CHEMICAL

THERMODYNAMICS

Mrs.Vaishali Mahajan

Spontaneous Processes (changes):

Once the process begins, it proceeds

automatically without the need to do work on

the system.

The opposite of every Spontaneous Process is

a

Non-Spontaneous Process

that can only proceed if external work is done

on the system.

• In a

Reversible Process,the system undergoes changes such that the system plus it’s surroundings can be put back in their original states by exactly reversing the process.

• In a

Reversible Process,

changes proceed in

infinitesimally small steps, so

that the system is infinitesimally close to equilibrium at every step.

The First Law of Thermodynamics states that energy can be

converted from one form to another with the interaction of

heat, work and internal energy, but it cannot be created nor

destroyed, under any circumstances. Mathematically, this is

represented as

ΔU=q+w

ΔU is the total change in internal energy of a system,

q is the heat exchanged between a system and its

surroundings, and

w is the work done by or on the system.

First Law of Thermodynamics

LIMITATIONS OF FIRST LAW OF

THERMODYNAMICS It doesn’t tell the extent and direction of the convertibility

of one form of energy to another.

It doesn’t tell why chemical reactions do not proceed to completion.

It doesn’t tell why natural processes are unidirectional.

It doesn’t explain the feasibility and spontaneity of a process.

It says the equivalency of work and heat. But it has been observed that the work can be completely transformed into heat but heat cannot be transformed into work without permanent change in the system or surrounding which is not explained by the law.

CARNOT CYCLE

The Carnot cycle consists of the following four processes:

A reversible isothermal gas expansion process. In this process, the ideal gas in the system absorbs qin amount heat from a heat source at a high temperature Th, expands and does work on surroundings.

A reversible adiabatic gas expansion process. In this process, the system is thermally insulated. The gas continues to expand and do work on surroundings, which causes the system to cool to a lower temperature, Tl.

A reversible isothermal gas compression process. In this process, surroundings do work to the gas at Tl, and causes a loss of heat, qout.

A reversible adiabatic gas compression process. In this process, the system is thermally insulated. Surroundings continue to do work to the gas, which causes the temperature to rise back to Th.

Figure 1: An ideal gas-piston model of the Carnot cycle.

The P-V diagram of the Carnot cycle is shown in Figure 2.

In isothermal processes I and III, ∆U=0 because ∆T=0. In adiabatic processes

II and IV, q=0.

EFFICIENCY OF CARNOT ENGINE

Hence, the total work done by the gas on the environment in one complete cycle is given by:

W=W1→2+W2→3+W3→4+W4→1W=μRT1lnv2v1−μRT2lnv3v4

Net efficiency = Net workdone by the gas Heat absorbed by the gas

Net efficiency = WQ1=Q1−Q2Q1=1−Q2Q1=1−T2T1lnv3v4lnv2v1

Since the step 2–>3 is an adiabatic process, we can write T1V2Ƴ-1 =

T2V3Ƴ-1

Or,

V2/v3=(T2/T1)1γ−1

Similarly, for the process 4–>1, we can write

V1/v2=(T2/T1)1γ−1

This implies,

V2/v3=v1/v2

So, the expression for net efficiency of carnot engine reduces to:

Net efficiency=1−T2/T1

CONCLUSION FROM CARNOT CYCLE

The Carnot cycle provides a simple method to determine

the efficiency of any heat engine.There is no engine

available which when working between the same two

temperature can convert thermal to mechanical energy

with grater efficiency than the Carnot engine .Thus

Carnot cycle efficiency is used to find out the maximum

conversion of thermal mechanical energy that can be

expected for any real engine.

No heat engine can be constructed that can have unit

efficiency.

SECOND LAW OF THERMODYNAMICS

Rudolf Clausius was a German physicist, and he developed the Clausius statement, which says "Heat generally cannot flow spontaneously from a material at a lower temperature to a material at a higher temperature.“

Lord Kelvin, formulated the Kelvin statement, which states "It is impossible to convert heat completely in a cyclic process." This means that there is no way for one to convert all the energy of a system into work, without losing energy.

The second law of thermodynamics can be

understood through considering these

processes:

Hot pans cool down when taken out from the

stove.

Ice cubes melt in a warm room.

ENTROPY

Entropy just measures the spontaneous

dispersal of energy: or how much energy is

spread out in a process as a function of

temperature.

Entropy a measure of disorder in the physical system.

the second law of thermodynamics – the universe, or in any isolated system, the degree of disorder (entropy) can only increase.

the movement towards a disordered state is a spontaneous process.

Entropy = “ energy dispersed”/ T

Entropy couldn't be expressed without the

inclusion of absolute temperature.

Entropy change ΔS shows us exactly how

important to a system is a dispersion of a

given amount of energy.

THIRD LAW OF THERMODYNAMICS

“It is impossible to reach a temperature of

absolute zero.”

On the Kelvin Temperature Scale,

T = 0 Kis often referred to as

“Absolute Zero”

THIRD LAW OF THERMODYNAMICS

The entropy of a true equilibrium state of a system at T = 0 K is zero.”

(Strictly speaking, this is true only if the quantum mechanical ground state is non-degenerate. If it is

degenerate, the entropy at T = 0 K is a small constant, not 0!)

This is Equivalent to:“It is impossible to reduce the temperature of a system to

T = 0 K using a finite number of processes.”

you can pump heat out of a refrigerator (to

make ice cubes), but the heat is placed in

the house and the entropy of the house

increases, even though the local entropy of

the ice cube tray decreases.

“Energy can neither be created nor

destroyed. It can only be changed from

one form to another.”

• The 1st Law of Thermodynamics is

Conservation of Total Energy!!!!

• It says nothing about

The Direction of Energy Transfer!

Rudolf Clausius, 1850

Summary

VARIOUS STATEMENTS OF THE SECOND LAW

1. “No series of processes is possible whose

sole result is the absorption of heat from a

thermal reservoir and the complete

conversion of this energy to work.” That is

There are no perfect engines!

2. “All processes occuring in nature are

spontaneous and therefore irreversible

unless interfered by some external agency.”

3. “It is impossible to transfer heat from a cold to

hot reservoir without at the same time

converting a certain amount of work to heat.”-

4. By Clausius.

3. “It is impossible to extract an amount of heat QH

from a hot reservoir and use it all to do work W.

Some amount of heat QC must be exhausted to

a cold reservoir.”

The Kelvin-Planck

THIRD LAW OF THERMODYNAMICS

“The entropy of a true equilibrium state of a

system at T = 0 K is zero.”

The entropy of a perfect crystal of every

element or compound is zero at 0 K.