NCERT Textbook: Thermodynamics

# NCERT Textbook: Thermodynamics | Chemistry Class 11 - NEET PDF Download

``` Page 1

Thermodynamics
It is the only physical theory of universal content concerning
which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown.
Albert Einstein
Chemical energy stored by molecules can be released as
heat during chemical reactions when a fuel like methane,
cooking gas or coal burns in air. The chemical energy may
also be used to do mechanical work when a fuel burns
in an engine or to provide electrical energy through a
galvanic cell like dry cell. Thus, various forms of energy
are interrelated and under certain conditions, these may
be transformed from one form into  another. The study
of these energy transformations forms the subject matter
of thermodynamics. The laws of thermodynamics deal
with energy changes of macroscopic systems involving
a large number of molecules rather than microscopic
systems containing a few molecules. Thermodynamics is
not concerned about how and at what rate these energy
transformations are carried out, but is based on initial and
final states of a system undergoing the change. Laws of
thermodynamics apply only when a system is in equilibrium
or moves from one equilibrium state to another equilibrium
state. Macroscopic properties like pressure and temperature
do not change with time for a system in equilibrium state.
In this unit, we would like to answer some of the important
questions through thermodynamics, like:
How do we determine the energy changes involved in a
chemical reaction/process? Will it occur or not?
What drives a chemical reaction/process?
To what extent do the chemical reactions proceed?
UniT 5
After studying this Unit, you will be
able to
• explain the terms : system and
surroundings;
• discriminate between close, open
and isolated systems;
• explain internal energy, work and
heat;
• state first law of thermodynamics
and express it mathematically;
• calculate energy changes as
work and heat contributions in
chemical systems;
• explain state functions: U, H.
• correlate  ?U and ?H;
• measure experimentally ?U and
?H;
• define standard states for ?H;
• calculate enthalpy changes for
various types of reactions;
• state and apply Hess’s law of
constant heat summation;
• differentiate between extensive
and intensive properties;
• define spontaneous and non-
spontaneous processes;
• explain entropy as a
thermodynamic state function
and apply it for spontaneity;
• explain Gibbs energy change (?G);
and
• establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
Unit 5.indd   136 9/12/2022   11:53:33 AM
2024-25
Page 2

Thermodynamics
It is the only physical theory of universal content concerning
which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown.
Albert Einstein
Chemical energy stored by molecules can be released as
heat during chemical reactions when a fuel like methane,
cooking gas or coal burns in air. The chemical energy may
also be used to do mechanical work when a fuel burns
in an engine or to provide electrical energy through a
galvanic cell like dry cell. Thus, various forms of energy
are interrelated and under certain conditions, these may
be transformed from one form into  another. The study
of these energy transformations forms the subject matter
of thermodynamics. The laws of thermodynamics deal
with energy changes of macroscopic systems involving
a large number of molecules rather than microscopic
systems containing a few molecules. Thermodynamics is
not concerned about how and at what rate these energy
transformations are carried out, but is based on initial and
final states of a system undergoing the change. Laws of
thermodynamics apply only when a system is in equilibrium
or moves from one equilibrium state to another equilibrium
state. Macroscopic properties like pressure and temperature
do not change with time for a system in equilibrium state.
In this unit, we would like to answer some of the important
questions through thermodynamics, like:
How do we determine the energy changes involved in a
chemical reaction/process? Will it occur or not?
What drives a chemical reaction/process?
To what extent do the chemical reactions proceed?
UniT 5
After studying this Unit, you will be
able to
• explain the terms : system and
surroundings;
• discriminate between close, open
and isolated systems;
• explain internal energy, work and
heat;
• state first law of thermodynamics
and express it mathematically;
• calculate energy changes as
work and heat contributions in
chemical systems;
• explain state functions: U, H.
• correlate  ?U and ?H;
• measure experimentally ?U and
?H;
• define standard states for ?H;
• calculate enthalpy changes for
various types of reactions;
• state and apply Hess’s law of
constant heat summation;
• differentiate between extensive
and intensive properties;
• define spontaneous and non-
spontaneous processes;
• explain entropy as a
thermodynamic state function
and apply it for spontaneity;
• explain Gibbs energy change (?G);
and
• establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
Unit 5.indd   136 9/12/2022   11:53:33 AM
2024-25
THERMODYNAMICS         137
5.1  Thermodynamic Terms
We are interested in chemical reactions and
the energy changes accompanying them. For
this we need to know certain thermodynamic
terms. These are discussed below.
5.1.1 The system and the surroundings
A system in thermodynamics refers to that
part of universe in which observations are
the surroundings. The surroundings include
everything other than the system. System
and the surroundings together constitute the
universe.
The universe = The system + The surroundings
However, the entire universe other than
the system is not affected by the changes
taking place in the system. Therefore, for all
practical purposes, the surroundings are that
portion of the remaining universe which can
interact with the system. Usually, the region
of space in the neighbourhood of the system
constitutes its surroundings.
For example, if we are studying the
reaction between two substances A and B
kept in a beaker, the beaker containing the
reaction mixture is the system and the room
where the beaker is kept is the surroundings
(Fig. 5.1).
Fig. 5.1 System and the surroundings
the system from the surroundings is called
boundary. This is designed to allow us to
control and keep track of all movements of
matter and energy in or out of the system.
5.1.2 Types of the system
We, further classify the systems according
to the movements of matter and energy in or
out of the system.
1. Open System
In an open system, there is exchange of energy
and matter between system and surroundings
[Fig. 5.2 (a)]. The presence of reactants in an
open beaker is an example of an open system*.
Here the boundary is an imaginary surface
enclosing the beaker and reactants.
2. Closed System
In a closed system, there is no exchange of
matter, but exchange of energy is possible
between system and the surroundings
[Fig. 5.2 (b)]. The presence of reactants in a
closed vessel made of conducting material
e.g., copper or steel is an example of a closed
system.
Fig. 5.2  Open, closed and isolated systems.
Note that the system may be defined
by physical boundaries, like beaker or test
tube, or the system may simply be defined
by a set of Cartesian coordinates specifying
a particular volume in space. It is necessary
to think of the system as separated from the
surroundings by some sort of wall which may
be real or imaginary. The wall that separates
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
Unit 5.indd   137 9/12/2022   11:53:34 AM
2024-25
Page 3

Thermodynamics
It is the only physical theory of universal content concerning
which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown.
Albert Einstein
Chemical energy stored by molecules can be released as
heat during chemical reactions when a fuel like methane,
cooking gas or coal burns in air. The chemical energy may
also be used to do mechanical work when a fuel burns
in an engine or to provide electrical energy through a
galvanic cell like dry cell. Thus, various forms of energy
are interrelated and under certain conditions, these may
be transformed from one form into  another. The study
of these energy transformations forms the subject matter
of thermodynamics. The laws of thermodynamics deal
with energy changes of macroscopic systems involving
a large number of molecules rather than microscopic
systems containing a few molecules. Thermodynamics is
not concerned about how and at what rate these energy
transformations are carried out, but is based on initial and
final states of a system undergoing the change. Laws of
thermodynamics apply only when a system is in equilibrium
or moves from one equilibrium state to another equilibrium
state. Macroscopic properties like pressure and temperature
do not change with time for a system in equilibrium state.
In this unit, we would like to answer some of the important
questions through thermodynamics, like:
How do we determine the energy changes involved in a
chemical reaction/process? Will it occur or not?
What drives a chemical reaction/process?
To what extent do the chemical reactions proceed?
UniT 5
After studying this Unit, you will be
able to
• explain the terms : system and
surroundings;
• discriminate between close, open
and isolated systems;
• explain internal energy, work and
heat;
• state first law of thermodynamics
and express it mathematically;
• calculate energy changes as
work and heat contributions in
chemical systems;
• explain state functions: U, H.
• correlate  ?U and ?H;
• measure experimentally ?U and
?H;
• define standard states for ?H;
• calculate enthalpy changes for
various types of reactions;
• state and apply Hess’s law of
constant heat summation;
• differentiate between extensive
and intensive properties;
• define spontaneous and non-
spontaneous processes;
• explain entropy as a
thermodynamic state function
and apply it for spontaneity;
• explain Gibbs energy change (?G);
and
• establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
Unit 5.indd   136 9/12/2022   11:53:33 AM
2024-25
THERMODYNAMICS         137
5.1  Thermodynamic Terms
We are interested in chemical reactions and
the energy changes accompanying them. For
this we need to know certain thermodynamic
terms. These are discussed below.
5.1.1 The system and the surroundings
A system in thermodynamics refers to that
part of universe in which observations are
the surroundings. The surroundings include
everything other than the system. System
and the surroundings together constitute the
universe.
The universe = The system + The surroundings
However, the entire universe other than
the system is not affected by the changes
taking place in the system. Therefore, for all
practical purposes, the surroundings are that
portion of the remaining universe which can
interact with the system. Usually, the region
of space in the neighbourhood of the system
constitutes its surroundings.
For example, if we are studying the
reaction between two substances A and B
kept in a beaker, the beaker containing the
reaction mixture is the system and the room
where the beaker is kept is the surroundings
(Fig. 5.1).
Fig. 5.1 System and the surroundings
the system from the surroundings is called
boundary. This is designed to allow us to
control and keep track of all movements of
matter and energy in or out of the system.
5.1.2 Types of the system
We, further classify the systems according
to the movements of matter and energy in or
out of the system.
1. Open System
In an open system, there is exchange of energy
and matter between system and surroundings
[Fig. 5.2 (a)]. The presence of reactants in an
open beaker is an example of an open system*.
Here the boundary is an imaginary surface
enclosing the beaker and reactants.
2. Closed System
In a closed system, there is no exchange of
matter, but exchange of energy is possible
between system and the surroundings
[Fig. 5.2 (b)]. The presence of reactants in a
closed vessel made of conducting material
e.g., copper or steel is an example of a closed
system.
Fig. 5.2  Open, closed and isolated systems.
Note that the system may be defined
by physical boundaries, like beaker or test
tube, or the system may simply be defined
by a set of Cartesian coordinates specifying
a particular volume in space. It is necessary
to think of the system as separated from the
surroundings by some sort of wall which may
be real or imaginary. The wall that separates
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
Unit 5.indd   137 9/12/2022   11:53:34 AM
2024-25
chemIstry 138
3. Isolated System
In an isolated system, there is no exchange
of energy or matter between the system and
the surroundings [Fig. 5.2 (c)]. The presence
of reactants in a thermos flask or any other
closed insulated vessel is an example of an
isolated system.
5.1.3 The state of the system
The system must be described in order to
make any useful calculations by specifying
quantitatively each of the properties such as
its pressure (p), volume (V), and temperature
(T ) as well as the composition of the system.
We need to describe the system by specifying
it before and after the change. You would
recall from your Physics course that the
state of a system in mechanics is completely
specified at a given instant of time, by the
position and velocity of each mass point of
the system. In thermodynamics, a different
and much simpler concept of the state of a
system is introduced. It does not need detailed
knowledge of motion of each particle  because,
we deal with average measurable properties of
the system. We specify the state of the system
by state functions or state variables.
The state of a thermodynamic system is
described by its measurable or macroscopic
(bulk) properties. We can describe the state
of a gas by quoting its pressure (p), volume
(V), temperature (T ), amount (n) etc. Variables
like p, V, T are called state variables or state
functions because their values depend only
on the state of the system and not on how it
is reached. In order to completely define the
state of a system it is not necessary to define
all the properties of the system; as only a
certain number of properties can be varied
independently. This number depends on the
nature of the system. Once these minimum
number of macroscopic properties are fixed,
others automatically have definite values.
The state of the surroundings can never
be completely specified; fortunately it is not
necessary to do so.
5.1.4 The internal energy as a state
Function
When we talk about our chemical system
losing or gaining energy, we need to introduce
a quantity which represents the total energy
of the system. It may be chemical, electrical,
mechanical or any other type of energy you
may think of, the sum of all these is the energy
of the system. In thermodynamics, we call it
the internal energy, U of the system, which
may change, when
•	 heat passes into or out of the system,
•	 work is done on or by the system,
•	 matter enters or leaves the system.
These systems are classified accordingly
as you have already studied in section 5.1.2.
(a) Work
Let us first examine a change in internal energy
by doing work. We take a system containing
some quantity of water in a thermos flask
or in an insulated beaker. This would not
allow exchange of heat between the system
and surroundings through its boundary and
we call this type of system as adiabatic. The
manner in which the state of such a system
may be changed will be called adiabatic
process. Adiabatic process is a process in
which there is no transfer of heat between
the system and surroundings. Here, the wall
separating the system and the surroundings
is called the adiabatic wall (Fig. 5.3).
Fig. 5.3 An adiabatic system which does not
permit the transfer of heat through its
boundary.
Let us bring the change in the internal
energy of the system by doing some work on
it. Let us call the  initial state of the system
as state A and its temperature as  T
A
. Let
the internal energy of the system in state A
be called U
A
. We can change the state of the
system in two different ways.
Unit 5.indd   138 9/12/2022   11:53:36 AM
2024-25
Page 4

Thermodynamics
It is the only physical theory of universal content concerning
which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown.
Albert Einstein
Chemical energy stored by molecules can be released as
heat during chemical reactions when a fuel like methane,
cooking gas or coal burns in air. The chemical energy may
also be used to do mechanical work when a fuel burns
in an engine or to provide electrical energy through a
galvanic cell like dry cell. Thus, various forms of energy
are interrelated and under certain conditions, these may
be transformed from one form into  another. The study
of these energy transformations forms the subject matter
of thermodynamics. The laws of thermodynamics deal
with energy changes of macroscopic systems involving
a large number of molecules rather than microscopic
systems containing a few molecules. Thermodynamics is
not concerned about how and at what rate these energy
transformations are carried out, but is based on initial and
final states of a system undergoing the change. Laws of
thermodynamics apply only when a system is in equilibrium
or moves from one equilibrium state to another equilibrium
state. Macroscopic properties like pressure and temperature
do not change with time for a system in equilibrium state.
In this unit, we would like to answer some of the important
questions through thermodynamics, like:
How do we determine the energy changes involved in a
chemical reaction/process? Will it occur or not?
What drives a chemical reaction/process?
To what extent do the chemical reactions proceed?
UniT 5
After studying this Unit, you will be
able to
• explain the terms : system and
surroundings;
• discriminate between close, open
and isolated systems;
• explain internal energy, work and
heat;
• state first law of thermodynamics
and express it mathematically;
• calculate energy changes as
work and heat contributions in
chemical systems;
• explain state functions: U, H.
• correlate  ?U and ?H;
• measure experimentally ?U and
?H;
• define standard states for ?H;
• calculate enthalpy changes for
various types of reactions;
• state and apply Hess’s law of
constant heat summation;
• differentiate between extensive
and intensive properties;
• define spontaneous and non-
spontaneous processes;
• explain entropy as a
thermodynamic state function
and apply it for spontaneity;
• explain Gibbs energy change (?G);
and
• establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
Unit 5.indd   136 9/12/2022   11:53:33 AM
2024-25
THERMODYNAMICS         137
5.1  Thermodynamic Terms
We are interested in chemical reactions and
the energy changes accompanying them. For
this we need to know certain thermodynamic
terms. These are discussed below.
5.1.1 The system and the surroundings
A system in thermodynamics refers to that
part of universe in which observations are
the surroundings. The surroundings include
everything other than the system. System
and the surroundings together constitute the
universe.
The universe = The system + The surroundings
However, the entire universe other than
the system is not affected by the changes
taking place in the system. Therefore, for all
practical purposes, the surroundings are that
portion of the remaining universe which can
interact with the system. Usually, the region
of space in the neighbourhood of the system
constitutes its surroundings.
For example, if we are studying the
reaction between two substances A and B
kept in a beaker, the beaker containing the
reaction mixture is the system and the room
where the beaker is kept is the surroundings
(Fig. 5.1).
Fig. 5.1 System and the surroundings
the system from the surroundings is called
boundary. This is designed to allow us to
control and keep track of all movements of
matter and energy in or out of the system.
5.1.2 Types of the system
We, further classify the systems according
to the movements of matter and energy in or
out of the system.
1. Open System
In an open system, there is exchange of energy
and matter between system and surroundings
[Fig. 5.2 (a)]. The presence of reactants in an
open beaker is an example of an open system*.
Here the boundary is an imaginary surface
enclosing the beaker and reactants.
2. Closed System
In a closed system, there is no exchange of
matter, but exchange of energy is possible
between system and the surroundings
[Fig. 5.2 (b)]. The presence of reactants in a
closed vessel made of conducting material
e.g., copper or steel is an example of a closed
system.
Fig. 5.2  Open, closed and isolated systems.
Note that the system may be defined
by physical boundaries, like beaker or test
tube, or the system may simply be defined
by a set of Cartesian coordinates specifying
a particular volume in space. It is necessary
to think of the system as separated from the
surroundings by some sort of wall which may
be real or imaginary. The wall that separates
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
Unit 5.indd   137 9/12/2022   11:53:34 AM
2024-25
chemIstry 138
3. Isolated System
In an isolated system, there is no exchange
of energy or matter between the system and
the surroundings [Fig. 5.2 (c)]. The presence
of reactants in a thermos flask or any other
closed insulated vessel is an example of an
isolated system.
5.1.3 The state of the system
The system must be described in order to
make any useful calculations by specifying
quantitatively each of the properties such as
its pressure (p), volume (V), and temperature
(T ) as well as the composition of the system.
We need to describe the system by specifying
it before and after the change. You would
recall from your Physics course that the
state of a system in mechanics is completely
specified at a given instant of time, by the
position and velocity of each mass point of
the system. In thermodynamics, a different
and much simpler concept of the state of a
system is introduced. It does not need detailed
knowledge of motion of each particle  because,
we deal with average measurable properties of
the system. We specify the state of the system
by state functions or state variables.
The state of a thermodynamic system is
described by its measurable or macroscopic
(bulk) properties. We can describe the state
of a gas by quoting its pressure (p), volume
(V), temperature (T ), amount (n) etc. Variables
like p, V, T are called state variables or state
functions because their values depend only
on the state of the system and not on how it
is reached. In order to completely define the
state of a system it is not necessary to define
all the properties of the system; as only a
certain number of properties can be varied
independently. This number depends on the
nature of the system. Once these minimum
number of macroscopic properties are fixed,
others automatically have definite values.
The state of the surroundings can never
be completely specified; fortunately it is not
necessary to do so.
5.1.4 The internal energy as a state
Function
When we talk about our chemical system
losing or gaining energy, we need to introduce
a quantity which represents the total energy
of the system. It may be chemical, electrical,
mechanical or any other type of energy you
may think of, the sum of all these is the energy
of the system. In thermodynamics, we call it
the internal energy, U of the system, which
may change, when
•	 heat passes into or out of the system,
•	 work is done on or by the system,
•	 matter enters or leaves the system.
These systems are classified accordingly
as you have already studied in section 5.1.2.
(a) Work
Let us first examine a change in internal energy
by doing work. We take a system containing
some quantity of water in a thermos flask
or in an insulated beaker. This would not
allow exchange of heat between the system
and surroundings through its boundary and
we call this type of system as adiabatic. The
manner in which the state of such a system
may be changed will be called adiabatic
process. Adiabatic process is a process in
which there is no transfer of heat between
the system and surroundings. Here, the wall
separating the system and the surroundings
is called the adiabatic wall (Fig. 5.3).
Fig. 5.3 An adiabatic system which does not
permit the transfer of heat through its
boundary.
Let us bring the change in the internal
energy of the system by doing some work on
it. Let us call the  initial state of the system
as state A and its temperature as  T
A
. Let
the internal energy of the system in state A
be called U
A
. We can change the state of the
system in two different ways.
Unit 5.indd   138 9/12/2022   11:53:36 AM
2024-25
THERMODYNAMICS         139
One way: We do some mechanical work, say
1 kJ, by rotating a set of small paddles and
thereby churning water. Let the new state
be called B state and its temperature, as
T
B
. It is found that T
B
> T
A
and the change
in temperature, ?T = T
B
–T
A
. Let the internal
energy of the system in state B be U
B
and the
change in internal energy, ?U =U
B
– U
A
.
Second way: We now do an equal amount
(i.e., 1kJ) electrical work with the help of an
immersion rod  and note down the temperature
change. We find that the change in temperature
is same as in the earlier case, say, T
B
– T
A
.
In fact, the experiments in the above
manner were done by J. P. Joule between
1840–50 and he was able to show that a
given amount of work done on the system,
no matter how it was done (irrespective of
path) produced the same change of state, as
measured by the change in the temperature
of the system.
So, it seems appropriate to define a
quantity, the internal energy U, whose value
is characteristic of the state of a system,
required to
bring about a change of state is equal to the
difference between the value of U in one state
and that in another state, ?U i.e.,
?U =U
2
–U
1
= w
Therefore, internal energy, U, of the
system is a state function.
By conventions of IUPAC in chemical
thermodynamics. The positive sign expresses
that w
is positive when work is done on the
system and the internal energy of system
increases. Similarly, if the work is done by the
system, w
will be negative because internal
energy of the system decreases.
Can you name some other familiar state
functions? Some of other familiar state
functions are V, p, and  T. For example, if we
bring a change in temperature of the system
from 25°C to 35°C, the change in temperature
is 35°C–25°C = +10°C, whether we go straight
up to 35°C or we cool the system for a few
degrees, then take the system to the final
temperature. Thus, T is a state function and
the change in temperature is independent of
the route taken. Volume of water in a pond, for
example, is a state function, because change
in volume of its water is independent of the
route by which water is filled in the pond,
either by rain or by tubewell or by both.
(b) Heat
We can also change the internal energy
of a system by transfer of heat from the
surroundings to the system or vice-versa
without expenditure of work. This exchange
of energy, which is a result of temperature
difference is called heat, q. Let us consider
bringing about the same change in temperature
(the same initial and final states as before
in section 5.1.4 (a) by transfer of heat
We take water at temperature, T
A
in a
container having thermally conducting walls,
say made up of copper and enclose it in a
huge heat reservoir at temperature, T
B
. The
heat absorbed by the system (water), q can be
measured in terms of temperature difference,
T
B
– T
A
. In this case change in internal energy,
?U = q, when no work is done at constant
volume.
By conventions of IUPAC in chemical
thermodynamics. The  q is positive, when heat
is transferred from the surroundings to the
system and the internal energy of the system
increases and q is negative when heat is
transferred from system to the surroundings
resulting in decrease of the internal energy of
the system.
* Earlier negative sign was assigned when the work is done on the system and positive sign when the work is done by the
system. This is still followed in physics books, although IUPAC has recommended the use of new sign convention.
Fig. 5.4 A system which allows heat transfer
through its boundary.
Unit 5.indd   139 9/12/2022   11:53:37 AM
2024-25
Page 5

Thermodynamics
It is the only physical theory of universal content concerning
which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown.
Albert Einstein
Chemical energy stored by molecules can be released as
heat during chemical reactions when a fuel like methane,
cooking gas or coal burns in air. The chemical energy may
also be used to do mechanical work when a fuel burns
in an engine or to provide electrical energy through a
galvanic cell like dry cell. Thus, various forms of energy
are interrelated and under certain conditions, these may
be transformed from one form into  another. The study
of these energy transformations forms the subject matter
of thermodynamics. The laws of thermodynamics deal
with energy changes of macroscopic systems involving
a large number of molecules rather than microscopic
systems containing a few molecules. Thermodynamics is
not concerned about how and at what rate these energy
transformations are carried out, but is based on initial and
final states of a system undergoing the change. Laws of
thermodynamics apply only when a system is in equilibrium
or moves from one equilibrium state to another equilibrium
state. Macroscopic properties like pressure and temperature
do not change with time for a system in equilibrium state.
In this unit, we would like to answer some of the important
questions through thermodynamics, like:
How do we determine the energy changes involved in a
chemical reaction/process? Will it occur or not?
What drives a chemical reaction/process?
To what extent do the chemical reactions proceed?
UniT 5
After studying this Unit, you will be
able to
• explain the terms : system and
surroundings;
• discriminate between close, open
and isolated systems;
• explain internal energy, work and
heat;
• state first law of thermodynamics
and express it mathematically;
• calculate energy changes as
work and heat contributions in
chemical systems;
• explain state functions: U, H.
• correlate  ?U and ?H;
• measure experimentally ?U and
?H;
• define standard states for ?H;
• calculate enthalpy changes for
various types of reactions;
• state and apply Hess’s law of
constant heat summation;
• differentiate between extensive
and intensive properties;
• define spontaneous and non-
spontaneous processes;
• explain entropy as a
thermodynamic state function
and apply it for spontaneity;
• explain Gibbs energy change (?G);
and
• establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
Unit 5.indd   136 9/12/2022   11:53:33 AM
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THERMODYNAMICS         137
5.1  Thermodynamic Terms
We are interested in chemical reactions and
the energy changes accompanying them. For
this we need to know certain thermodynamic
terms. These are discussed below.
5.1.1 The system and the surroundings
A system in thermodynamics refers to that
part of universe in which observations are
the surroundings. The surroundings include
everything other than the system. System
and the surroundings together constitute the
universe.
The universe = The system + The surroundings
However, the entire universe other than
the system is not affected by the changes
taking place in the system. Therefore, for all
practical purposes, the surroundings are that
portion of the remaining universe which can
interact with the system. Usually, the region
of space in the neighbourhood of the system
constitutes its surroundings.
For example, if we are studying the
reaction between two substances A and B
kept in a beaker, the beaker containing the
reaction mixture is the system and the room
where the beaker is kept is the surroundings
(Fig. 5.1).
Fig. 5.1 System and the surroundings
the system from the surroundings is called
boundary. This is designed to allow us to
control and keep track of all movements of
matter and energy in or out of the system.
5.1.2 Types of the system
We, further classify the systems according
to the movements of matter and energy in or
out of the system.
1. Open System
In an open system, there is exchange of energy
and matter between system and surroundings
[Fig. 5.2 (a)]. The presence of reactants in an
open beaker is an example of an open system*.
Here the boundary is an imaginary surface
enclosing the beaker and reactants.
2. Closed System
In a closed system, there is no exchange of
matter, but exchange of energy is possible
between system and the surroundings
[Fig. 5.2 (b)]. The presence of reactants in a
closed vessel made of conducting material
e.g., copper or steel is an example of a closed
system.
Fig. 5.2  Open, closed and isolated systems.
Note that the system may be defined
by physical boundaries, like beaker or test
tube, or the system may simply be defined
by a set of Cartesian coordinates specifying
a particular volume in space. It is necessary
to think of the system as separated from the
surroundings by some sort of wall which may
be real or imaginary. The wall that separates
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
Unit 5.indd   137 9/12/2022   11:53:34 AM
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chemIstry 138
3. Isolated System
In an isolated system, there is no exchange
of energy or matter between the system and
the surroundings [Fig. 5.2 (c)]. The presence
of reactants in a thermos flask or any other
closed insulated vessel is an example of an
isolated system.
5.1.3 The state of the system
The system must be described in order to
make any useful calculations by specifying
quantitatively each of the properties such as
its pressure (p), volume (V), and temperature
(T ) as well as the composition of the system.
We need to describe the system by specifying
it before and after the change. You would
recall from your Physics course that the
state of a system in mechanics is completely
specified at a given instant of time, by the
position and velocity of each mass point of
the system. In thermodynamics, a different
and much simpler concept of the state of a
system is introduced. It does not need detailed
knowledge of motion of each particle  because,
we deal with average measurable properties of
the system. We specify the state of the system
by state functions or state variables.
The state of a thermodynamic system is
described by its measurable or macroscopic
(bulk) properties. We can describe the state
of a gas by quoting its pressure (p), volume
(V), temperature (T ), amount (n) etc. Variables
like p, V, T are called state variables or state
functions because their values depend only
on the state of the system and not on how it
is reached. In order to completely define the
state of a system it is not necessary to define
all the properties of the system; as only a
certain number of properties can be varied
independently. This number depends on the
nature of the system. Once these minimum
number of macroscopic properties are fixed,
others automatically have definite values.
The state of the surroundings can never
be completely specified; fortunately it is not
necessary to do so.
5.1.4 The internal energy as a state
Function
When we talk about our chemical system
losing or gaining energy, we need to introduce
a quantity which represents the total energy
of the system. It may be chemical, electrical,
mechanical or any other type of energy you
may think of, the sum of all these is the energy
of the system. In thermodynamics, we call it
the internal energy, U of the system, which
may change, when
•	 heat passes into or out of the system,
•	 work is done on or by the system,
•	 matter enters or leaves the system.
These systems are classified accordingly
as you have already studied in section 5.1.2.
(a) Work
Let us first examine a change in internal energy
by doing work. We take a system containing
some quantity of water in a thermos flask
or in an insulated beaker. This would not
allow exchange of heat between the system
and surroundings through its boundary and
we call this type of system as adiabatic. The
manner in which the state of such a system
may be changed will be called adiabatic
process. Adiabatic process is a process in
which there is no transfer of heat between
the system and surroundings. Here, the wall
separating the system and the surroundings
is called the adiabatic wall (Fig. 5.3).
Fig. 5.3 An adiabatic system which does not
permit the transfer of heat through its
boundary.
Let us bring the change in the internal
energy of the system by doing some work on
it. Let us call the  initial state of the system
as state A and its temperature as  T
A
. Let
the internal energy of the system in state A
be called U
A
. We can change the state of the
system in two different ways.
Unit 5.indd   138 9/12/2022   11:53:36 AM
2024-25
THERMODYNAMICS         139
One way: We do some mechanical work, say
1 kJ, by rotating a set of small paddles and
thereby churning water. Let the new state
be called B state and its temperature, as
T
B
. It is found that T
B
> T
A
and the change
in temperature, ?T = T
B
–T
A
. Let the internal
energy of the system in state B be U
B
and the
change in internal energy, ?U =U
B
– U
A
.
Second way: We now do an equal amount
(i.e., 1kJ) electrical work with the help of an
immersion rod  and note down the temperature
change. We find that the change in temperature
is same as in the earlier case, say, T
B
– T
A
.
In fact, the experiments in the above
manner were done by J. P. Joule between
1840–50 and he was able to show that a
given amount of work done on the system,
no matter how it was done (irrespective of
path) produced the same change of state, as
measured by the change in the temperature
of the system.
So, it seems appropriate to define a
quantity, the internal energy U, whose value
is characteristic of the state of a system,
required to
bring about a change of state is equal to the
difference between the value of U in one state
and that in another state, ?U i.e.,
?U =U
2
–U
1
= w
Therefore, internal energy, U, of the
system is a state function.
By conventions of IUPAC in chemical
thermodynamics. The positive sign expresses
that w
is positive when work is done on the
system and the internal energy of system
increases. Similarly, if the work is done by the
system, w
will be negative because internal
energy of the system decreases.
Can you name some other familiar state
functions? Some of other familiar state
functions are V, p, and  T. For example, if we
bring a change in temperature of the system
from 25°C to 35°C, the change in temperature
is 35°C–25°C = +10°C, whether we go straight
up to 35°C or we cool the system for a few
degrees, then take the system to the final
temperature. Thus, T is a state function and
the change in temperature is independent of
the route taken. Volume of water in a pond, for
example, is a state function, because change
in volume of its water is independent of the
route by which water is filled in the pond,
either by rain or by tubewell or by both.
(b) Heat
We can also change the internal energy
of a system by transfer of heat from the
surroundings to the system or vice-versa
without expenditure of work. This exchange
of energy, which is a result of temperature
difference is called heat, q. Let us consider
bringing about the same change in temperature
(the same initial and final states as before
in section 5.1.4 (a) by transfer of heat
We take water at temperature, T
A
in a
container having thermally conducting walls,
say made up of copper and enclose it in a
huge heat reservoir at temperature, T
B
. The
heat absorbed by the system (water), q can be
measured in terms of temperature difference,
T
B
– T
A
. In this case change in internal energy,
?U = q, when no work is done at constant
volume.
By conventions of IUPAC in chemical
thermodynamics. The  q is positive, when heat
is transferred from the surroundings to the
system and the internal energy of the system
increases and q is negative when heat is
transferred from system to the surroundings
resulting in decrease of the internal energy of
the system.
* Earlier negative sign was assigned when the work is done on the system and positive sign when the work is done by the
system. This is still followed in physics books, although IUPAC has recommended the use of new sign convention.
Fig. 5.4 A system which allows heat transfer
through its boundary.
Unit 5.indd   139 9/12/2022   11:53:37 AM
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chemIstry 140
(c) The general case
Let us consider the general case in which
a change of state is brought about both by
doing work and by transfer of heat. We write
change in internal energy for this case as:
?U = q  + w           (5.1)
For a given change in state, q and w can
vary depending on how the change is carried
out. However, q +w = ?U will depend only on
initial and final state. It will be independent
of the way the change is carried out. If there
is no transfer of energy as heat or as work
(isolated system) i.e., if w = 0 and q = 0, then
? U = 0.
The equation 5.1 i.e., ?U = q + w is
mathematical statement of the first law of
thermodynamics, which states that
The energy of an isolated system is
constant.
It is commonly stated as the law of conservation
of energy i.e., energy can neither be created
nor be destroyed.
note: There is considerable difference between
the character of the thermodynamic property
energy and that of a mechanical property such
as volume. We can specify an unambiguous
(absolute) value for volume of a system in a
particular state, but not the absolute value of
the internal energy. However, we can measure
only the changes in the internal energy, ?U
of the system.
Problem 5.1
Express the change in internal energy
of a system when
(i) No heat is absorbed by the system
from the surroundings, but work
(w) is done on the system. What
type of wall does the system have ?
(ii) No work is done on the system,
but q  amount of heat is taken out
from the system and given to the
surroundings. What type of wall
does the system have?
(iii) w amount of work is done by the
system and q amount of heat is
supplied to the system. What type
of system would it be?
solution
(i)  ? U = w
(ii) ? U = – q, thermally conducting
walls
(iii) ? U = q – w, closed system.
5.2 aPPlica Tions
Many chemical reactions involve the generation
of gases capable of doing mechanical work or
the generation of heat. It is important for us
to quantify these changes and relate them
to the changes in the internal energy. Let us
see how!
5.2.1 Work
First of all, let us concentrate on the nature of
work a system can do. We will consider only
mechanical work i.e., pressure-volume work.
For understanding pressure-volume
work, let us consider a cylinder which
contains one mole of an ideal gas fitted with
a frictionless piston. Total volume of the gas
is V
i
and pressure of the gas inside is p. If
external pressure is p
ex
which is greater  than
p, piston is moved inward till the pressure
Fig. 5.5 (a) Work done on an ideal gas in a
cylinder when it is compressed by
a constant external pressure, p
ex

(in single step) is equal to the shaded
area.
Unit 5.indd   140 9/12/2022   11:53:37 AM
2024-25
```

## Chemistry Class 11

172 videos|306 docs|152 tests

## FAQs on NCERT Textbook: Thermodynamics - Chemistry Class 11 - NEET

 1. What is thermodynamics?
Ans. Thermodynamics is a branch of physics that deals with the study of energy and its transformation from one form to another. It focuses on understanding the behavior of heat and work in relation to different systems.
 2. What are the laws of thermodynamics?
Ans. The laws of thermodynamics are fundamental principles that govern energy and its interactions in various systems. They include: - The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transferred or transformed. - The second law of thermodynamics states that the entropy of an isolated system always increases over time. - The third law of thermodynamics states that it is impossible to reach absolute zero temperature through a finite number of processes.
 3. How does thermodynamics apply to real-life situations?
Ans. Thermodynamics has numerous real-life applications. It is used in the design and optimization of engines, refrigeration systems, power plants, and chemical processes. It helps in understanding the efficiency and performance of these systems and enables engineers to make improvements and reduce energy wastage.
 4. What is the difference between heat and work in thermodynamics?
Ans. In thermodynamics, heat and work are two forms of energy transfer. Heat is the transfer of energy due to a temperature difference, while work is the transfer of energy due to a force acting through a distance. Heat transfer occurs spontaneously, following the temperature gradient, while work transfer requires an external agent to apply a force.
 5. How is the efficiency of an engine calculated in thermodynamics?
Ans. The efficiency of an engine is calculated using the formula: Efficiency = (Useful work output / Heat input) * 100% The useful work output is the work done by the engine, and the heat input is the heat energy supplied to the engine. The efficiency gives a measure of how effectively the engine converts heat energy into useful work, with a higher percentage indicating better efficiency.

## Chemistry Class 11

172 videos|306 docs|152 tests

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