CHEMISTRY160
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); and
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
2020-21
THERMODYNAMICS 161
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.
2020-21