NCERT Textbook - Thermodynamics, Chemistry | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


CHEMISTRY 154
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
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);
• • • • • establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
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 6
© NCERT
not to be republished
Page 2


CHEMISTRY 154
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
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);
• • • • • establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
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 6
© NCERT
not to be republished
THERMODYNAMICS 155
6.1  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.
6.1.1 The System and the Surroundings
A system in thermodynamics refers to that
part of universe in which observations are
made and remaining universe constitutes 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. 6.1).
be real or imaginary. The wall that separates
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.
6.1.2 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. 6.2 (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. 6.2 (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. 6.2  Open, closed and isolated systems.
Fig. 6.1 System and the surroundings
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
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
© NCERT
not to be republished
Page 3


CHEMISTRY 154
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
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);
• • • • • establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
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 6
© NCERT
not to be republished
THERMODYNAMICS 155
6.1  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.
6.1.1 The System and the Surroundings
A system in thermodynamics refers to that
part of universe in which observations are
made and remaining universe constitutes 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. 6.1).
be real or imaginary. The wall that separates
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.
6.1.2 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. 6.2 (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. 6.2 (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. 6.2  Open, closed and isolated systems.
Fig. 6.1 System and the surroundings
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
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
© NCERT
not to be republished
CHEMISTRY 156
3. Isolated System
In an isolated system, there is  no exchange of
energy or matter between the system and the
surroundings [Fig. 6.2 (c)]. The presence of
reactants in a thermos flask or any other closed
insulated vessel is an example of an isolated
system.
6.1.3 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. W e 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.
6.1.4 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 6.1.2.
(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 6.3).
Fig. 6.3 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
© NCERT
not to be republished
Page 4


CHEMISTRY 154
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
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);
• • • • • establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
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 6
© NCERT
not to be republished
THERMODYNAMICS 155
6.1  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.
6.1.1 The System and the Surroundings
A system in thermodynamics refers to that
part of universe in which observations are
made and remaining universe constitutes 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. 6.1).
be real or imaginary. The wall that separates
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.
6.1.2 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. 6.2 (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. 6.2 (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. 6.2  Open, closed and isolated systems.
Fig. 6.1 System and the surroundings
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
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
© NCERT
not to be republished
CHEMISTRY 156
3. Isolated System
In an isolated system, there is  no exchange of
energy or matter between the system and the
surroundings [Fig. 6.2 (c)]. The presence of
reactants in a thermos flask or any other closed
insulated vessel is an example of an isolated
system.
6.1.3 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. W e 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.
6.1.4 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 6.1.2.
(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 6.3).
Fig. 6.3 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
© NCERT
not to be republished
THERMODYNAMICS 157
internal energy of the system in state A be
called U
A
. We can change the state of the system
in two different ways.
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,
whereby the adiabatic work, w
ad
 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.,
2 1 ad
= = w ? - U U U
Therefore, internal energy, U, of the system
is a state function.
The positive sign expresses that w
ad  
is
positive when work is done on the system.
Similarly, if the work is done by the system,w
ad
will be negative.
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 6.1.4 (a) by transfer of heat through
thermally conducting walls instead of
adiabatic walls (Fig. 6.4).
Fig. 6.4 A system which allows heat transfer
through its boundary.
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.
The  q is positive, when heat is transferred
from the surroundings to the system and q is
negative when heat is transferred from
system to the surroundings.
(c)  The general case
Let us consider the general case in which a
change of state is brought about both by
© NCERT
not to be republished
Page 5


CHEMISTRY 154
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
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);
• • • • • establish relationship between
?G and spontaneity, ?G and
equilibrium constant.
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 6
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THERMODYNAMICS 155
6.1  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.
6.1.1 The System and the Surroundings
A system in thermodynamics refers to that
part of universe in which observations are
made and remaining universe constitutes 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. 6.1).
be real or imaginary. The wall that separates
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.
6.1.2 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. 6.2 (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. 6.2 (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. 6.2  Open, closed and isolated systems.
Fig. 6.1 System and the surroundings
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
*  We could have chosen only the reactants as system then walls of the beakers will act as boundary.
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CHEMISTRY 156
3. Isolated System
In an isolated system, there is  no exchange of
energy or matter between the system and the
surroundings [Fig. 6.2 (c)]. The presence of
reactants in a thermos flask or any other closed
insulated vessel is an example of an isolated
system.
6.1.3 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. W e 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.
6.1.4 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 6.1.2.
(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 6.3).
Fig. 6.3 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
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THERMODYNAMICS 157
internal energy of the system in state A be
called U
A
. We can change the state of the system
in two different ways.
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,
whereby the adiabatic work, w
ad
 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.,
2 1 ad
= = w ? - U U U
Therefore, internal energy, U, of the system
is a state function.
The positive sign expresses that w
ad  
is
positive when work is done on the system.
Similarly, if the work is done by the system,w
ad
will be negative.
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 6.1.4 (a) by transfer of heat through
thermally conducting walls instead of
adiabatic walls (Fig. 6.4).
Fig. 6.4 A system which allows heat transfer
through its boundary.
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.
The  q is positive, when heat is transferred
from the surroundings to the system and q is
negative when heat is transferred from
system to the surroundings.
(c)  The general case
Let us consider the general case in which a
change of state is brought about both by
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CHEMISTRY 158
doing work and by transfer of heat. We write
change in internal energy for this case as:
      ?U = q  + w (6.1)
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 6.1 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 6.1
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 
ad
, wall is adiabatic
(ii) ? U = – q, thermally conducting walls
(iii) ? U = q – w, closed system.
6.2 APPLICATIONS
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!
6.2.1 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
inside becomes equal to p
ex
. Let this change
Fig. 6.5(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.
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