the
surrounding atmosphere.
Absolute Pressures
o gauge pressure are converted to
absolute pressures by addition of the
local barometric pressure
o are used in thermodynamic calculations
The pressure to which a fluid height
corresponds is determined by the density of the
fluid (which depends on its identity and
temperature) and the local acceleration of
gravity.
A unit of pressure in common use (but not an SI
unit) is the standard atmosphere, representing
the average pressure exerted by the earth’s
atmosphere at sea level, and defined as 101.325
kPa
ENERGY
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Kinetic Energy
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Potential Energy
HEAT
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temperature as the driving force for the transfer
of energy as heat
INTERNAL ENERGY
WORK
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work. When the piston moves into the cylinder
so as to compress the fluid, the applied force
and its displacement are in the same direction;
the work is therefore positive. The minus sign is
required because the volume change is
negative.
For an expansion process, the applied force and
its displacement are in opposite directions. The
volume change in this case is positive, and the
minus sign is again required to make the work
negative
performed whenever a force acts through a
distance
By its definition, the quantity of work is given by
the equation dW=Fdl
The SI unit of work is the newton·meter or
joule, symbol J
By convention, work is regarded as positive
when the displacement is in the same direction
as the applied force and negative when they are
in opposite directions
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This energy is named internal to distinguish it
from the kinetic and potential energy
associated with a substance because of its
macroscopic position, configuration, or motion,
which can be thought of as external forms of
energy
Internal energy has no concise thermodynamic
definition. It is a thermodynamic primitive. It
cannot be directly measured; there are no
internal-energy meters. As a result, absolute
values are unknown
FIRST LAW OF THERMODYNAMICS
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The minus signs in these equations are made
necessary by the sign convention adopted for
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Although energy assumes many forms, the
total quantity of energy is constant, and when
energy disappears in one form it appears
simultaneously in other forms.
The total energy of any system and its
surroundings is conserved.
For any process, the first law requires
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dEnergy of system + dEnergy of surr = 0
heat and work represent energy in transit
across the boundary dividing the system from
its surroundings, and are never stored or
contained in the system.
Heat and work represent energy flows to or
from a system, while potential, kinetic, and
internal energy represent quantities of energy
associated with a system
QUIZ #2
FIRST LAW OF THERMODYNAMICS FOR OPEN SYSTEMS
Measures of Flow
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Open systems are characterized by flowing
streams; there are four common measures of
flow:
a. Mass Flow rate, m
b. Molar Flow rate, n
c. Volumetric Flow rate, q
d. Velocity, u
The measures of flow are interrelated
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where ℳ is molar mass and A is the crosssectional area for flow
mass and molar flow rates relate to velocity:
Mass Balance for Open Systems
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Although velocity is a vector quantity, its scalar
magnitude u is used here as the average speed
of a stream in the direction normal to A.
A is the cross sectional area that is normal to
the direction of flow
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Example:
Liquid n-hexane flows at a rate of 0.75 kg/s in a pipe
with inside diameter D = 5 cm. What are q, n, v? What
would these quantities be for the same m if D = 2 cm?
Assume for liquid n-hexane that ρ= 659 kg/m3.
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region of space identified for analysis of open
systems is called a control volume
it is separated from its surroundings by a control
surface
The fluid within the control volume is the
thermodynamic system for which mass and
energy balances are written.
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When there is a single entrance and a single exit
stream, the mass flow rate m˙ is the same for
both streams
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specific volume is the reciprocal of density
General Energy Balance
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Since mass is conserved, the rate of change of
mass within the control volume, dmcv/dt,
equals the net rate of flow of mass into the
control volume
The convention is that flow is positive when
directed into the control volume and negative
when directed out
mass balance is:
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Energy is conserved
rate of change of energy within c.v. equals the
net rate of flow of energy into the c.v.
Streams flowing into & out of the control
volume have associated with them energy in its
internal, potential and kinetic forms
Each stream transports energy at the rate
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Net energy transported into the system
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Other ways to change energy in the system is
application of heat and work
Rate of energy accumulation:
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Continuity Equation
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net rate of flow in mass stream:
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mass flow rate:
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Steady-state flow processes are those for
which conditions within the control volume do
not change with time
In a steady-state process, the control volume
contains a constant mass of fluid, and the first
or accumulation term is zero
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Work: associated w/ the flowing streams, shaft
work, & associated w/expansion & contraction
of control volume
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Enthalpy, H = U +PV
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If kinetic & potential energy changes are
negligible
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The velocity u in the kinetic-energy terms is the
bulk-mean velocity as defined by the equation
u = m/(ρA)
Example:
An insulated, electrically heated tank for hot
water contains 190 kg of liquid water at 333.15
K (60°C) when a power outage occurs. If
water is withdrawn from the tank at a steady
rate of m = 0.2 kg/s, how long will it take for the
temperature of the water in the tank to drop
from 333.15 K to 308.15 K (60 to 35°C)? Assume
that cold water enters the tank at 283.15 K
(10°C), and that heat loss from the tank is
negligible. For liquid water, Cv = Cp = C,
Energy Balances for Steady-State Flow Processes
independent of T & P.
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Flow processes for which the accumulation
term is zero is said to occur at steady state
No expansion of the control volume is possible
under these circumstances
The only work of the process is shaft work, and
the general energy balance:
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If potential & kinetic energy are negligible
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