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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
Rationalised 2023-24
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
Rationalised 2023-24
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 
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. 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
Rationalised 2023-24
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
Rationalised 2023-24
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 
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. 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
Rationalised 2023-24
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
Rationalised 2023-24
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
Rationalised 2023-24
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 
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. 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
Rationalised 2023-24
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
Rationalised 2023-24
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, 
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.,
?U =U
2
 –U
1
= w
ad
Therefore, internal energy, U, of the 
system is a state function. 
By conventions of IUPAC in chemical 
thermodynamics. The positive sign expresses 
that w
ad  
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
ad
 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 
through thermally conducting walls instead 
of adiabatic walls (Fig. 5.4). 
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
Rationalised 2023-24
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
Rationalised 2023-24
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 
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. 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
Rationalised 2023-24
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
Rationalised 2023-24
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, 
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.,
?U =U
2
 –U
1
= w
ad
Therefore, internal energy, U, of the 
system is a state function. 
By conventions of IUPAC in chemical 
thermodynamics. The positive sign expresses 
that w
ad  
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
ad
 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 
through thermally conducting walls instead 
of adiabatic walls (Fig. 5.4). 
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
Rationalised 2023-24
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 
ad
, wall is adiabatic
(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
Rationalised 2023-24
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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.
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