Download Laws of Thermodynamics

Lecture - 1
CHE 208
Course Instructor: Sheikh Ahmad Shah
Semester: Fall 2016
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Thermodynamics
Etymology
“Thermo- + dynamics”
Thermo = “hot, heat, temperature”
Dynamics = “active, energetic, forceful”
“The theory of relationship between heat and mechanical
energy.”
Scientific Definition: “Thermodynamics is the study of the
laws governing the transformation of heat energy to and
from other forms of energy.
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Laws and Theories
What are Laws?
A descriptive generalization about how some aspects of
the natural world behaves under stated circumstances.
What are Theories?
A well-substantiated explanation of some aspect of the
natural world, based on a body of facts that have been
repeatedly confirmed through observation and
experiment.
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Laws of Thermodynamics
There are four laws of thermodynamics:
1. First Law of Thermodynamics
2. Second Law of Thermodynamics
3. Third Law of Thermodynamics
4. Zeroth Law of Thermodynamics
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Brief History of Thermodynamics
• (1700s): Heat = a fluid like substance transferred from one substance
to another substance called “Calorie”.
• (1850): Heat = a form of energy.
During that time, 1st Law of Thermodynamics was established.
• (1872 – 1930): Establishment of the 2nd Law of Thermodynamics.
• (1906 – 1912): Development of the 3rd Law of Thermodynamics.
• 1935: Development of the 0th Law of Thermodynamics.
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Few Basic Concepts
A thermodynamic system is that part of the physical universe that
is under consideration.
A system is separated from the rest of the universe by a real or
idealized Boundary.
The part of the universe outside the boundary of the system is
referred to as the Surroundings.
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Types of System
On the basis of boundary, there can be three types of system:
If the boundary around a system prevents interaction of the system
with its surroundings, the system is called an isolated system. If
matter can be transferred from the surroundings to the system, or
vice versa, the system is referred to as an open system; otherwise, it
is a closed system.
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State Variables of a System
The macroscopic state of a system at equilibrium can be specified
by the values of a small number of macroscopic variables.
These variables, which include, for example, temperature (T) ,
pressure (P), and volume (V), are referred to as State Variables or
Thermodynamic Variables.
Two samples of a substance that have the same state variables are
said to be in the same state.
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Types of State Variables
State variables or thermodynamics variables are either intensive
or extensive.
Intensive variables are independent of the size of the system;
examples are pressure, density, and temperature.
Extensive variables are dependent on the size of the system and
double if the system is duplicated and added to itself; examples
are volume, mass, internal energy, and entropy.
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Types of State Variables
When a system is in a certain state with its properties
independent of time and having no fluxes (e.g., no heat flowing
through the system), then the system is said to be at equilibrium.
When a thermodynamic system is at equilibrium, its state is
defined entirely by the state variables, not by the history of
previous conditions of the system.
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0th Law of Thermodynamics
If two thermodynamic systems are each in thermal
equilibrium with a third, then they are in thermal
equilibrium with each other.
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Few More Concepts
Physically, work is performed on an object when the
object moves some distance s due to the application of
a force F.
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Few More Concepts
Energy of a system is the capacity or ability to do work.
When any work is done, transfer of energy happens. So,
work is a way to transfer energy.
When energy is transferred due to temperature
difference, that energy is termed as “heat”.
Hot Object
Energy
Cold Object
“Heat”
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Molecular Basis of Heat and Work
Heating is a kind of energy
transfer that makes use of
disorderly molecular motion
in the surroundings.
Whereas, work is a kind of
energy transfer that makes of
organized molecular motion
in the surroundings.
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Concept of Work
The most common form of work studied
by basic thermodynamics involves the
changing volume of a system. In the
picture, A frictionless piston confines a
sample of a gas in an initial volume Vi. The
gas inside the chamber also has an initial
pressure pi. Initially, what keeps the piston
at a fixed position is the external pressure
of the surroundings, pext.
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Concept of Work
If the piston moves out, then the system is
doing work on the surroundings. That
means that the system is losing energy in
the form of work.
The infinitesimal amount of work dw lost
by the system to the surroundings for an
infinitesimal change in volume dV while
acting against a constant external pressure
pext is defined as:
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Concept of Work
By using calculus, we find that:
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Concept of Internal Energy
The total energy of a system is defined as the internal energy.
The internal energy is composed of energy from different sources,
like chemical, electronic, nuclear, and kinetic energies.
It is denoted as “U”.
When any system transfers heat or does work to its surroundings,
Change in Internal energy happens.
ΔU = Ufinal - Uinitial
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1st Law of Thermodynamics
Statement: “For an isolated system, the total energy of the
system remains constant.”
For an isolated system, ΔU = 0
In all investigations of energy changes in systems, it has been
found that when the total energy of a system changes, the energy
change goes into either work or heat, nothing else.
Mathematically, this is written as
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1st Law of Thermodynamics
Mathematical Expression:
ΔU = q + w
Where,
ΔU = Change of the internal energy of the system
q = Energy transferred as heat to the system
w = Work done on a system
In an isolated system,
q = 0 and w = 0
So, ΔU = 0
“For an isolated system, the total
energy of the system remains
constant.”
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Concept of Enthalpy
From the 1st law of thermodynamics, we get:
ΔU = q + w
or, q = ΔU – w
Now, we know,
So, q = ΔU – (- Pext ·ΔV)
Or, q = ΔU + Pext ·ΔV
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Concept of Enthalpy
Enthalpy is the amount of heat content used or released in a system
at constant pressure. It can defined as:
“Enthalpy is a thermodynamic quantity equivalent to the total heat
content of a system. It is equal to the internal energy of the system
plus the product of pressure and volume.”
When ΔH is Positive:
Endothermic
When ΔH is Negative:
Exothermic
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Hess’s Law
“The standard enthalpy of an overall reaction is the sum of the
standard enthalpies of the individual reactions into which a reaction
may be divided.”
That means:
If, A + B = D + E; ΔH1
and, A + B = C; ΔH2
and, C = D + E; ΔH3
According to Hess’s Law:
ΔH1 = ΔH2 + ΔH3
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Hess’s Law
ΔH° = ?
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