Thermodynamics is one of the most important and scoring chapters of Class 12 Chemistry. This chapter explains how energy is transferred, stored, and converted during physical and chemical processes. Many numerical questions, theory questions, and concept-based questions are directly asked from this chapter in CBSE board exams, NEET, and other competitive exams.
In simple words, thermodynamics deals with heat, work, energy, and their interconversion. Understanding this chapter helps students easily solve numericals, understand chemical reactions, and score well in exams.
Class 12 Chemistry Thermodynamics Notes | Important Questions, Formulas & Numericals
What is Thermodynamics?
Thermodynamics is the branch of science that studies:
- Heat
- Work
- Energy
- Relationship between heat and work
It tells us:
- Whether a reaction will occur or not
- How much energy is absorbed or released
- Whether a process is spontaneous
Thermodynamic Terms and Definitions
Understanding basic terms is very important before studying laws of thermodynamics.
1. System
The part of the universe which is under study is called a system.
2. Surroundings
Everything other than the system is called the surroundings.
3. Universe
System + Surroundings = Universe
Types of Systems
1. Open System
- Exchange of mass and energy with surroundings
- Example: Open beaker with water
2. Closed System
- Exchange of energy only, not mass
- Example: Water in a closed container
3. Isolated System
- No exchange of mass or energy
- Example: Thermos flask
State of a System
The condition of a system described by measurable properties such as:
- Temperature (T)
- Pressure (P)
- Volume (V)
is called the state of the system.
State Functions
State functions depend only on initial and final states, not on the path.
Examples of State Functions
- Internal energy (U)
- Enthalpy (H)
- Entropy (S)
- Pressure, Volume, Temperature
Path Functions
Path functions depend on the path followed.
Examples
- Heat (q)
- Work (w)
Thermodynamic Processes
1. Isothermal Process
- Temperature remains constant
- ΔT = 0
2. Adiabatic Process
- No heat exchange
- q = 0
3. Isobaric Process
- Pressure remains constant
4. Isochoric Process
- Volume remains constant
- w = 0
Internal Energy (U)
Internal energy is the total energy possessed by a system, including:
- Kinetic energy
- Potential energy
- Energy of molecules
Change in Internal Energy
\Delta U = U_{final} - U_{initial}
Heat (q) and Work (w)
Heat (q)
Energy transferred due to temperature difference.
Work (w)
Energy transferred when force is applied over distance.
First Law of Thermodynamics
Statement
Energy can neither be created nor destroyed; it can only be converted from one form to another.
Mathematical Expression
\Delta U = q + w
Where:
- ΔU = change in internal energy
- q = heat supplied
- w = work done
Sign Convention
| Process | q | w |
|---|---|---|
| Heat absorbed | + | |
| Heat released | – | |
| Work done on system | + | |
| Work done by system | – |
Work Done in Different Processes
Isothermal Process
w = -2.303 nRT \log \frac{V_2}{V_1}
Isochoric Process
w = 0
Enthalpy (H)
Enthalpy is defined as:
H = U + PV
Change in Enthalpy
\Delta H = \Delta U + \Delta (PV)
Exothermic and Endothermic Reactions
Exothermic Reaction
- Heat is released
- ΔH < 0
- Example: Combustion reactions
Endothermic Reaction
- Heat is absorbed
- ΔH > 0
- Example: Photosynthesis
Enthalpy Changes
1. Enthalpy of Reaction (ΔH)
Heat change during a chemical reaction.
2. Enthalpy of Formation
Heat change when 1 mole of compound is formed from elements.
3. Enthalpy of Combustion
Heat released when 1 mole of substance is burnt.
4. Enthalpy of Neutralization
Heat change when acid and base react.
Hess’s Law of Constant Heat Summation
Statement
Total enthalpy change of a reaction is same whether it occurs in one step or multiple steps.
Importance
- Helps calculate unknown enthalpy changes
- Based on first law of thermodynamics
Bond Enthalpy
Energy required to break one mole of bonds in gaseous state.
\Delta H = \sum \text{Bond energies of reactants} - \sum \text{Bond energies of products}
Entropy (S)
Entropy is the measure of disorder or randomness.
Key Points
- Higher disorder → higher entropy
- Solid < Liquid < Gas
Change in Entropy
\Delta S = S_{final} - S_{initial}
Second Law of Thermodynamics
Statement
Entropy of the universe always increases for a spontaneous process.
\Delta S_{universe} > 0
Gibbs Free Energy (G)
G = H - TS
Change in Gibbs Free Energy
\Delta G = \Delta H - T \Delta S
Spontaneity of Reaction
| ΔG Value | Nature |
|---|---|
| ΔG < 0 | Spontaneous |
| ΔG = 0 | Equilibrium |
| ΔG > 0 | Non-spontaneous |
Important Thermodynamic Formulas (Class 12)
- ΔU = q + w
- w = −PΔV
- H = U + PV
- ΔG = ΔH − TΔS
- ΔS = q / T
Important Exam-Oriented Points
- Thermodynamics is numerical-heavy chapter
- First law questions are frequently repeated
- Gibbs free energy is very important for theory
- Hess law questions often come in boards
Why Thermodynamics is Important for Class 12 Students?
- High weightage in board exams
- Base for physical chemistry
- Useful for competitive exams like NEET
Conclusion-Class 12 Chemistry Thermodynamics Notes
The chapter Class 12 Chemistry Thermodynamics explains the fundamental laws governing energy changes in chemical reactions. By understanding concepts like internal energy, enthalpy, entropy, Gibbs free energy, and thermodynamic laws, students can easily solve both theoretical and numerical questions.
Regular practice of numericals, clear understanding of formulas, and revision of important concepts will help students score maximum marks in exams.
Class 12 Chemistry Thermodynamics Notes– Important & Repeated Questions
🔴 Very Short Answer Questions (1 Mark)
- What is thermodynamics?
- Define a thermodynamic system.
- What is meant by surroundings?
- Write the mathematical expression of the first law of thermodynamics.
- Is heat a state function or path function?
- Is internal energy a state function?
- Define enthalpy.
- What is an isolated system?
- What is entropy?
- Write the unit of entropy.
- What is Gibbs free energy?
- Write the condition for spontaneity of a reaction.
- What is an adiabatic process?
- State Hess’s law.
- What is the sign of ΔH for an exothermic reaction?
🔵 Short Answer Questions (2–3 Marks)
- Differentiate between open system, closed system, and isolated system.
- Explain state function and path function with examples.
- Define internal energy. How does it change during a chemical reaction?
- Write the first law of thermodynamics and explain its significance.
- What is an isothermal process? Write the expression for work done.
- Explain exothermic and endothermic reactions with examples.
- Define enthalpy of formation.
- What is enthalpy of combustion?
- Define bond enthalpy.
- Explain the term entropy and its physical meaning.
- Write the formula of Gibbs free energy and explain each term.
- What happens to entropy when a solid changes into gas?
- State the second law of thermodynamics.
- Explain why entropy of the universe always increases.
🟢 Long Answer Questions (4–5 Marks) – Most Repeated
- Explain the first law of thermodynamics. Derive its mathematical form.
- Define enthalpy. Derive the relationship between ΔH and ΔU.
- What is Hess’s law of constant heat summation? Explain with an example.
- Explain the second law of thermodynamics in terms of entropy.
- What is Gibbs free energy? Explain the spontaneity of reactions using ΔG.
- Derive the expression for work done during an isothermal reversible process.
- Explain different types of thermodynamic processes.
- Define entropy. Explain entropy change during:
- Melting of ice
- Vaporization of water
🧮 Important Numerical Questions (Repeated in Exams)
- Calculate the work done when a gas expands from volume V₁ to V₂ at constant pressure.
- Calculate ΔU when q = +200 J and w = −50 J.
- If ΔH = −100 kJ and ΔS = −200 J K⁻¹ at 300 K, calculate ΔG.
- Calculate the entropy change when 500 J of heat is absorbed at 250 K.
- Calculate ΔH of a reaction using bond enthalpy values.
- Calculate work done in an isochoric process.
- Determine whether a reaction is spontaneous at a given temperature using ΔG.
⭐ Most Frequently Repeated Board Questions (High Priority)
✔ First law of thermodynamics (theory + numericals)
✔ Hess’s law numericals
✔ Gibbs free energy & spontaneity
✔ Difference between ΔH and ΔU
✔ Entropy and second law
✔ Exothermic vs Endothermic reactions
✔ Work done in isothermal process
📌 Exam Tips for Thermodynamics
- Always write formula first in numericals
- Mention sign convention clearly
- Draw energy level diagrams where required
- Learn definitions word-to-word
- Practice previous year questions
We are Completed Class 12 Chemistry Thermodynamics Notes.