# Thermodynamics

William Thomson coined the term thermodynamics in 1749. It is derived from two Greek words “thermes” meaning heat, and “dynamikos” meaning powerful. When we say the word dynamic we think of motion or movement and energy. Thus, the term thermodynamics means heat movement or heat flow.

## What is Thermodynamics?

Thermodynamics is the branch of physics which is concerned with the relationship between other forms of energy and heat. To be specific, it explains how the thermal energy is converted to or from other forms of energy and how matter is affected by this process. Thermal energy is the energy that comes from heat. This heat is generated by the movement of tiny particles within an object. The faster these particles move, the more heat is generated.

### Thermodynamics Timeline:

Thermodynamics has many sections under it and is considered as a broad subject because it deals with topics that exist all around us and thus classification becomes necessary.

• Classical Thermodynamics – In this section, the behaviour of matter is analyzed with a macroscopic approach. Units such as temperature and pressure are taken into consideration which helps the individuals to calculate other properties and to predict the characteristics of the matter that is undergoing the process.
• Statistical Thermodynamics – In this section, every molecule is under the spotlight i.e. the properties of each and every molecule and ways in which they interact are taken into consideration to characterize the behaviour of a group of molecules.
• Pure Component Thermodynamics – As the name itself states, this section tries to describe the behaviour of a system that has an unadulterated or pure constituent.
• Solution Thermodynamics – This section attempts to describe the behaviour of a system that contains more than one chemical in the mixture.

Here, we will focus on the Classical part of the thermodynamics in our curriculum.

## Laws of Thermodynamics:

The laws of thermodynamics define the fundamental physical quantities like energy, temperature and entropy that characterise thermodynamic systems at thermal equilibrium. The laws represent how these quantities behave under various circumstances. The four laws of thermodynamics are given below:

To learn in details about all the laws of thermodynamics visit the links given below.

### Zeroth Law of Thermodynamics:

The Zeroth Law is the basis for the measurement of temperature. It states that “two bodies which are in thermal equilibrium with a third body are in thermal equilibrium with each other.” As an example of the zeroth law of thermodynamics, 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. Now 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. From 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 which is also known as the conservation of energy principle states that energy can neither be created nor destroyed, but it can be changed from one form to another. This law may seem abstract but if we look at a few examples of the first law of thermodynamics, we will get a clearer idea. Following are a few examples:

• Fans convert the electrical energy to mechanical energy.
• 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 when we scroll through this page.

### Second Law of Thermodynamics:

The second law of thermodynamics states that Energy in the form of heat only flows from regions of higher temperature to that of lower temperature. Many individuals take this statement lightly and for granted, but it has an extensive impact and consequence. This is why it costs money to run an air conditioner. The human body obeys the second law of thermodynamics too. One of the examples of the second law of thermodynamics can be sweating in a crowded room. Assume yourself to be in a small room full of people. You are very likely to feel warm and start sweating. Sweating is a mechanism the human body uses to cool itself. Here, the heat from your body is transferred to sweat. As the sweat absorbs more and more heat from the body it evaporates and transfers heat to the surrounding air, thereby, heating up the temperature of the room.

### Third Law of Thermodynamics:

The Third Law states, “The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).” Entropy is sometimes called “waste energy,” i.e., the energy that is unable to do work, and since there is no heat energy whatsoever at absolute zero, there can be no waste energy.

Let us consider steam as an example for the third law of thermodynamics. We know that steam is a gaseous state of water at higher temperatures. In this state, the molecules within it move freely and have a high entropy. 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. 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. As the temperature of the ice further reduces, the movement of the molecules in them are restricted more and the entropy of the substance goes on decreasing. 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.

#### What is Enthalpy?

Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals the total content of heat 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, E, and the product of the pressure, P, and volume, V, of the system.

H = E + PV

#### What is Entropy?

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 of a system. 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.

### Different Measures of Energy:

Internal Energy: $U=\int TdS\;-\;PdV+\sum _{i}\mu _{i}dN_{i}$

Helmholtz free energy:  F = U – TS

Enthalpy: H = U + PV

Gibbs Free Energy: G = U + PV – TS

### Solved Examples:

Example 1: Calculate $\Delta G$

at 290 K for the following reaction:

$2NO_{(g)}+O_{2(g)}\rightarrow 2NO_{2(g)}$

Given

$\Delta H$ = -120kJ

$\Delta S=-150JK^{-1}$

Solution: To make the unit of $\Delta S$ same as $\Delta H$, we have to convert the unit of $\Delta S$ as follows,

$\Delta S\;=\;-150:J/K(\frac{1: kJ}{1000: J}) \Rightarrow \Delta S\;=\;-0.15kJ/K$

We know that,

$G\;=\;U\;+\;PV\;-\;TS \Rightarrow \Delta G\;=\;\Delta H\;-\;T\Delta S$

So,

$\Delta G\;=\;-120: kJ\;-\;(290K)\;(-0.150: kJ/K) \Rightarrow \Delta G\;=\;-120: kJ\;+\;43kJ \Rightarrow \Delta G=-77: kJ$

Therefore, $\Delta G$  is -77kJ.

### Important Questions in Thermodynamics

Q.1) What are the different laws of thermodynamics?

• The first law of thermodynamics, which is also known as the Law of Conservation of Energy, states that energy can neither be created nor be destroyed, it can only be transferred from one form to another.
• The second law of thermodynamics says that the entropy of any isolated system always increases.
• The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.
• The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

Q.2) What is basic thermodynamics?

In simple terms, thermodynamics deals with the transfer of energy from one form to another.

Q.3) What are the four laws of thermodynamics?

[Refer to Question Number 1]

Q.4) Who is the father of thermodynamics?

Nicolas Léonard Sadi Carnot is often described as the “Father of Thermodynamics.”

Q.5) What is entropy in simple terms?

Entropy is the measure of the number of possible arrangements the atoms in a system can have.

Q.6) How does thermodynamics work?

[Refer to Question Number 2]

Q.7) What is entropy in thermodynamics?

[Refer to Question Number 5]

Q.8) What is the second law of thermodynamics in simple terms?

[Refer to Question Number 1]

Q.9) Can energy be destroyed or lost?

[Refer to Question Number 1]

Q.10) What relationship is discussed in the Zeroth Law of Thermodynamics?

[Refer to Question Number 2]

Q.11) What is an example of negative work?

When you’re pushing an object along the floor, the work done by Kinetic Friction is negative.

Q.12) What are the thermodynamic process?

A thermodynamic process is a passage of a thermodynamic system from an initial to a final state of thermodynamic equilibrium.

Q.13) What are the thermodynamic properties?

Thermodynamic properties may be extensive or intensive. Intensive properties are properties that do not depend on the quantity of matter. For example, pressure and temperature are intensive properties. volume, energy, and enthalpy are examples of extensive properties. Their value depends on the mass of the system.

#### Practise This Question

One mole of an ideal gas is taken from a to b along two paths denoted by the solid and the dashed lines as shown in the graph below. If the work done along the solid line path is Ws and that along the dotted line path is Wd, then the integter closest to the ratio Wd/Ws is
(2010)