The four laws of thermodynamics provide a comprehensive framework for understanding all energy exchanges: Zeroth Law: If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. This defines temperature. First Law (Conservation of Energy): Energy cannot be created or destroyed. The total change in internal energy equals heat added to the system minus the work done by the system. Second Law: Energy conversions are inherently inefficient. In any spontaneous process, total entropy (disorder) increases, and it is impossible for heat to flow from a colder body to a hotter one without work input. Third Law: As a system approaches absolute zero temperature, the entropy approaches a minimum constant; for a perfect crystal, this minimum is zero. Understanding and applying these laws is critical to solving energy, heat, and work problems across science and engineering.

The four laws of thermodynamics provide a comprehensive framework for understanding all energy exchanges: Zeroth Law: If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. This defines temperature. First Law (Conservation of Energy): Energy cannot be created or destroyed. The total change in internal energy equals heat added to the system minus the work done by the system. Second Law: Energy conversions are inherently inefficient. In any spontaneous process, total entropy (disorder) increases, and it is impossible for heat to flow from a colder body to a hotter one without work input. Third Law: As a system approaches absolute zero temperature, the entropy approaches a minimum constant; for a perfect crystal, this minimum is zero. Understanding and applying these laws is critical to solving energy, heat, and work problems across science and engineering.

To explain the four laws of thermodynamics to a beginner audience, it’s essential to use relatable language, simple analogies, and clear step-by-step logic. Here’s an approach tailored for classroom, tutoring, or self-study contexts:The Four Laws of Thermodynamics: Beginner-Friendly Explanations0. Zeroth Law of Thermodynamics: The “Basic Temperature Rule” What it Means: If two objects are each at the same temperature as a third object, then they are at the same temperature as each other. Analogy: Imagine you have three cups of water. Cups A and B feel just as warm as cup C. That means A, B, and C all have the same temperature—even if you never compared A and B directly. Why it Matters: It lets us use thermometers and understand what temperature means. 1. First Law of Thermodynamics: Conservation of Energy What it Means: Energy can’t be created or destroyed; it can only change from one form to another. The increase in internal energy of a system equals the heat added to it minus the work done by the system. Analogy: Like a bank account—you can deposit money (heat) or withdraw it (work), but you can’t make money appear from nothing or vanish without action. Formula (for context): Δ U = Q − W Why it Matters: This law explains why when you heat a pot of water, the energy you put in (heat) either raises the water’s temperature (internal energy) or causes it to boil (doing work). 2. Second Law of Thermodynamics: Direction of Processes & Entropy What it Means: Heat naturally flows from hot things to cold things, never the reverse (unless you work at it). The universe tends toward more “disorder,” called entropy. Analogy: If you spill a box of marbles, they scatter everywhere on their own but never neatly arrange themselves without help—things move from order to disorder. Why it Matters: This is why refrigerators need electricity to move heat from inside (cold) to outside (warm) and why you can’t make a machine that’s 100% efficient. 3. Third Law of Thermodynamics: Absolute Zero and Perfect Order What it Means: As something gets closer and closer to absolute zero (the coldest temperature possible, -273.15°C), everything slows down and disorder (entropy) drops to a minimum. But absolute perfection can never be reached. Analogy: Imagine cleaning a messy room—no matter how organized you get, absolute, molecular-level perfection is impossible; there’s always a little disorder left. Why it Matters: This sets a limit for how cold things can really get and is important in fields like cryogenics and quantum physics.

To explain the four laws of thermodynamics to a beginner audience, it’s essential to use relatable language, simple analogies, and clear step-by-step logic. Here’s an approach tailored for classroom, tutoring, or self-study contexts:The Four Laws of Thermodynamics: Beginner-Friendly Explanations0. Zeroth Law of Thermodynamics: The “Basic Temperature Rule” What it Means: If two objects are each at the same temperature as a third object, then they are at the same temperature as each other. Analogy: Imagine you have three cups of water. Cups A and B feel just as warm as cup C. That means A, B, and C all have the same temperature—even if you never compared A and B directly. Why it Matters: It lets us use thermometers and understand what temperature means. 1. First Law of Thermodynamics: Conservation of Energy What it Means: Energy can’t be created or destroyed; it can only change from one form to another. The increase in internal energy of a system equals the heat added to it minus the work done by the system. Analogy: Like a bank account—you can deposit money (heat) or withdraw it (work), but you can’t make money appear from nothing or vanish without action. Formula (for context): Δ U = Q − W Why it Matters: This law explains why when you heat a pot of water, the energy you put in (heat) either raises the water’s temperature (internal energy) or causes it to boil (doing work). 2. Second Law of Thermodynamics: Direction of Processes & Entropy What it Means: Heat naturally flows from hot things to cold things, never the reverse (unless you work at it). The universe tends toward more “disorder,” called entropy. Analogy: If you spill a box of marbles, they scatter everywhere on their own but never neatly arrange themselves without help—things move from order to disorder. Why it Matters: This is why refrigerators need electricity to move heat from inside (cold) to outside (warm) and why you can’t make a machine that’s 100% efficient. 3. Third Law of Thermodynamics: Absolute Zero and Perfect Order What it Means: As something gets closer and closer to absolute zero (the coldest temperature possible, -273.15°C), everything slows down and disorder (entropy) drops to a minimum. But absolute perfection can never be reached. Analogy: Imagine cleaning a messy room—no matter how organized you get, absolute, molecular-level perfection is impossible; there’s always a little disorder left. Why it Matters: This sets a limit for how cold things can really get and is important in fields like cryogenics and quantum physics.

Thermodynamics & Thermochemistry Class 11 Chemistry Notes

Q: Thermodynamics is the scientific study of energy, heat, work, and how they interact within physical systems. It investigates how energy is transformed, transferred, and conserved, especially between heat and mechanical work. Central to thermodynamics are key properties like temperature, pressure, internal energy, and entropy. Fundamentally, thermodynamics explains why engines work, how refrigerators cool, and why chemical reactions release or absorb heat.By understanding the laws of thermodynamics, scientists and engineers can design systems from engines to power plants that convert energy efficiently and sustainably. Whether in chemistry, physics, or engineering, thermodynamics underpins much of modern technology and is vital for innovations in energy, environment, and materials science.

Thermodynamics is the scientific study of energy, heat, work, and how they interact within physical systems. It investigates how energy is transformed, transferred, and conserved, especially between heat and mechanical work. Central to thermodynamics are key properties like temperature, pressure, internal energy, and entropy. Fundamentally, thermodynamics explains why engines work, how refrigerators cool, and why chemical reactions release or absorb heat.By understanding the laws of thermodynamics, scientists and engineers can design systems from engines to power plants that convert energy efficiently and sustainably. Whether in chemistry, physics, or engineering, thermodynamics underpins much of modern technology and is vital for innovations in energy, environment, and materials science.

Q: Efficient study of thermodynamics involves reviewing your textbook and class notes, practicing a diverse set of problems, and regularly self-testing to identify weak areas. Break complex topics into basics—like laws, definitions, and key equations—before tackling more advanced applications or calculations. Time your practice under exam conditions to build confidence. Use summary sheets, diagrams, and concept maps to visualize relationships and flows. If you’re stuck, visit office hours or join a study group for discussion and clarification. Relate concepts to real-life systems to cement your understanding and make learning more engaging.

Efficient study of thermodynamics involves reviewing your textbook and class notes, practicing a diverse set of problems, and regularly self-testing to identify weak areas. Break complex topics into basics—like laws, definitions, and key equations—before tackling more advanced applications or calculations. Time your practice under exam conditions to build confidence. Use summary sheets, diagrams, and concept maps to visualize relationships and flows. If you’re stuck, visit office hours or join a study group for discussion and clarification. Relate concepts to real-life systems to cement your understanding and make learning more engaging.

Q: Thermodynamics is foundational for countless engineering systems and processes. In mechanical engineering, it powers car engines, turbines, and heating/cooling systems. Chemical engineers use thermodynamic principles to design reactors and optimize chemical production, ensuring maximum efficiency and safety. Electrical engineers apply thermodynamics in power generation and battery technology.Thermodynamics is also the basis for renewable energy systems, such as solar panels and wind turbines, and helps make buildings energy efficient through better insulation. In everyday life, thermodynamics explains how refrigerators keep food cold, air conditioners cool rooms, and why hot drinks cool down over time. Its principles are vital for technological innovation, energy conservation, and sustainability.

Thermodynamics is foundational for countless engineering systems and processes. In mechanical engineering, it powers car engines, turbines, and heating/cooling systems. Chemical engineers use thermodynamic principles to design reactors and optimize chemical production, ensuring maximum efficiency and safety. Electrical engineers apply thermodynamics in power generation and battery technology.Thermodynamics is also the basis for renewable energy systems, such as solar panels and wind turbines, and helps make buildings energy efficient through better insulation. In everyday life, thermodynamics explains how refrigerators keep food cold, air conditioners cool rooms, and why hot drinks cool down over time. Its principles are vital for technological innovation, energy conservation, and sustainability.

Q: Understanding thermodynamics is critical for designing efficient machines, reducing energy waste, and solving key scientific and engineering problems. It helps optimize processes—leading to lower costs and reduced environmental impacts. Thermodynamics is essential in developing new materials, advancing alternative energy sources, and ensuring that industrial processes are environmentally responsible. Additionally, it provides the tools to investigate and improve technologies ranging from engines and power plants to batteries and environmental control systems.

Understanding thermodynamics is critical for designing efficient machines, reducing energy waste, and solving key scientific and engineering problems. It helps optimize processes—leading to lower costs and reduced environmental impacts. Thermodynamics is essential in developing new materials, advancing alternative energy sources, and ensuring that industrial processes are environmentally responsible. Additionally, it provides the tools to investigate and improve technologies ranging from engines and power plants to batteries and environmental control systems.

Q: Entropy is a measure of a system’s disorder, randomness, or unavailability of energy to do work. Physically, it reflects the number of ways the internal energy can be distributed among the system's molecules. According to the second law, in any real process, the total entropy of an isolated system always increases, meaning natural processes are irreversible and tend toward greater disorder. Entropy is fundamental for predicting spontaneity of chemical reactions and for analyzing energy efficiency in machines and natural phenomena.

Entropy is a measure of a system’s disorder, randomness, or unavailability of energy to do work. Physically, it reflects the number of ways the internal energy can be distributed among the system's molecules. According to the second law, in any real process, the total entropy of an isolated system always increases, meaning natural processes are irreversible and tend toward greater disorder. Entropy is fundamental for predicting spontaneity of chemical reactions and for analyzing energy efficiency in machines and natural phenomena.

Q: Studying thermodynamics gives you a powerful lens to understand how the universe works—from the smallest chemical reaction to vast engineering systems. Mastering thermodynamics hones critical thinking and problem-solving skills, opens up career opportunities in engineering, chemistry, physics, and environmental fields, and enables you to design solutions for global energy and sustainability challenges.Exploring thermodynamics enriches your understanding of both everyday phenomena—like boiling water or keeping your house warm and complex, cutting-edge technologies like renewable energy and advanced materials. It’s a truly interdisciplinary subject that connects theory to application and invites lifelong curiosity.

Studying thermodynamics gives you a powerful lens to understand how the universe works—from the smallest chemical reaction to vast engineering systems. Mastering thermodynamics hones critical thinking and problem-solving skills, opens up career opportunities in engineering, chemistry, physics, and environmental fields, and enables you to design solutions for global energy and sustainability challenges.Exploring thermodynamics enriches your understanding of both everyday phenomena—like boiling water or keeping your house warm and complex, cutting-edge technologies like renewable energy and advanced materials. It’s a truly interdisciplinary subject that connects theory to application and invites lifelong curiosity.

Q: In thermodynamics, work refers to energy transfer that results from a force acting over a distance within a system. When a gas expands in a piston, for example, it performs work by pushing against atmospheric pressure. Thermodynamic work is crucial because it measures how much useful energy can be extracted from a system or needs to be provided to achieve a process such as compressing a gas, moving a piston, or running a turbine.Unlike heat (which relates to energy transferred due to temperature difference), work is organized energy transfer directly related to mechanical or electrical actions. Mastery of this concept is key for analyzing processes in engines, chemical reactions, and power production.

In thermodynamics, work refers to energy transfer that results from a force acting over a distance within a system. When a gas expands in a piston, for example, it performs work by pushing against atmospheric pressure. Thermodynamic work is crucial because it measures how much useful energy can be extracted from a system or needs to be provided to achieve a process such as compressing a gas, moving a piston, or running a turbine.Unlike heat (which relates to energy transferred due to temperature difference), work is organized energy transfer directly related to mechanical or electrical actions. Mastery of this concept is key for analyzing processes in engines, chemical reactions, and power production.

Q: While both physics and chemistry study thermodynamics, their focus often differs. Physics emphasizes large-scale energy transformations—like engines, heat flow, and power systems—typically in context of physical processes. Chemistry applies thermodynamics to understand how and why chemical reactions occur, predict reaction spontaneity, and analyze energy changes at the molecular level. Despite these emphases, the core laws and principles are universal, underpinning both disciplines with shared rules for energy conservation and entropy.

While both physics and chemistry study thermodynamics, their focus often differs. Physics emphasizes large-scale energy transformations—like engines, heat flow, and power systems—typically in context of physical processes. Chemistry applies thermodynamics to understand how and why chemical reactions occur, predict reaction spontaneity, and analyze energy changes at the molecular level. Despite these emphases, the core laws and principles are universal, underpinning both disciplines with shared rules for energy conservation and entropy.

Q: Many students find thermodynamics challenging because it combines abstract principles, mathematical reasoning, and real-world application. The topic introduces unfamiliar ideas like entropy, energy states, and thermodynamic cycles. Success in learning thermodynamics starts by mastering core definitions (such as temperature, heat, work, and entropy), understanding the four fundamental laws, and visualizing processes through diagrams and real-life examples.Practice is essential: work through a variety of problems, gradually increasing in difficulty. Don’t hesitate to seek help from peers or instructors, use visual aids, and relate principles to practical scenarios (like engines or refrigerators). Building a foundation of clear concepts before moving to advanced topics is key, as each principle builds on the last.

Many students find thermodynamics challenging because it combines abstract principles, mathematical reasoning, and real-world application. The topic introduces unfamiliar ideas like entropy, energy states, and thermodynamic cycles. Success in learning thermodynamics starts by mastering core definitions (such as temperature, heat, work, and entropy), understanding the four fundamental laws, and visualizing processes through diagrams and real-life examples.Practice is essential: work through a variety of problems, gradually increasing in difficulty. Don’t hesitate to seek help from peers or instructors, use visual aids, and relate principles to practical scenarios (like engines or refrigerators). Building a foundation of clear concepts before moving to advanced topics is key, as each principle builds on the last.

INTRODUCTION

The branch of science which deals with energy changes during chemical reaction is called chemical energetics and the branch which deals with heat changes in a chemical reaction is called thermochemistry.
This branch also help us to predict spontaneous nature of any thermodynamic process.

TERMS USED IN THERMODYNAMICS

System

The system is any region of space being investigated.
(i)   Closed system: A system which can only exchange energy but not matter with surroundings.
(ii)   Open system: A system which can exchange both matter and energy with the surrounding.
(iii)   Isolated system: A system which can neither exchange matter nor energy with the surroundings.

Surrounding

The surroundings are considered to be all other matter that can interact with the system.
System + surrounding = Universe

THERMODYNAMIC PROPERTIES

(i) Extensive property: Those variables whose values in any part of the divided system are different from the values of the entire system are called extensive properties, e.g. volume, energy, heat capacity, enthalpy, entropy etc.

STATE FUNCTIONS

Fundamental properties which determine the state of a system are referred to as state functions. The state properties do not depend upon the path followed, the change in these properties depend only on initial and final state of system, e.g. pressure, volume, temperature, enthalpy etc.

THERMODYNAMIC PROCESSES

Any systematic change in thermodynamic state functions of any system is known as thermodynamic process
(i)   Isothermal process: This occurs under constant temperature condition.
(ii)   Isobaric process: This occurs under constant pressure condition.
(iii)   Isochoric process: This occurs under constant volume condition.
(iv)   Adiabatic process: This occurs under the condition that heat can neither be added to nor removed from the system. These processes are also isoentropic processes.

INTERNAL ENERGY

It is the sum of all forms of energy of all the particles present in the system at a particular temperature. It includes translational energy, vibrational energy, rotation energy etc. The exact value of this energy is not known as it includes all types of energies of molecules constituting the given mass of matter. It is denoted by U(SI) or E.

FIRST LAW OF THERMODYNAMICS

The net energy change of a closed system is equal to heat absorbed plus the work done on the system. In other words the total energy of an isolated system remains conserved
ΔE = ΔQ + ΔW
where, ΔQ = heat absorbed
ΔE = change in internal energy
ΔW = work done on the system

ENTHALPY OR HEAT CONTENT

The energy stored in a substance that can be available in the form of heat.
H = E + PV
ΔH = ΔE + PΔV = ΔE + Δn_gRT
Δn_g = (n_p - n_r)
where n_p = number of moles of gaseous products
n_r = number of moles of gaseous reactants

Example 1 :
   If ΔH is the enthalpy change and ΔE the change in internal energy accompanying a gaseous reaction then
   (A)   ΔH is always less than ΔE
   (B)   ΔH is always greater than ΔE
   (C)   ΔH is less than ΔE if the number of moles of gaseous products is greater than the number of moles of gaseous reactants.
   (D)   ΔH is less than ΔE if the number of moles of gaseous products is less than the number of moles of gaseous reactants

Solution:   (D). As, ΔH = ΔE + Δn_gRT
       ΔH value is less than or greater than ΔE depending on the value of Δn_g which is the change in number of moles of the gaseous components.
       As result (A), (B) cannot be true
       ΔH<ΔE when Δn_g <0, hence (C) is also false only (D) is correct

HEAT CAPACITY

Heat capacity of a system is defined as the quantity of heat required to raise the temperature of the system by one degree.

where, C = Heat capacity
q = Heat absorbed
T₁ = Initial temperature
T₂ = Final temperature

If the amount of substance taken is 1 gm then C is called specific heat capacity and if the amount of substance taken is 1 mole then C is called molar heat capacity.
at constant volume
at constant pressure
Note:     (i)   Lighter elements have higher specific heat capacity and vice – versa. Specific heat of same substance in solid, liquid or gaseous state will be different.
(ii)   Specific heat of water in cal / goC is 1, that of ice is 0.5, and that of steam is 0.47.
(iii)   Boiling point of a liquid rises on increasing pressure whereas melting point can increases in some cases and can decreases also depending upon contraction and extension on melting. e.g. Melting points of ice and rubber (which contract on melting) fall with increase in pressure. Melting point of wax and sulphur (which expand on melting) rise with rise in pressure.
(iv)   At Boyle’s temperature, all real gases behave like a perfect gas.
(v)   At low pressure and high temperature a gas may show behaviour of ideal gas.
(vi)   A real gas spread in a very large volume also behaves as a real gas.
(vii) Saturated vapours do not obey gas laws whereas unsaturated vapours obey gas laws.

WORK DONE IN ISOTHERMAL REVERSIBLE PROCESS

where, V₁ = initial volume and V₂ = final volume

WORK DONE IN ADIABATIC REVERSIBLE EXPANSION

Adiabatic relations

PV^γ = constant
TV^γ-1 = constant

where,
   C_p = specific heat capacity at constant pressure
   C_v = specific heat capacity at constant volume
   R = universal gas constant
   γ = specific heat ratio of gas

KIRCHHOFF’S EQUATION

   ΔH₂ - ΔH₁ = ΔC_p(T₁ - T₂)(at constant pressure)
   ΔE₂ - ΔE₁ = ΔC_p(T₂ - T₁)(at constant volume)


Example 2 : A gas expands from 1.5 L to 6.5 L against a constant pressure of
0.5 atm and during this process the gas also absorbs 100 J of heat.
The change in the internal energy of the gas is
(A) 153.3 J   (B)   353.3 J
(C) –153.3 J   (D)   –353.3 J

Solution:   (C). ΔE = q-w
   =100J - (0.5 atm)(6.5L - 1.5L)
   =100J - (2.5 atm L)
   = –153.3 J

Example 3 : If a gas adiabatically changes from 1.0 L to 12.0 L against a constant
pressure of 0.75 atm. The ΔE of the gas is
(A) –853.9 J (B) 835.9 J
C) 853.9 J (D) –835.9 J

Solution:    (D).
   ΔE = W = -p_extΔV for adiabatic process
= -(0.75 atm)(12.0 L - 1.0 L)

Example 4 : Which of the statement is correct?
   (A) Slope of adiabatic P–V curve will be same as that of isothermal
one
   (B) Slope of adiabatic P–V curve is smaller than that in isothermal one
   (C) Slope of adiabatic P–V curve is larger than that in isothermal one
   (D) Slope of adiabatic P–V curve may be smaller or larger depending
on the value V

Solution: (C).

   Slope of PV curve, for isothermal process
   Slope of PV curve for adiabatic process
   As eg > 1;

TYPES OF ENTHALPIES

Enthalpy of combustion

CH₄(g) + 2O₂(g)⟶CO₂(g) + 2H₂O(g); ΔH = -890.4 kJ mol^-1
Enthalpy of combustion of CH₄ = –890.4 kJ mol^–1
i.e. heat released for 1 mole of substance.

Example 5 : The molar enthalpies of combustion of C_2H2(g), C(graphite) and H₂(g)
are –1300, –394 and –286 kJ mol^–1 respectively. The standard
enthalpy of formation of C₂H₂(g) is
(A) 226 kJ mol^–1 (B) –226 kJ mol^–1
(C) –620 kJ mol^–1 (D) 620 kJ mol^–1

Solution:    (A). It is given that,

       Hence, for the equation
       2C(graphite)+ H₂(g)⟶C₂H₂(g)
       ΔH = 2VH₂+ ΔH₃ - ΔH₁ =[-2 x 394 - 286 +(1300) ]kJ mol^-1] = 226 kJ mo^l–1

Enthalpy of neutralization

H⁺(aq)⁺OH^-(aq)⟶H₂O(ℓ); ΔH = -57.1 kJ mol^-1
Heat evolved / absorbed when 1 gm equivalent of both reacted.

Enthalpy of hydration

Heat evolved/ absorbed when 1 mol of anhydrous substance convert into hydrated formed
CuSO₄(s)+ 5H₂O――⟶CuSO₄.5H₂O(s);ΔH = -78.2 kJ mol^-1

Enthalpy change during phase transitions

Heat of fusion

Heat evolved / absorbed when 1 mol of solid directly converts liquid at constant temperature.

Heat of vaporization

Heat evolved / absorbed when 1 mol of liquid converts into gas (vapour) at constant temperature.

Heat of sublimation

Heat evolved / absorbed when 1 mol of solid directly converts into gas (vapour) at constant temperature.
I₂(S)⟶I₂(g); ΔH = 62.39 kJ mol-1

HESS’s LAW OF CONSTANT HEAT SUMMATION

This law states that “the resultant enthalpy change in a reaction is the same whether it occurs one or several steps”.

BOND ENERGY (B.E.)

It is the defined as the enthalpy change when a mole of gaseous molecules separates into constituent gas atoms.
H₂(g)――⟶2H₁ ΔH_H-H = 436 kJ mol^-1

Note:

Example 6 : The enthalpy of combustion of H₂(g) to give H₂O(g) is –249 kJ mol^–1
and bond enthalpies of H–H and O=O are 433 kJ mol^–1 and 492 kJ
mol^–1 respectively. The bond enthalpy of O–H is
(A) 464 kJ mol^–1 (B) –464 kJ mol^–1
(C) 232 kJ mol^–1 (D) –232 kJ mol^–1

Solution: (A). For the equation,

ENTROPY

Entropy is a thermodynamic quantity which is a measure of the randomness or disorder of the molecules of the system.

where, q – amount of heat evolved and absorbed at constant temperature
T – temperature
Entropy is a measure of randomness of a system.
ΔS(total) = ΔS(system)+ ΔS(surroundings)
(i)   Spontaneous process,ΔS(total) > 0
(ii)   Non – spontaneous process,ΔS(total)> 0
(iii)   Equilibrium, ΔS(total)= 0

SECOND LAW OF THERMODYNAMICS

This law states that –
The entropy of universe is always increasing in the course of every spontaneous process.

It is impossible to construct a machine functioning in a cycle which can convert heat completely into equivalent amount of work without producing change elsewhere, i.e. perpetual motions are not allowed.

Spontaneous process

A process which can take place either by itself or by initiation.

Non – spontaneous process

A process which can neither take place by itself nor by initiations.
The overall driving force for a process is the resultant of
(i) tendency for minimum energy and
(ii) tendency for maximum randomness.
Whenever a spontaneous process takes place, it is accompanied by an increase in the total entropy of the universe.

Example 7 : Which of the following reactions is attained to by an increase in
entropy?
(A) N₂(g) + 3H₂(g)──⟶ 2NH₃(g) (B) 2h₂(g) + O₂(g)──⟶2H₂O(ℓ)
(C) H₂(g) + I₂(g)──⟶2HL(g) (D) C(graphite)+ H₂O(g)──⟶CO(g)+ H₂O

Solution: (D). Increase in entropy will occur if Δn_g is positive, i.e., there is more number of gaseous products than the gaseous reactants.

Example 8 : Given that:
H₂O(ℓ,373.5 K,1 atm)──⟶H₂O(g,373.15 k,1 atm);ΔS₁
H₂O(s,273.5 K,1 atm)──⟶H₂O(ℓ,273.15 k,1 atm);ΔS₂
   Predict which of the following is true
   (A) ΔS₁ = S₂ (B) ΔS₁> ΔS₂
   (C) ΔS₁ < S₂ (D) ΔS1may be greater or smaller than

Solution: (B). Vaporization of water creates more randomness.

Gibb’s energy

In chemical system, the substance, moves in a direction in which there is a decrease in free energy, i.e. ΔG is negative.
G = H – TS
where, G – Gibbs energy, H – Enthalpy constant, S – Entropy, T – Temperature
ΔG = ΔH ─ TΔS(Gibb’s Helmholtz equation)
Free energy change
   (a)   If ΔG < 0,the process is spontaneous
   (b)   If ΔG > 0 the process is non – spontaneous
   (c)   If ΔG = 0 the process is in equilibrium

   ΔG = ΔG^o + 2.203 RT log Q
where, ΔG ─ free energy change;ΔG^o ─ standard free energy change; Q – reaction quotient

Example 9 : For the reaction, 2A(g) + B(g)──⟶2D(g),
   ΔU^o300k = -10.0 kJ mol^-1 and ΔS^o300k = -45J K^-1mol^-1. The reaction
   (A) Will be spontaneous   (B)   Will not be spontaneous
   (C) Will be at equilibrium   (D)   is endothermic

Solution:    (B). ΔH^o = ΔE^o + (Δn_g)RT
   =-10.0kJ mol^-1 +(-1)(8.314 x 10^-3kJ K^-1 mol^-1)(300K)
   = –12.49 kJ mol^–1
       ΔG^o = ΔH^o - TΔS^o
   =-12.49 kJ mol^-1 -(300K)(-45 x 10^-3)kJ K^-1 mol^-1
= (–12.49 + 13.5) kJ mol^–1= positive

ANSWER TO EXCERCISE

Exercise 1. C

Exercise 2. D

Exercise 3. A

Exercise 4. C

POINTS TO PONDER

1. Internal energy: It is sum of all forms of energy of all the particles present in the system at a particlar temperautre.

2. First law of thermodynamics:
ΔQ = ΔE + ΔW

3.   Enthalpy or heat constant:
   H = E + PV
   ΔH = ΔE + PΔV = ΔE + Δn_gRT

4.   Heat capacity: It is defined as the quantity of heat required to raise the temperatuer of the system by one degree.

5. and

6.   Work done in isothermal reversible process:
   W = -2.303nRTlog
   = -2.303nRTlog

7. Work done in adiabatic reversible expansion:
W = C_v(T₂ - T₁)

8.   Adiabatic relations:
   PV^γ = constant
   TV^γ-1 = constant


   where γ = sepecific heat ratio of gas.

9.   Kirchoff’s equation
   ΔH2 - ΔH1 = ΔCP(T2 - T1)   (at constant pressure)
   ΔE2 - ΔE1 = ΔCV(T2 - T1)   (at constant volume)


10.   Entropy: Entropy is thermodynamic quantity which is a measure of the randomness or disorders of the molecules of the system.

   Spontaneous process S_(total) > 0
   Non-spontaneous processS_(total) < 0
   At equilibrium S_(total) = 0

11. Second law of thermodynamics: It states that the entropy of universe is always increasing in the course of every spontaneous process.

12. Gibb’s energy: G = H - TS

SOLVED EXAMPLES

1.   Which of the following reactions correspond to the definition of enthalpy of combustion at 25oC?
   (A) ΔH₁ + ΔH₂ - ΔH₃      (B) ΔH₁ - ΔH₂ + ΔH₃
   (C) 2.303 x 2 x 298 log2   (D) CH₄(ℓ)+ 2O₂(ℓ)――⟶CO₂(ℓ)+ 2H₂O(ℓ)

Sol. (A).
In enthalpy of combustion, the stoichiometric number of the species being combusted is equal to one. The products are CO₂(g) and H₂O(ℓ) when temperature is 25^oC.

2.   The enthalpy of vaporization of water at 100^oC is 40.63 kJ mol^–1. Its entropy of vaporization would be
   (A)   406.3 J K^–1 mol^–1   (B)   108.9 J K^–1 mol^–1
   (C)   606.3 J K^–1 mol^–1   (D)   808.3 J K^–1 mol^–1

Sol.   (B).

3.   For a spontaneous process at all conditions of temperature which of the following is true?
   (A)   ΔS positive and ΔH positive   (B)   ΔS positive and ΔH negative
   (C)   ΔS negative and ΔH positive   (D)   ΔS negative and ΔH negative

Sol.   (B). For a spontaneous process, ΔG is negative. Since
   ΔG = ΔH - TΔS
   ΔG will always be negative if and only if ΔH is negative and ΔS is positive.

4.   Which of the following results in a decrease in entropy?
   (A)   Crystallization of sucrose from solution
   (B)   Rusting of iron
   (C)   Conversion of ice into water
   (D)   Vaporization of camphor

Sol. (A). Decrease in disorderliness would result into the decrease in entropy.

5.   If the standard free energy change for a reaction is 1.546 kJ mol^–1 at 500^oC, then the value of standard equilibrium constant for the reaction is
   (A)   Antilog (0.105)   (B)   Antilog (–0.105)
   (C)   Antilog (0.241)   (D)   Antilog (–0.241)

Sol.   (B). ΔG^o = -RT In K^o
   1.546 x 103 J mol^-1 = -(8.314 J K^-1 mol^-1)(773K)In K

   K^o =antilog(-0.105)

6.   5 mole of an ideal gas expand reversibly from a volume of 8 dm³ sp 80 dm³ at a temperature of 27°C. Calculate the change in entropy.
   (A)   70.26 JK^–1   (B)   82.55 JK^–1
   (C) 95.73 JK^–1 (D)   107.11 JK^–1

Sol.   (C). Entropy change (ΔS) = 2.303 nR log = 2.303 x 5 x 8.314 x = 95.73 JK^–1

7.   The molar heat capacity for a gas at constant T and P is
   (A)    (B)
   (C)   depends upon the atomicity of the gas   (D)   infinity

Sol.   (D).
   since at constant T, dT = 0

8.   Given: H₂S(g) ⟶ HS(g) + H(g), ΔH^o = x₁
   ΔH^o_f(H₂S,g) = x₂ and ΔH^o_f(H,g)= x₃
   Hence ΔH^o_f(HS,g) is
   (A)   x₁ + x₂ – x₃   (B)   x₃ + x₁ + x₂
   (C)   x₁ – x₂ – x₃   (D)   x₃ – x₁ + x₂

Sol.   (A). I. H₂S(g) ⟶ HS(g) + H(g),    ΔH^o = x₁
II. H₂(g) + S(g)⟶ H₂S(g), ΔH^o = x₂
III. H₂(g) ⟶ H(g) ΔHo = x₃
– – –
       ––––––––––––––––––––––––––––––––––––––––––––––––
   I + II + III: H₂(g) + S(g) ⟶ HS(g)    ΔH^o = x₁ + x₂ – x₃

9.   The dissociation energy of CH4 and C2H6 to convert them into gaseous atoms are 360 and 620 kcals mol–1 respectively. The bond energy of C – C bond is
   (A)   260 kcals mol–1   (B)   180 kcals mol–1
   (C)   130 kcals mol–1   (D)   80 kcals mol–1

Sol. (D). S_C–H =
= 90 kcals mole^–1
Dissociation energy of C₂H₆ = E_C–C + 6 E_C–H
620 = E_C–C + 6 x 90
ΔE_C–C= 80 kcals mole^–1

10.   Calculate the maximum efficiency of an engine operating between 110°C & 25°C.
(A)   11.1%   (B)   22.2%
(C) 33.3%   (D)   44.4%

Sol. (B). Maximum efficiency of an engine working between temperature T₂ & T₁ is given by
h = = 0.222% = 22.2%

ASSIGNMENT PROBLEMS

1.   Change in enthalpy for the reaction
   2H₂O₂(ℓ)⟶2H₂O(ℓ) + O₂(g)
   If heat of formation of H₂O₂(ℓ) and H₂O(ℓ) are –188 kJ mol^–1 and –286 kJ mol^–1 respectively, is
   (A)   –196 kJ mol^–1   (B)   +196 kJ mol^–1
   (C)   +948 kJ mol^–1 (D)   –948 kJ mol^–1

2.   In order to decompose 9g water, 142.5 kJ heat is required. Hence the enthalpy of formation of water is
   (A)   +285 kJ   (B)   –285 kJ
   (C)   +142.6 kJ   (D)   –142.5 kJ

3.   Enthalpy of neutralization of HCl with NaOH is x. The heat evolved when 500 ml of 2 N HCl are mixed with 250 ml of 4 N NaOH will be
   (A)   500 x   (B)   100 x
   (C)   x   (D)   10 x

4.   If the bond energies of H – H, Br – Br and H – Br are 433, 192 and 364 kJ mol^–1 respectively, the ΔHo for the reaction
H₂(g) + Br₂(g)⟶2HBr(g)is
(A)   –261 kJ   (B)   +103 kJ
(C)   +261 kJ   (D)   –103 kJ

5. The enthalpy of monoatomic gas at T Kelvin is

6.   All the natural processes in this universe produce:
   (A)   a decrease in entropy of the universe
   (B)   an increase in entropy of the universe
   (C)   no change in entropy
   (D)   some times increase or sometimes decrease in entropy

7.   ΔHf of graphite is 0.23 kJ / mole and ΔH_f for diamond is 1.896 kJ / mole. ΔH_transition from graphite to diamond is
   (A)   1.66 kJ / mole   (B)   2.1 kJ / mole
   (C)   2.33 kJ / mole   (D)   1.5 kJ / mole

8.   One gram of sample of NH₄NO₃ is decomposed in a bomb calorimeter. The temperature of the calorimeter increases by 6.12 K. The heat capacity of the system is 1.23 kJ / g / deg. What is the molar heat of decomposition for NH₄NO₃?
   (A)   –7.53 kJ / mol   (B)   –398.1 kJ / mol
   (C)   –16.1 kJ / mol   (D)   –602 kJ / mol

9. Consider the reactions,
C(s) + 2H₂O ⟶ CH₄(g)₁ ΔH= -x kcal

C(g) + 4H(g)⟶ CH₄(g), ΔH= -x₁ kcal

CH₄(g) ⟶ CH₄(g), ΔH= +y kcal

The bond energy of C – H bonds is

10.   Given the bond energies of N - N, H – H and N – H bonds are 945, 436 and 391 kJ/mol. respectively, the enthalpy of the reaction
N₂(g) + 3H₂(g) ⟶ 2NH₃(g) is
(A)   – 93 kJ (B)   102 kJ
(C)   90 kJ   (D)   105 kJ

11.   Given ΔH_ioniz (HCN) = 45.2 kJ mol^–1& ΔH_ioniz (CH₃COOH) = 2.1 kJ/mol
(A)   pKa (HCN) = pKa (CH₃COOH)
(B)   pKa (HCN) > pKa (CH₃COOH)
(C)   pKa (HCN) < pKa (CH₃COOH)
(D)   pKa (HCN) = pKa (CH₃COOH)

12.   For isothermal process which of the following is not correct?
   (A)    ΔE = 0
   (B)   ΔH = 0
   (C)   In isothermal expansion,
   (D)   In isothermal compression,

13.   Heat of hydrogenation of is x₁ and that of

is x₂, the resonance energy is equal to
(A)   x₁ + x₂   (B)   x₁ – x₂
(C)   2(x₁ – x₂)   (D)   2x₁ – x₂

14.   For the reaction,

   Bond energies are given as under
   (i)   C – C, 346 kJ / mol   (ii)   C – H, 413 kJ / mol
   (iii)   H – H, 437 kJ / mol and   (iv)   C = C, 611 kJ / mol

   What will be the value of ΔH ,25oC for the above reaction?
   (A)   –289 kJ mol^–1   (B)   –124 kJ mol^–1
   (C)   +124 kJ mol^–1   (D)   +289 kJ mol^–1

15.   CH₂= CH₂(g) + H₂(g)⟶CH₃ - CH₃(g).The bond energy of C–C=80 kcal, C=C=145 kcal, C–H = 98 kcal, H–H = 103 kcal, the heat of reaction is
    (A)   –14 kcal   (B)   –28 kcal
   (C)   –42 kcal   (D)   –56 kcal

ANSWER TO ASSIGNMENT PROBLEMS

1. A 2. B 3. C

4. D 5. C 6. B

7. A 8. D 9. C

10. A 11. C 12. C

13. D 14. B 15. B

Frequently Asked Questions

Thermodynamics is the scientific study of energy, heat, work, and how they interact within physical systems. It investigates how energy is transformed, transferred, and conserved, especially between heat and mechanical work. Central to thermodynamics are key properties like temperature, pressure, internal energy, and entropy. Fundamentally, thermodynamics explains why engines work, how refrigerators cool, and why chemical reactions release or absorb heat.

By understanding the laws of thermodynamics, scientists and engineers can design systems from engines to power plants that convert energy efficiently and sustainably. Whether in chemistry, physics, or engineering, thermodynamics underpins much of modern technology and is vital for innovations in energy, environment, and materials science.

Efficient study of thermodynamics involves reviewing your textbook and class notes, practicing a diverse set of problems, and regularly self-testing to identify weak areas. Break complex topics into basics—like laws, definitions, and key equations—before tackling more advanced applications or calculations. Time your practice under exam conditions to build confidence. Use summary sheets, diagrams, and concept maps to visualize relationships and flows. If you’re stuck, visit office hours or join a study group for discussion and clarification. Relate concepts to real-life systems to cement your understanding and make learning more engaging.

Thermodynamics is foundational for countless engineering systems and processes. In mechanical engineering, it powers car engines, turbines, and heating/cooling systems. Chemical engineers use thermodynamic principles to design reactors and optimize chemical production, ensuring maximum efficiency and safety. Electrical engineers apply thermodynamics in power generation and battery technology.

Thermodynamics is also the basis for renewable energy systems, such as solar panels and wind turbines, and helps make buildings energy efficient through better insulation. In everyday life, thermodynamics explains how refrigerators keep food cold, air conditioners cool rooms, and why hot drinks cool down over time. Its principles are vital for technological innovation, energy conservation, and sustainability.

Understanding thermodynamics is critical for designing efficient machines, reducing energy waste, and solving key scientific and engineering problems. It helps optimize processes—leading to lower costs and reduced environmental impacts. Thermodynamics is essential in developing new materials, advancing alternative energy sources, and ensuring that industrial processes are environmentally responsible. Additionally, it provides the tools to investigate and improve technologies ranging from engines and power plants to batteries and environmental control systems.

The four laws of thermodynamics provide a comprehensive framework for understanding all energy exchanges:

Zeroth Law: If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. This defines temperature.
First Law (Conservation of Energy): Energy cannot be created or destroyed. The total change in internal energy equals heat added to the system minus the work done by the system.
Second Law: Energy conversions are inherently inefficient. In any spontaneous process, total entropy (disorder) increases, and it is impossible for heat to flow from a colder body to a hotter one without work input.
Third Law: As a system approaches absolute zero temperature, the entropy approaches a minimum constant; for a perfect crystal, this minimum is zero.
Understanding and applying these laws is critical to solving energy, heat, and work problems across science and engineering.

Entropy is a measure of a system’s disorder, randomness, or unavailability of energy to do work. Physically, it reflects the number of ways the internal energy can be distributed among the system's molecules. According to the second law, in any real process, the total entropy of an isolated system always increases, meaning natural processes are irreversible and tend toward greater disorder. Entropy is fundamental for predicting spontaneity of chemical reactions and for analyzing energy efficiency in machines and natural phenomena.

Studying thermodynamics gives you a powerful lens to understand how the universe works—from the smallest chemical reaction to vast engineering systems. Mastering thermodynamics hones critical thinking and problem-solving skills, opens up career opportunities in engineering, chemistry, physics, and environmental fields, and enables you to design solutions for global energy and sustainability challenges.

Exploring thermodynamics enriches your understanding of both everyday phenomena—like boiling water or keeping your house warm and complex, cutting-edge technologies like renewable energy and advanced materials. It’s a truly interdisciplinary subject that connects theory to application and invites lifelong curiosity.

To explain the four laws of thermodynamics to a beginner audience, it’s essential to use relatable language, simple analogies, and clear step-by-step logic. Here’s an approach tailored for classroom, tutoring, or self-study contexts:

The Four Laws of Thermodynamics: Beginner-Friendly Explanations

0. Zeroth Law of Thermodynamics: The “Basic Temperature Rule”

What it Means: If two objects are each at the same temperature as a third object, then they are at the same temperature as each other.
Analogy: Imagine you have three cups of water. Cups A and B feel just as warm as cup C. That means A, B, and C all have the same temperature—even if you never compared A and B directly.
Why it Matters: It lets us use thermometers and understand what temperature means.

1. First Law of Thermodynamics: Conservation of Energy

What it Means: Energy can’t be created or destroyed; it can only change from one form to another. The increase in internal energy of a system equals the heat added to it minus the work done by the system.
Analogy: Like a bank account—you can deposit money (heat) or withdraw it (work), but you can’t make money appear from nothing or vanish without action.
Formula (for context): ΔU=Q−W
Why it Matters: This law explains why when you heat a pot of water, the energy you put in (heat) either raises the water’s temperature (internal energy) or causes it to boil (doing work).

2. Second Law of Thermodynamics: Direction of Processes & Entropy

What it Means: Heat naturally flows from hot things to cold things, never the reverse (unless you work at it). The universe tends toward more “disorder,” called entropy.
Analogy: If you spill a box of marbles, they scatter everywhere on their own but never neatly arrange themselves without help—things move from order to disorder.
Why it Matters: This is why refrigerators need electricity to move heat from inside (cold) to outside (warm) and why you can’t make a machine that’s 100% efficient.

3. Third Law of Thermodynamics: Absolute Zero and Perfect Order

What it Means: As something gets closer and closer to absolute zero (the coldest temperature possible, -273.15°C), everything slows down and disorder (entropy) drops to a minimum. But absolute perfection can never be reached.
Analogy: Imagine cleaning a messy room—no matter how organized you get, absolute, molecular-level perfection is impossible; there’s always a little disorder left.
Why it Matters: This sets a limit for how cold things can really get and is important in fields like cryogenics and quantum physics.

In thermodynamics, work refers to energy transfer that results from a force acting over a distance within a system. When a gas expands in a piston, for example, it performs work by pushing against atmospheric pressure. Thermodynamic work is crucial because it measures how much useful energy can be extracted from a system or needs to be provided to achieve a process such as compressing a gas, moving a piston, or running a turbine.

Unlike heat (which relates to energy transferred due to temperature difference), work is organized energy transfer directly related to mechanical or electrical actions. Mastery of this concept is key for analyzing processes in engines, chemical reactions, and power production.

While both physics and chemistry study thermodynamics, their focus often differs. Physics emphasizes large-scale energy transformations—like engines, heat flow, and power systems—typically in context of physical processes. Chemistry applies thermodynamics to understand how and why chemical reactions occur, predict reaction spontaneity, and analyze energy changes at the molecular level. Despite these emphases, the core laws and principles are universal, underpinning both disciplines with shared rules for energy conservation and entropy.

Many students find thermodynamics challenging because it combines abstract principles, mathematical reasoning, and real-world application. The topic introduces unfamiliar ideas like entropy, energy states, and thermodynamic cycles. Success in learning thermodynamics starts by mastering core definitions (such as temperature, heat, work, and entropy), understanding the four fundamental laws, and visualizing processes through diagrams and real-life examples.

Practice is essential: work through a variety of problems, gradually increasing in difficulty. Don’t hesitate to seek help from peers or instructors, use visual aids, and relate principles to practical scenarios (like engines or refrigerators). Building a foundation of clear concepts before moving to advanced topics is key, as each principle builds on the last.

S.no	Formulas List
1.	Atomic Structure
2.	Some Basic Concepts of chemistry
3.	Gaseous State
4.	Periodic Properties
5.	Chemical Bonding
6.	Thermodynamics & Thermochemistry
7.	Chemical Equilibrium
8.	Ionic Equilibrium
9.	Surface Chemistry
10.	S-block elements and Hydrogen

Thermodynamics & Thermochemistry