UNIT-4, Biochemistry, 2021-2022, NEP syllabus.

Thermodynamics:
Thermodynamics is the study of the relations between heat, work, temperature, and
energy. The laws of thermodynamics describe how the energy in a system changes and whether
the system can perform useful work on its surroundings.
Or
Thermodynamics deals with the concepts of heat and temperature and the inter-conversion of
heat and other forms of energy. The four laws of thermodynamics govern the behaviour of
these quantities and provide a quantitative description. William Thomson, in 1749, coined the
term thermodynamics.
Definition: Thermodynamics in physics is a branch that deals with heat, work and temperature,
and their relation to energy, radiation and physical properties of matter .
Thermodynamic systems:
1. Systems
2. Boundary
3. Surroundings

System: A thermodynamics system is defined a definite space or area on which the study of
energy transfer and energy conversations is made.
Boundary: The system and surroundings are seated by boundary. It may be fixed or movable
or imaginary. It will not occupy any volume or mass in space.
Surroundings: Both mass and energy can cross the boundary of a control volume which is
called control surface.
Classifications of thermodynamics systems

Isolated System – An isolated system cannot exchange both energy and mass with its
surroundings. The universe is considered an isolated system. Thermo flask
Closed System – Across the boundary of the closed system, the transfer of energy takes place
but the transfer of mass doesn’t take place. A refrigerator is an example of closed system.
Open System – In an open system, the mass and energy both may be transferred between the
system and surroundings. An open system can exchange both energy and matter with its
surroundings. The stovetop example would be an open system, because heat and water vapor
can be lost to the air.
What is Enthalpy?
Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy
equals the total heat content of a system, equivalent to the system’s internal energy plus the
product of volume and pressure.
Mathematically, the enthalpy, H, equals the sum of the internal energy (Matter enters or
leaves the system/Heat passes into or out of the system/Work is done on or by the system.
), E, and the product of the pressure, P, and volume, V, of the system.
H = E + PV
What is Entropy?
The entropy is a thermodynamic quantity whose value depends on the physical state or
condition of a system. In other words, it is a thermodynamic function used to measure the
randomness or disorder.
For example, the entropy of a solid, where the particles are not free to move, is less than the
entropy of a gas, where the particles will fill the container
Laws of Thermodynamics
Thermodynamics laws define the fundamental physical quantities like energy, temperature, and
entropy that characterize thermodynamic systems at thermal equilibrium. These
thermodynamics laws represent how these quantities behave under various circumstances.
There are four laws of thermodynamics and are given below:
 Zeroth law of thermodynamics
 First law of thermodynamics
 Second law of thermodynamics
 Third law of thermodynamics

Zeroth Law of Thermodynamics

The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium
with a separate third body, then the first two bodies are also in thermal equilibrium with each
other.
This means that if system A is in thermal equilibrium with system C and system B is also in
equilibrium with system C, then system A and B are also in thermal equilibrium.

An example demonstrating the Zeroth Law

Consider two cups A and B with boiling water. When a thermometer is placed in cup A, it gets
warmed up by the water until it reads 100 °C. When it read 100 °C, we say that the thermometer
is in equilibrium with cup A. When we move the thermometer to cup B to read the temperature,
it continues to read 100 °C. The thermometer is also in equilibrium with cup B. By keeping in
mind the zeroth law of thermodynamics; we can conclude that cup A and cup B are in
equilibrium with each other.
The zeroth law of thermodynamics enables us to use thermometers to compare the temperature
of any two objects that we like.
First Law of Thermodynamics
The first law of thermodynamics, also known as the law of conservation of energy, states that
energy can neither be created nor destroyed, but it can be changed from one form to another.
The first law of thermodynamics may seem abstract, but we will get a clearer idea if we look
at a few examples of the first law of thermodynamics.
First Law of Thermodynamics Examples:
1. Plants convert the radiant energy of sunlight to chemical energy through
photosynthesis. We eat plants and convert the chemical energy into kinetic energy while
we swim, walk, breathe, and scroll through this page.
2. Switching on light may seem to produce energy, but it is electrical energy that is
converted.
According to this law, some heat given to the system is used to change the internal energy
while the rest is used in doing work by the system.
It can be represented mathematically as

Where,

 ΔQ is the heat given or lost

 ΔU is the change in internal energy
 W is the work done

Second Law of Thermodynamics

The second law of thermodynamics states that the entropy in an isolated system always
increases. Any isolated system spontaneously evolves towards thermal equilibrium—the state
of maximum entropy of the system.
The entropy of the universe only increases and never decreases. Many individuals take this
statement lightly and for granted, but it has an extensive impact and consequence.
Visualizing the second law of thermodynamics
If a room is not tidied or cleaned, it invariably becomes more messy and disorderly with time.
When the room is cleaned, its entropy decreases, but the effort to clean it has resulted in
increased entropy outside the room exceeding the entropy lost.

Third Law of Thermodynamics

The third law of thermodynamics states that the entropy of a system approaches a constant
value as the temperature approaches absolute zero.
The entropy of a pure crystalline substance (perfect order) at absolute zero temperature is zero.
This statement holds true if the perfect crystal has only one state with minimum energy.
Third Law of Thermodynamics Examples:
Let us consider steam as an example to understand the third law of thermodynamics step
by step:
1. The molecules within it move freely and have high entropy.
2. If one decreases the temperature below 100 °C, the steam gets converted to water,
where the movement of molecules is restricted, decreasing the entropy of water.
3. When water is further cooled below 0 °C, it gets converted to solid ice. In this state, the
movement of molecules is further restricted and the entropy of the system reduces more.
4. As the temperature of the ice further reduces, the movement of the molecules in them
is restricted further and the entropy of the substance goes on decreasing.
5. When the ice is cooled to absolute zero, ideally, the entropy should be zero. But in
reality, it is impossible to cool any substance to zero.
Thermodynamics Examples in Daily Life
Whether we are sitting in an air-conditioned room or travelling in any vehicle, the application
of thermodynamics is everywhere. We have listed a few of these applications below:
 Different types of vehicles such as planes, trucks and ships work on the basis of the 2nd
law of thermodynamics.
 The three modes of heat transfer work on the basis of thermodynamics. The heat
transfer concepts are widely used in radiators, heaters and coolers.
 Thermodynamics is involved in the study of different types of power plants such as
nuclear power plants, thermal power plants.
Internal Energy (U): Absolute value of internal energy cannot be determined. Internal energy,
U of the system may change, when, Matter enters or leaves the system/Heat passes into or out
of the system/Work is done on or by the system.
Therefore Total energy of the system is constant.
If energy is released by system U is negative
If energy is absorbed by system U is positive.
U change in the internal energy.
Q is heat is transferred due to the difference in temperature.
U=q+W
Difference between Enthalpy and Entropy

Enthalpy is the measure of total heat present in the thermodynamic system where the pressure
is constant.
It is represented as ΔH=ΔE+PΔV where E is the internal energy.
Entropy is the measure of disorder in a thermodynamic system. It is represented
as ΔS=ΔQ/T where Q is the heat content and T is the temperature.