Class 11 Chemistry Chapter 5: Chemical Thermodynamics
Chemical Thermodynamics explains the energy changes that accompany physical and chemical processes. This chapter introduces important concepts such as the system and surroundings, different types of thermodynamic processes, internal energy, work, heat, enthalpy, Hess's Law, bond enthalpy, entropy, spontaneity, and Gibbs free energy. Understanding these concepts is essential for predicting the feasibility of chemical reactions and solving energy-based numerical problems. The chapter carries significant weightage in JEE Main, JEE Advanced, NEET, and CBSE Board examinations.
At ALLEN, Chapter 5 Chemical Thermodynamics smart study notes are prepared by expert chemistry faculty to simplify thermodynamic concepts through concise theory, energy diagrams, important formulas, solved numerical examples, shortcut techniques, previous years' question (PYQ) analysis, and exam-oriented revision tips. These notes help students strengthen both conceptual understanding and numerical problem-solving skills in Physical Chemistry.
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question 1 of 5
Which of the following is a state function?
1.Heat
2.Work
3.Enthalpy
4.Path length
Thermodynamics
1.0What is Thermodynamics?
Thermodynamics is the branch of science that deals with the energy changes taking place in all physical and chemical processes, but Chemical thermodynamics is the branch of thermodynamics that deals with the study of energy changes taking place in chemical processes.
To simplify thermodynamics, a clear boundary is established around a system, encompassing everything within it. The surroundings are everything outside this boundary. This setup helps describe the movement and transfer of energy across the system's boundaries. The system and its surroundings constitute the "universe" in thermodynamics.
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2.0Thermodynamic Terms
System: A system is defined as a specific part of the universe or a specified portion of matter that is under experimental investigation.
Surroundings: The rest of the universe, excluding the system, is called the surroundings.
Universe: The universe is the sum of the system and its surroundings.
Universe= System + Surroundings
Boundary: Anything that separates the system from its surroundings is called a boundary.
Surroundings
The surroundings of a thermodynamic system include everything external to the system that can interact with it. Based on the nature of the interaction between a system and its surroundings, thermodynamic systems can be classified into three types:
Open System: An open system can exchange energy and matter with its surroundings. This type of system is not isolated and allows for transferring heat, work, and mass across its boundaries. Examples include boiling water in an open pot.
Closed System: In a closed system, energy can be exchanged with the surroundings, but the transfer of matter is restricted. While heat and work can cross the boundaries, mass remains constant within the system. A sealed steam radiator is an example of a closed system.
behaviour: An isolated system cannot exchange either energy or matter with its surroundings. It is completely insulated from its environment. A perfect example is a thermos bottle that keeps its contents at a constant temperature for an extended period.
The most frequently tested topics include the First Law of Thermodynamics, enthalpy change, Hess's Law, bond enthalpy, entropy, and Gibbs free energy. Numerical questions from these concepts are common in JEE Main and JEE Advanced, while NEET focuses more on conceptual understanding.
Internal energy (ΔU) represents the total energy contained within a system, whereas enthalpy (ΔH) accounts for both the internal energy and the pressure-volume work of the system. At constant pressure, the heat exchanged by the system is equal to the enthalpy change.
Hess's Law allows the calculation of the enthalpy change of a reaction that cannot be measured directly. It is widely used in competitive examinations to solve reaction enthalpy problems by combining multiple chemical equations.
Develop a clear understanding of sign conventions, write the appropriate formula before substitution, maintain consistent units, and practice a variety of numerical problems involving work, heat, enthalpy, and Gibbs free energy.
After mastering Chemical Thermodynamics, continue with Equilibrium, where thermodynamic concepts such as Gibbs free energy and spontaneity help explain the direction and extent of chemical reactions.
Thermodynamics is classified into several branches, given below:
Classical Thermodynamics: This branch deals with the macroscopic properties of systems and uses laws and concepts without requiring knowledge of the behaviour of individual particles.
Statistical Thermodynamics: This branch bridges the gap between macroscopic and microscopic worlds by explaining thermodynamic properties regarding the statistical behaviour, of the molecules that compose a system.
Chemical Thermodynamics: This branch examines how heat and work interact with chemical reactions or physical state changes, adhering to the principles of thermodynamics.
Equilibrium Thermodynamics: This branch studies systems in thermodynamic equilibrium, where macroscopic variables do not change over time, and there are no net flows of matter or energy.
Non-Equilibrium Thermodynamics: This branch deals with systems that are not in thermodynamic equilibrium and where macroscopic variables can change over time, often involving the study of transport processes and the rate of irreversible processes.
2.0Thermodynamic Properties
State Variables (or Thermodynamic Variables/Quantities): These properties define a system's state, which is characterized by measurable properties such as temperature, pressure, and volume. Any change in these properties indicates a change in the system's state.
State Functions: These are state variables that depend only on the system's current state, not on the path taken to reach that state. State functions are denoted by capital letters. Examples include internal energy (E), enthalpy (H), entropy (S), Gibbs free energy (G), temperature (T), pressure (P), and volume (V).
Path Functions: These are properties that depend on the path or mechanism followed by the system to reach its final state and the initial and final states. Path functions are denoted by lowercase letters. Examples include work done (w) and heat (q).
3.0Types of Thermodynamic Variables
Intensive Variables: These variables are independent of the size or mass of the system. Examples include temperature, pressure, and specific heat capacity.
Extensive Variables: These variables depend on the size or mass of the system. Examples include volume, energy, entropy, heat capacity, and enthalpy.
4.0Types of Thermodynamic Processes
Thermodynamic processes are the various ways a thermodynamic system can change its state. These processes can be defined based on how certain properties, such as pressure, volume, and temperature, are held constant or how energy is transferred within the system. Here are the main types of thermodynamic processes:
Isothermal Process
A process that occurs at a constant temperature.
Heat is exchanged with the surroundings to ensure the temperature remains constant.
The internal energy of an ideal gas does not change.
Example: Slow compression or expansion of a gas in a cylinder with a heat reservoir.
Adiabatic Process
A process that occurs without any heat exchange between the system and its surroundings.
The system is thermally insulated.
Changes in internal energy result solely from work done on or by the system.
Example: Rapid compression or expansion of a gas in an insulated cylinder.
Isobaric Process
A process that occurs at a constant pressure.
Volume changes while pressure remains constant.
Heat added to or removed from the system changes the internal energy and performs work.
Example: Heating water at atmospheric pressure.
Isochoric Process
A process that occurs at a constant volume.
No work is done since volume does not change.
Any heat added to the system changes its internal energy.
Example: Heating a gas in a rigid, sealed container.
Cyclic Process
A process where the system reverts to its initial state following a sequence of alterations.
The net change in internal energy over one cycle is zero.
Work done over one cycle is equal to the net heat added to the system.
Example: The operation of a heat engine.
Reversible Process
A process that can be reversed without leaving any change in both the system and the surroundings.
It occurs infinitely slowly, allowing the system to always remain in equilibrium.
Idealization; no real process is truly reversible.
Example: Quasi-static processes in thermodynamic cycles.
Irreversible Process
A process that cannot be reversed without leaving changes in the system and surroundings.
Involves dissipative effects like friction, turbulence, and unrestrained expansion.
Real processes are typically irreversible.
Example: Spontaneous mixing of gases, real-world heat transfer
5.0Thermodynamic Potentials
Thermodynamic potentials measure a system's stored energy and how it transforms from its initial state to its final state. Different potentials are used depending on the system's constraints, such as temperature and pressure. Here are the main forms of thermodynamic potentials:
Internal Energy (U): The total energy contained within the system, comprising both the capacity for performing work and the potential for releasing heat..
Gibbs Energy (G): The energy associated with a system that can be used to do non-mechanical work. It is particularly useful in processes occurring at constant temperature and pressure.
Enthalpy (H): The overall heat content within a system. representing the ability to do non-mechanical work and the ability to emit heat. It is especially useful for processes occurring at constant pressure.
Thermodynamics): The energy that can be converted to both mechanical and non-mechanical work. It is particularly relevant for processes occurring at constant volume and temperature.
Enthalpy in Thermodynamics
Enthalpy is a measure of a system's total heat content. It encompasses the system's internal energy along with the product of its pressure and volume.
Enthalpy is a property or state function, akin to energy, and shares the same dimensions, typically measured in joules or ergs.
Its value solely depends on the temperature, pressure, and composition of the system, irrespective of its past states.
Entropy in Thermodynamics
Entropy represents the amount of thermal energy per temperature unit in a system that cannot be harnessed for useful work.
It quantifies the molecular disorder or randomness within a system, contrasting with the ordered molecular motion that generates work.
Entropy theory provides insights into the spontaneous direction of change for various common phenomena.
6.0The Laws of Thermodynamics
The principles of thermodynamics are foundational laws that dictate the behaviourbehaviour of thermodynamic systems. These laws form the foundation of classical thermodynamics and have broad applications across various fields of science and engineering.
There are indeed four laws of thermodynamics:
Zeroth Law of Thermodynamics: This law asserts that if two systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other It establishes the concept of temperature and provides a basis for temperature measurement.
First Law of Thermodynamics: Also known as the law of energy conservation, it can only change forms. It establishes the principle of energy conservation.
Second Law of Thermodynamics: This law introduces the concept of entropy, stating that in any natural process, the total entropy of a system and its surroundings always increases (or at least remains constant in an idealized reversible process). It implies the directionality of processes and the irreversibility of natural phenomena.
Third Law of Thermodynamics: This law states that as the temperature of a system approaches absolute zero, the entropy of the system approaches a minimum value. It provides a baseline for the absolute entropy of substances at zero Kelvin and offers insights into the behaviour of systems at low temperatures.
After studying these revision notes, you will be able to:
Differentiate between system, surroundings, and boundary.
Classify thermodynamic processes as isothermal, adiabatic, isobaric, and isochoric.
Apply the First Law of Thermodynamics to solve numerical problems.
Calculate internal energy and enthalpy changes in chemical reactions.
Use Hess's Law and bond enthalpy to determine reaction enthalpy.
Explain entropy and Gibbs free energy in terms of spontaneity.
Interpret energy profile diagrams for exothermic and endothermic reactions.
Develop a strong foundation for advanced Physical Chemistry topics.
2.0Quick Revision Checklist
Revise the following concepts before solving numerical and conceptual questions.
Must-Revise Concepts
System, Surroundings and Boundary
Types of Systems
State Functions and Path Functions
Internal Energy
Heat and Work
First Law of Thermodynamics
Enthalpy
Heat Capacity
Specific Heat Capacity
Standard Enthalpy Change
Hess's Law
Enthalpy of Formation
Enthalpy of Combustion
Bond Enthalpy
Entropy
Gibbs Free Energy
Spontaneous and Non-Spontaneous Processes
Exothermic and Endothermic Reactions
Formula & Calculation Checklist
Quickly revise these important formulas and relationships before the exam.
First Law of Thermodynamics
Work Done Formula
Enthalpy Change
Heat Capacity Equation
Hess's Law
Bond Enthalpy Equation
Gibbs Free Energy Equation
Entropy Change
Heat at Constant Pressure
Heat at Constant Volume
Relationship between ΔH and ΔU
Sign Convention for Heat and Work
3.0High Weightage Topics
Topic
Key Theoretical Focus
First Law of Thermodynamics
Energy conservation, heat, work, and internal energy
Thermodynamic Processes
Isothermal, adiabatic, isobaric, and isochoric processes
Enthalpy Change
Standard enthalpy, heat of reaction, sign conventions
Hess's Law
Enthalpy calculations using multiple reaction steps
Bond Enthalpy
Bond breaking, bond formation, and reaction enthalpy
Entropy & Spontaneity
Disorder, spontaneity, and driving force of reactions
Gibbs Free Energy
Relationship between enthalpy, entropy, and spontaneity
4.0Formula Handbook
The following formulas and thermodynamic relationships are essential for solving numerical and conceptual questions from Chemical Thermodynamics in JEE Main, JEE Advanced, NEET, and CBSE examinations.
5.0Common Mistakes & Exam Tips
Chemical Thermodynamics contains both conceptual and numerical questions. Most errors occur due to incorrect sign conventions, misuse of formulas, and confusion between thermodynamic quantities.
Common Mistake
Exam Tip
Confusing heat (q) with enthalpy (ΔH)
Remember that ΔH equals heat only at constant pressure.
Using incorrect sign conventions for heat and work
Always follow the thermodynamic sign convention before solving numericals.
Mixing internal energy and enthalpy
Revise the difference between ΔU and ΔH along with their applications.
Forgetting the relation between ΔH and ΔU
Use ( \Delta H=\Delta U+\Delta n_gRT ) only when gaseous moles change.
Incorrect bond enthalpy calculations
Count every bond broken and formed carefully before substitution.
Applying Hess's Law incorrectly
Reverse reactions and multiply equations only after changing the sign or magnitude of enthalpy accordingly.
Ignoring units
Maintain consistency in units (J, kJ, mol⁻¹, K) throughout calculations.
Confusing spontaneity with reaction rate
A spontaneous reaction is not necessarily a fast reaction.
6.0ALLEN Expert Tips
Understand the physical meaning of every thermodynamic quantity instead of memorizing formulas.
Prepare a separate summary sheet containing all important equations and sign conventions.
Practice numerical problems involving the First Law of Thermodynamics regularly.
Revise Hess's Law and bond enthalpy calculations using multiple examples.
Learn the conditions under which ΔG becomes positive, negative, or zero.
Focus on conceptual differences between heat, work, internal energy, and enthalpy.
Solve NCERT examples and exemplar questions before attempting advanced-level problems.
Practice Previous Years' Questions to improve speed and accuracy in numerical calculations.
7.0PYQ Trend Analysis
Chemical Thermodynamics is one of the highest-scoring chapters in Physical Chemistry. Questions are frequently asked from the First Law of Thermodynamics, enthalpy changes, Hess's Law, bond enthalpy, Gibbs free energy, and spontaneity.
Note: The trend below is based on the analysis of previous years' JEE Main and JEE Advanced question papers. The exact number of questions may vary each year.
Topic
Questions Asked (Last 5 Years)
Average Difficulty
First Law of Thermodynamics
2–3
Moderate
Work, Heat & Internal Energy
1–2
Easy–Moderate
Enthalpy & Heat Capacity
2–3
Moderate
Hess's Law
2–3
Moderate
Bond Enthalpy
1–2
Moderate
Gibbs Free Energy
1–2
Moderate
Entropy & Spontaneity
1–2
Moderate
8.0Most Common Question Types
Question Type
What is Commonly Asked?
Numerical Problems
Calculating work, heat, internal energy, and enthalpy changes
Hess's Law Questions
Determining reaction enthalpy using multiple chemical equations
Bond Enthalpy Calculations
Estimating enthalpy change from bond dissociation energies
Formula-Based Questions
Applying ΔU, ΔH, ΔG, heat capacity, and work equations
Assertion & Reason Questions
Thermodynamic processes, spontaneity, and sign conventions
Conceptual MCQs
State functions, path functions, entropy, and Gibbs free energy
Graph-Based Questions
Interpreting energy profile diagrams and thermodynamic processes
9.0Smart Revision Plan
Revise this chapter in the following sequence for effective conceptual understanding and numerical accuracy:
System, Surroundings, and Thermodynamic Terminology
Types of Thermodynamic Processes
Heat, Work, and Internal Energy
First Law of Thermodynamics
Enthalpy and Heat Capacity
Hess's Law and Enthalpy Calculations
Bond Enthalpy
Entropy and Spontaneity
Gibbs Free Energy
Formula Handbook, NCERT Examples, and Previous Years' Questions
