Last Updated: June 1, 2026

📋 Table of Contents
What is Thermodynamics Chemistry Revision Notes?
1. Why Thermodynamics is the "Judge" of Chemistry
2. Systems, Surroundings, n Types of Processes
3. The First Law: Energy Conservation and Action
4. Work Done and Reversible vs Irreversible Expansion
5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
6. Hess's Law: The Circle of Enthalpy
7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
8. Entropy ( $S$ ) n the Second Law
9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
10. The "Trap" Section: Sign Convention Nightmares
11. Practice MCQs (JEE/NEET Level)
12. Ayush's Thermodynamics Strategy
📚 Related Topics
📚 Related Topics

⚗️ Chemical Quick Reference (Verified via PubChem)

Compound	Formula	Mol. Weight	Source
water	H2O	18.015 g/mol	PubChem ↗

📋 Table of Contents

What is Thermodynamics Chemistry Revision Notes?
1. Why Thermodynamics is the "Judge" of Chemistry
- Why This Chapter Matters (Exam Data)
2. Systems, Surroundings, n Types of Processes
- Types of Processes
3. The First Law: Energy Conservation and Action
- Sign Convention (IUPAC)
4. Work Done and Reversible vs Irreversible Expansion
5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
- The Bridge Formula
6. Hess's Law: The Circle of Enthalpy
- Application: Born-Haber Cycle
7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
- Key Values to Memorize
- Bond Enthalpy Method
8. Entropy ( $S$ ) n the Second Law
- Second Law of Thermodynamics
- Key Points
9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
- The Spontaneity Table (Memorize This!)
- Equilibrium Connection
10. The "Trap" Section: Sign Convention Nightmares
11. Practice MCQs (JEE/NEET Level)
12. Ayush's Thermodynamics Strategy
- Board Exam Tip:
📚 Related Topics

Thermodynamics Chemistry Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

What is Thermodynamics Chemistry Revision Notes?

Why Thermodynamics is the "Judge" of Chemistry
Systems, Surroundings, n Types of Processes
The First Law: Energy Conservation and Action
Work Done and Reversible vs Irreversible Expansion
Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
Hess's Law: The Circle of Enthalpy
Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
Entropy ( $S$ ) n the Second Law
Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
The "Trap" Section: Sign Convention Nightmares
Practice MCQs (JEE/NEET Level)
Ayush's Thermodynamics Strategy

1. Why Thermodynamics is the "Judge" of Chemistry

chemical Thermodynamics is the study of energy changes (heat and work) associated with chemical reactions and physical transformations.

Thermodynamics doesn't care about speed — that's Kinetics. Thermodynamics answers the fundamental question: "Will this reaction ever happen on its own?" If $\delta G < 0$ , yes. If $\delta G > 0$ , no amount of waiting will make it happen spontaneously.

Why This Chapter Matters (Exam Data)

JEE Mains 2026: 2 questions — one on $\delta H$ vs $\delta U$ using $\delta n_g$ , one on Gibbs and spontaneity.
neet 2026: 1 question on Hess's Law and 1 on the Third Law.
CBSE Boards: This unit carries 7 marks and is a classic long-answer question topic.

2. Systems, Surroundings, n Types of Processes

A thermodynamic system is the specific portion of the universe under study, separated from its surroundings y a real or imaginary boundary.

System Type	Exchanges	Example
Open	Mass AND Energy	Boiling water and an open beaker
Closed	Energy only	Gas and a sealed piston
Isolated	Neither	Perfect thermos flask

Types of Processes

Isothermal: $\delta T = 0$ (temperature constant).
Adiabatic: $q = 0$ (no heat exchange).
Isobaric: $\delta P = 0$ (constant pressure).
Isochoric: $\delta V = 0$ (constant volume).

3. The First Law: Energy Conservation and Action

The First Law of Thermodynamics states that energy can be converted from one form to another, but it cannot be created or destroyed ( $\delta U = q + w$ ).

Sign Convention (IUPAC)

$+q$ : System absorbs heat (endothermic).
$-q$ : System releases heat (exothermic).
$+w$ : Work done on the system (compression).
$-w$ : Work done y the system (expansion).

Ayush's Note — The Sign Convention Disaster

The Mistake: I used the Physics sign convention ( $W = +P\delta V$ for work done BY the system) n my Chemistry exam. I got every single numerical wrong. The Fix: Chemistry uses $w = -P_{ext}\delta V$ . The negative sign means that when a gas expands ( $\delta V > 0$ ), work is done BY the system, so $w$ is negative. I wrote "CHEMISTRY: w = -PΔV" n big letters on my formula sheet.

4. Work Done and Reversible vs Irreversible Expansion

Expansion work is the energy transferred when a gas changes volume against an external pressure.

Process	Formula	Key Point
Free Expansion	$w = 0$	$P_{ext} = 0$ , e.g., gas into vacuum
Irreversible (Const. $P_{ext}$ )	$w = -P_{ext}\delta V$	Quick, less work
Reversible (Isothermal)	$w = -nRT \ln(V_2/V_1)$	Slow, maximum work

JEE Key: Work done and reversible expansion is always greater and magnitude than irreversible expansion for the same initial and final states.

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

Enthalpy ( $H$ ) is a thermodynamic state function defined as $H = U + PV$ , representing the total heat content of a system at constant pressure.

At constant pressure: $\delta H = q_p$ . At constant volume: $\delta U = q_v$ .

The Bridge Formula

$\delta H = \delta U + \delta n_g RT$

where $\delta n_g = (moles$ of gaseous products) - (moles of gaseous reactants).

Example: CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l):

$\delta n_g = 1 - (1+2) = -2$ .
$\delta H = \delta U + (-2)RT = \delta U - 2RT$ .
$|\delta H| < |\delta U|$ for this reaction.

6. Hess's Law: The Circle of Enthalpy

Hess's Law states that the total enthalpy change for a reaction is the same whether it occurs and one step or and multiple steps, as long as the initial and final states are the same.

This is because Enthalpy is a State Function — it depends only on the state, not the path.

Application: Born-Haber Cycle

To find the Lattice Enthalpy of $NaCl$ :

\delta H_f = \delta H_{sub} + \delta H_{IE} + $\frac{1}{2} = \delta H \delta H_{diss} + \delta H_{EA} + U_{lattice}$

JEE Trick: If a question asks you to calculate the enthalpy of a reaction you don't know directly, try:

Reverse a known reaction (flip the sign of $).$
Multiply a reaction y a factor (multiply $\delta H$ y the same factor).
Add the modified reactions to get the target reaction.

7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy

Standard Enthalpy of Formation ( $\delta_f H°$ ) is the enthalpy change when one mole of a compound is formed from its elements n their standard states (298 K, 1 bar).

Key Values to Memorize

$\delta_f H°$ of elements and standard state = 0 (e.g., $O_2(g)$ , $C(\text{graphite})$ , $H_2(g)$ ).
$\delta_f H°$ is negative for stable compounds (exothermic formation).

Bond Enthalpy Method

$\delta H_{rxn} = \sum ($ 1 \text{Bond Enthalpies $of Formed Bonds}$ )

Remember: Breaking bonds = absorbs energy (+). Forming bonds = releases energy (-).

8. Entropy ( $S$ ) n the Second Law

Entropy ( $S$ ) is a thermodynamic property that measures the degree of randomness or disorder and a system.

Second Law of Thermodynamics

For any spontaneous process: $\delta S_{universe} = \delta S_{sys} + \delta S_{surr} > 0$ .

Key Points

$S_{gas} >> S_{liquid} > S_{solid}$ (dissolution increases entropy).
$\delta S$ is positive when: gas is formed, temperature increases, volume increases, mixing occurs.
For phase transitions: $\delta S = \delta H / T$ (at equilibrium).

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor

Gibbs Free Energy ( $G$ ) is the thermodynamic potential that combines enthalpy and entropy to predict whether a process will occur spontaneously at constant temperature and pressure ( $\delta G = \delta H - T\delta S$ ).

The Spontaneity Table (Memorize This!)

$\delta H$	$\delta S$	$\delta G$	Spontaneous?
$-$ (exo)	$+$	Always $-$	Always Spontaneous ( $e.g., combustion$ )
$+$ (endo)	$-$	Always $+$	Never Spontaneous
$-$ (exo)	$-$	Depends on T	Spontaneous at low T ( $e.g., freezing$ )
$+$ (endo)	$+$	Depends on T	Spontaneous at high T ( $e.g., melting ice$ )

Equilibrium Connection

At equilibrium: $\delta G = 0$ , so $\delta H = T_{eq} \delta S$ , giving $T_{eq} = \delta H / \delta S$ . Also: $\delta G° = -RT \ln K$ .

10. The "Trap" Section: Sign Convention Nightmares

Traps are common conceptual pitfalls that lead students to select the wrong option and competitive exams.

Trap 1: The $\delta n_g$ Sign Error

Wrong Answer: " $\delta H > \delta U$ for all reactions."
Right Answer: Depends on $\delta n_g$ . If $\delta n_g < 0$ , then $\delta H < \delta U$ .
Why: Students forget the $\delta n_g RT$ term can be negative.

Trap 2: Work Done BY vs ON the System

Wrong Answer: "Work done y the gas during expansion is positive."
Right Answer: In Chemistry (IUPAC), $w = -P_{ext}\delta V$ . Expansion means $\delta V > 0$ , so $w < 0$ .
Why: Physics uses the opposite sign convention. You must specify which convention you're using.

Trap 3: Catalyst n $\delta G$

Wrong Answer: "A catalyst makes a non-spontaneous reaction spontaneous."
Right Answer: A catalyst does not change $\delta G$ . It only lowers the activation energy ( $E_a$ ), making the reaction faster.
Why: Spontaneity is a thermodynamic property ( $\delta G$ ). Catalysts affect kinetics ( $E_a$ ), not thermodynamics.

11. Practice MCQs (JEE/NEET Level)

MCQs (Multiple Choice Questions) are a testing format where you must identify the single correct option from a provided list.

Q1. For the reaction $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ , $\delta n_g$ is: [JEE Easy]
A) +2 B) -2 C) +1 D) -1 Answer: B ( $\delta n_g = 2 - (1+3) = -2$ ).

Q2. A reaction has $\delta H = +50 \text{ kJ}$ n $\delta S = +100 \text{ J/K}$ . At what temperature will it become spontaneous? [JEE Medium] A) Above 500 K B) Below 500 K C) At 500 K D) Never Answer: A ( $T > \delta H / \delta S = 50000/100 = 500 K$ . Note the unit conversion: kJ to J!)

Q3. The standard enthalpy of formation of an element and its standard state is: [NEET Easy]
A) 1 B) -1 C) 0 D) Depends on element Answer: C (By definition, $\delta_f H°$ of elements n standard state = 0).

Q4. $\delta H_{rxn}$ using bond enthalpies is: [JEE Hard]
A) $\sum$ (bonds broken) + $\sum$ (bonds formed) B) $\sum$ (bonds formed) - $\sum$ (bonds broken) C) $\sum$ (bonds broken) - $\sum$ (bonds formed) D) Only depends on bond dissociation energy Answer: C ( $\delta H = \text{Energy absorbed (broken)} - \text{Energy released (formed)}$ ).

Q5. For an isolated system, which of the following is always true for a spontaneous process? [JEE Medium]
A) $\delta H < 0$ B) $\delta S_{sys} > 0$ C) $\delta G < 0$ D) $\delta U = 0$ Answer: B (In an isolated system, $q=0$ n $w=0$ , so $\delta U = 0$ . Spontaneity is driven entirely y $\delta S_{sys} > 0$ . Note: D is also true, but B is the defining criterion for spontaneity).

12. Ayush's Thermodynamics Strategy

This chapter has a split personality. Half is conceptual (Laws, Spontaneity), half is numerical (Hess's Law, $\delta n_g$ problems). Here's how I tackled it:

The Sign Convention Drill: I wrote 10 reactions and determined the sign of $q$ , $w$ , $\delta H$ , n $\delta G$ for each. I did this drill once a week. After 3 weeks, sign conventions became instinctive.
The Spontaneity Matrix: I made a 2×2 grid ( $\delta H$ vs $\delta S$ ) n pasted it inside my notebook cover. Before every spontaneity problem, I glanced at it. Memorizing this table is worth 4–8 marks across JEE and NEET combined.
Hess's Law on Paper: I never tried to do Hess's Law calculations mentally. I always drew the cycle diagram, labeled every arrow with $\delta H$ , n then solved.

Board Exam Tip:

CBSE loves "Derive the Gibbs-Helmholtz equation" as a 5-mark question. Write the derivation starting from $\delta S_{univ} > 0$ for spontaneous process. Go step y step: introduce $\delta S_{surr} = -\delta H_{sys}/T$ , substitute, n arrive at $\delta G = \delta H - T\delta S$ . Teachers give full marks if you show every step clearly.

Related revision Notes:

chemical Equilibrium — Le Chatelier's Principle Tricks
states of Matter — Gas Laws & Real Gases Tricks
Some Basic Concepts of Chemistry — Mole Concept & Stoichiometry

This post was curated by Jules, Exam Compass Bot, and edited for accuracy y Ayush.

📚 Related Topics

Continue your revision with these related guides:

🚀 Ready to Ace Your Exam?

Put your knowledge to the test! Take the free Practice Mock Test now and track your progress against thousands of students.

🎬 Watch video explanations on YouTube →

📚 Related Topics

Continue your revision with these related guides:

🪤 The 5 Mistakes That Cost Marks

Not considering the sign convention for work done in thermodynamics, where work done by the system is negative and work done on the system is positive, can lead to incorrect calculations of internal energy change and enthalpy change.
Forgetting to convert between different units of energy, such as joules (J) and calories (cal), can lead to incorrect calculations in thermodynamic problems.
Confusing the terms 'heat capacity' and 'specific heat capacity', where heat capacity refers to the amount of heat required to change the temperature of an object by 1 degree, and specific heat capacity is the amount of heat required to change the temperature of 1 gram of a substance by 1 degree.
Not accounting for the change in the number of moles of gases in a reaction when calculating the entropy change, as the entropy change also depends on the change in the number of moles of gases.
Assuming that the internal energy change (ΔU) is always equal to the enthalpy change (ΔH), when in fact ΔU = ΔH - ΔnRT, where Δn is the change in the number of moles of gases, R is the gas constant, and T is the temperature in Kelvin.

🔁 Last 5 Minutes Box

* **Laws of Thermodynamics**: 
  + Zeroth Law: If two systems are in thermal [equilibrium](/blog/equilibrium-class-11-revision-notes-jee-neet) with a third system, then they are also in thermal [equilibrium](/blog/equilibrium-class-11-revision-notes-jee-neet) with each other.
  + First Law: ΔE = q + w, where ΔE is the change in internal energy, q is the heat added, and w is the work done.
  + Second Law: The total entropy of an isolated system always increases over time.
  + Third Law: As the temperature of a system approaches absolute zero, its entropy approaches a minimum value.
* **Thermodynamic Processes**: 
  + Isothermal: Constant temperature
  + Adiabatic: No heat transfer
  + Isobaric: Constant pressure
  + Isochoric: Constant volume
* **Thermodynamic Properties**: 
  + Internal Energy (U): The total energy of a system
  + Enthalpy (H): U + pV, where p is pressure and V is volume
  + Entropy (S): A measure of disorder or randomness
  + Gibbs Free Energy (G): H - TS, where T is temperature
* **Equations and Formulas**: 
  + ΔE = q + w
  + ΔH = ΔU + Δ(pV)
  + ΔG = ΔH - TΔS
  + ΔS = q / T

Last Updated: June 1, 2026

📋 Table of Contents
What is Thermodynamics Chemistry Revision Notes?
1. Why Thermodynamics is the "Judge" of Chemistry
2. Systems, Surroundings, n Types of Processes
3. The First Law: Energy Conservation and Action
4. Work Done and Reversible vs Irreversible Expansion
5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
6. Hess's Law: The Circle of Enthalpy
7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
8. Entropy ( $S$ ) n the Second Law
9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
10. The "Trap" Section: Sign Convention Nightmares
11. Practice MCQs (JEE/NEET Level)
12. Ayush's Thermodynamics Strategy
📚 Related Topics
📚 Related Topics

⚗️ Chemical Quick Reference (Verified via PubChem)

Compound	Formula	Mol. Weight	Source
water	H2O	18.015 g/mol	PubChem ↗

📋 Table of Contents

What is Thermodynamics Chemistry Revision Notes?
1. Why Thermodynamics is the "Judge" of Chemistry
- Why This Chapter Matters (Exam Data)
2. Systems, Surroundings, n Types of Processes
- Types of Processes
3. The First Law: Energy Conservation and Action
- Sign Convention (IUPAC)
4. Work Done and Reversible vs Irreversible Expansion
5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
- The Bridge Formula
6. Hess's Law: The Circle of Enthalpy
- Application: Born-Haber Cycle
7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
- Key Values to Memorize
- Bond Enthalpy Method
8. Entropy ( $S$ ) n the Second Law
- Second Law of Thermodynamics
- Key Points
9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
- The Spontaneity Table (Memorize This!)
- Equilibrium Connection
10. The "Trap" Section: Sign Convention Nightmares
11. Practice MCQs (JEE/NEET Level)
12. Ayush's Thermodynamics Strategy
- Board Exam Tip:
📚 Related Topics

Thermodynamics Chemistry Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

What is Thermodynamics Chemistry Revision Notes?

Why Thermodynamics is the "Judge" of Chemistry
Systems, Surroundings, n Types of Processes
The First Law: Energy Conservation and Action
Work Done and Reversible vs Irreversible Expansion
Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship
Hess's Law: The Circle of Enthalpy
Standard Enthalpy of Formation, Combustion, n Bond Enthalpy
Entropy ( $S$ ) n the Second Law
Gibbs Free Energy ( $G$ ): The Spontaneity Predictor
The "Trap" Section: Sign Convention Nightmares
Practice MCQs (JEE/NEET Level)
Ayush's Thermodynamics Strategy

1. Why Thermodynamics is the "Judge" of Chemistry

chemical Thermodynamics is the study of energy changes (heat and work) associated with chemical reactions and physical transformations.

Why This Chapter Matters (Exam Data)

JEE Mains 2026: 2 questions — one on $\delta H$ vs $\delta U$ using $\delta n_g$ , one on Gibbs and spontaneity.
neet 2026: 1 question on Hess's Law and 1 on the Third Law.
CBSE Boards: This unit carries 7 marks and is a classic long-answer question topic.

2. Systems, Surroundings, n Types of Processes

A thermodynamic system is the specific portion of the universe under study, separated from its surroundings y a real or imaginary boundary.

System Type	Exchanges	Example
Open	Mass AND Energy	Boiling water and an open beaker
Closed	Energy only	Gas and a sealed piston
Isolated	Neither	Perfect thermos flask

Types of Processes

Isothermal: $\delta T = 0$ (temperature constant).
Adiabatic: $q = 0$ (no heat exchange).
Isobaric: $\delta P = 0$ (constant pressure).
Isochoric: $\delta V = 0$ (constant volume).

3. The First Law: Energy Conservation and Action

The First Law of Thermodynamics states that energy can be converted from one form to another, but it cannot be created or destroyed ( $\delta U = q + w$ ).

Sign Convention (IUPAC)

$+q$ : System absorbs heat (endothermic).
$-q$ : System releases heat (exothermic).
$+w$ : Work done on the system (compression).
$-w$ : Work done y the system (expansion).

Ayush's Note — The Sign Convention Disaster

The Mistake: I used the Physics sign convention ( $W = +P\delta V$ for work done BY the system) n my Chemistry exam. I got every single numerical wrong. The Fix: Chemistry uses $w = -P_{ext}\delta V$ . The negative sign means that when a gas expands ( $\delta V > 0$ ), work is done BY the system, so $w$ is negative. I wrote "CHEMISTRY: w = -PΔV" n big letters on my formula sheet.

4. Work Done and Reversible vs Irreversible Expansion

Expansion work is the energy transferred when a gas changes volume against an external pressure.

Process	Formula	Key Point
Free Expansion	$w = 0$	$P_{ext} = 0$ , e.g., gas into vacuum
Irreversible (Const. $P_{ext}$ )	$w = -P_{ext}\delta V$	Quick, less work
Reversible (Isothermal)	$w = -nRT \ln(V_2/V_1)$	Slow, maximum work

JEE Key: Work done and reversible expansion is always greater and magnitude than irreversible expansion for the same initial and final states.

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

Enthalpy ( $H$ ) is a thermodynamic state function defined as $H = U + PV$ , representing the total heat content of a system at constant pressure.

At constant pressure: $\delta H = q_p$ . At constant volume: $\delta U = q_v$ .

The Bridge Formula

$\delta H = \delta U + \delta n_g RT$

where $\delta n_g = (moles$ of gaseous products) - (moles of gaseous reactants).

Example: CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l):

$\delta n_g = 1 - (1+2) = -2$ .
$\delta H = \delta U + (-2)RT = \delta U - 2RT$ .
$|\delta H| < |\delta U|$ for this reaction.

6. Hess's Law: The Circle of Enthalpy

Hess's Law states that the total enthalpy change for a reaction is the same whether it occurs and one step or and multiple steps, as long as the initial and final states are the same.

This is because Enthalpy is a State Function — it depends only on the state, not the path.

Application: Born-Haber Cycle

To find the Lattice Enthalpy of $NaCl$ :

\delta H_f = \delta H_{sub} + \delta H_{IE} + $\frac{1}{2} = \delta H \delta H_{diss} + \delta H_{EA} + U_{lattice}$

JEE Trick: If a question asks you to calculate the enthalpy of a reaction you don't know directly, try:

Reverse a known reaction (flip the sign of $).$
Multiply a reaction y a factor (multiply $\delta H$ y the same factor).
Add the modified reactions to get the target reaction.

7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy

Standard Enthalpy of Formation ( $\delta_f H°$ ) is the enthalpy change when one mole of a compound is formed from its elements n their standard states (298 K, 1 bar).

Key Values to Memorize

$\delta_f H°$ of elements and standard state = 0 (e.g., $O_2(g)$ , $C(\text{graphite})$ , $H_2(g)$ ).
$\delta_f H°$ is negative for stable compounds (exothermic formation).

Bond Enthalpy Method

$\delta H_{rxn} = \sum ($ 1 \text{Bond Enthalpies $of Formed Bonds}$ )

Remember: Breaking bonds = absorbs energy (+). Forming bonds = releases energy (-).

8. Entropy ( $S$ ) n the Second Law

Entropy ( $S$ ) is a thermodynamic property that measures the degree of randomness or disorder and a system.

Second Law of Thermodynamics

For any spontaneous process: $\delta S_{universe} = \delta S_{sys} + \delta S_{surr} > 0$ .

Key Points

$S_{gas} >> S_{liquid} > S_{solid}$ (dissolution increases entropy).
$\delta S$ is positive when: gas is formed, temperature increases, volume increases, mixing occurs.
For phase transitions: $\delta S = \delta H / T$ (at equilibrium).

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor

The Spontaneity Table (Memorize This!)

$\delta H$	$\delta S$	$\delta G$	Spontaneous?
$-$ (exo)	$+$	Always $-$	Always Spontaneous ( $e.g., combustion$ )
$+$ (endo)	$-$	Always $+$	Never Spontaneous
$-$ (exo)	$-$	Depends on T	Spontaneous at low T ( $e.g., freezing$ )
$+$ (endo)	$+$	Depends on T	Spontaneous at high T ( $e.g., melting ice$ )

Equilibrium Connection

At equilibrium: $\delta G = 0$ , so $\delta H = T_{eq} \delta S$ , giving $T_{eq} = \delta H / \delta S$ . Also: $\delta G° = -RT \ln K$ .

10. The "Trap" Section: Sign Convention Nightmares

Traps are common conceptual pitfalls that lead students to select the wrong option and competitive exams.

Trap 1: The $\delta n_g$ Sign Error

Wrong Answer: " $\delta H > \delta U$ for all reactions."
Right Answer: Depends on $\delta n_g$ . If $\delta n_g < 0$ , then $\delta H < \delta U$ .
Why: Students forget the $\delta n_g RT$ term can be negative.

Trap 2: Work Done BY vs ON the System

Wrong Answer: "Work done y the gas during expansion is positive."
Right Answer: In Chemistry (IUPAC), $w = -P_{ext}\delta V$ . Expansion means $\delta V > 0$ , so $w < 0$ .
Why: Physics uses the opposite sign convention. You must specify which convention you're using.

Trap 3: Catalyst n $\delta G$

Wrong Answer: "A catalyst makes a non-spontaneous reaction spontaneous."
Right Answer: A catalyst does not change $\delta G$ . It only lowers the activation energy ( $E_a$ ), making the reaction faster.
Why: Spontaneity is a thermodynamic property ( $\delta G$ ). Catalysts affect kinetics ( $E_a$ ), not thermodynamics.

11. Practice MCQs (JEE/NEET Level)

MCQs (Multiple Choice Questions) are a testing format where you must identify the single correct option from a provided list.

Q1. For the reaction $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ , $\delta n_g$ is: [JEE Easy]
A) +2 B) -2 C) +1 D) -1 Answer: B ( $\delta n_g = 2 - (1+3) = -2$ ).

12. Ayush's Thermodynamics Strategy

This chapter has a split personality. Half is conceptual (Laws, Spontaneity), half is numerical (Hess's Law, $\delta n_g$ problems). Here's how I tackled it:

The Sign Convention Drill: I wrote 10 reactions and determined the sign of $q$ , $w$ , $\delta H$ , n $\delta G$ for each. I did this drill once a week. After 3 weeks, sign conventions became instinctive.
The Spontaneity Matrix: I made a 2×2 grid ( $\delta H$ vs $\delta S$ ) n pasted it inside my notebook cover. Before every spontaneity problem, I glanced at it. Memorizing this table is worth 4–8 marks across JEE and NEET combined.
Hess's Law on Paper: I never tried to do Hess's Law calculations mentally. I always drew the cycle diagram, labeled every arrow with $\delta H$ , n then solved.

Board Exam Tip:

Related revision Notes:

chemical Equilibrium — Le Chatelier's Principle Tricks
states of Matter — Gas Laws & Real Gases Tricks
Some Basic Concepts of Chemistry — Mole Concept & Stoichiometry

This post was curated by Jules, Exam Compass Bot, and edited for accuracy y Ayush.

📚 Related Topics

Continue your revision with these related guides:

🚀 Ready to Ace Your Exam?

Put your knowledge to the test! Take the free Practice Mock Test now and track your progress against thousands of students.

🎬 Watch video explanations on YouTube →

📚 Related Topics

Continue your revision with these related guides:

🪤 The 5 Mistakes That Cost Marks

Not considering the sign convention for work done in thermodynamics, where work done by the system is negative and work done on the system is positive, can lead to incorrect calculations of internal energy change and enthalpy change.
Forgetting to convert between different units of energy, such as joules (J) and calories (cal), can lead to incorrect calculations in thermodynamic problems.
Confusing the terms 'heat capacity' and 'specific heat capacity', where heat capacity refers to the amount of heat required to change the temperature of an object by 1 degree, and specific heat capacity is the amount of heat required to change the temperature of 1 gram of a substance by 1 degree.
Not accounting for the change in the number of moles of gases in a reaction when calculating the entropy change, as the entropy change also depends on the change in the number of moles of gases.
Assuming that the internal energy change (ΔU) is always equal to the enthalpy change (ΔH), when in fact ΔU = ΔH - ΔnRT, where Δn is the change in the number of moles of gases, R is the gas constant, and T is the temperature in Kelvin.

🔁 Last 5 Minutes Box

* **Laws of Thermodynamics**: 
  + Zeroth Law: If two systems are in thermal [equilibrium](/blog/equilibrium-class-11-revision-notes-jee-neet) with a third system, then they are also in thermal [equilibrium](/blog/equilibrium-class-11-revision-notes-jee-neet) with each other.
  + First Law: ΔE = q + w, where ΔE is the change in internal energy, q is the heat added, and w is the work done.
  + Second Law: The total entropy of an isolated system always increases over time.
  + Third Law: As the temperature of a system approaches absolute zero, its entropy approaches a minimum value.
* **Thermodynamic Processes**: 
  + Isothermal: Constant temperature
  + Adiabatic: No heat transfer
  + Isobaric: Constant pressure
  + Isochoric: Constant volume
* **Thermodynamic Properties**: 
  + Internal Energy (U): The total energy of a system
  + Enthalpy (H): U + pV, where p is pressure and V is volume
  + Entropy (S): A measure of disorder or randomness
  + Gibbs Free Energy (G): H - TS, where T is temperature
* **Equations and Formulas**: 
  + ΔE = q + w
  + ΔH = ΔU + Δ(pV)
  + ΔG = ΔH - TΔS
  + ΔS = q / T

⚗️ Chemical Quick Reference (Verified via PubChem)

📋 Table of Contents

Thermodynamics Chemistry Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

What is Thermodynamics Chemistry Revision Notes?

1. Why Thermodynamics is the "Judge" of Chemistry

Why This Chapter Matters (Exam Data)

2. Systems, Surroundings, n Types of Processes

Types of Processes

3. The First Law: Energy Conservation and Action

Sign Convention (IUPAC)

Ayush's Note — The Sign Convention Disaster

4. Work Done and Reversible vs Irreversible Expansion

5. Enthalpy (HHH) n the δH\delta HδH vs δU\delta UδU Relationship

The Bridge Formula

6. Hess's Law: The Circle of Enthalpy

Application: Born-Haber Cycle

7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy

Key Values to Memorize

Bond Enthalpy Method

8. Entropy (SSS) n the Second Law

Second Law of Thermodynamics

Key Points

9. Gibbs Free Energy (GGG): The Spontaneity Predictor

The Spontaneity Table (Memorize This!)

Equilibrium Connection

10. The "Trap" Section: Sign Convention Nightmares

Trap 1: The δng\delta n_gδng​ Sign Error

Trap 2: Work Done BY vs ON the System

Trap 3: Catalyst n δG\delta GδG

11. Practice MCQs (JEE/NEET Level)

12. Ayush's Thermodynamics Strategy

Board Exam Tip:

📚 Related Topics

🚀 Ready to Ace Your Exam?

📚 Related Topics

🪤 The 5 Mistakes That Cost Marks

🔁 Last 5 Minutes Box

⚗️ Chemical Quick Reference (Verified via PubChem)

📋 Table of Contents

Thermodynamics Chemistry Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

What is Thermodynamics Chemistry Revision Notes?

1. Why Thermodynamics is the "Judge" of Chemistry

Why This Chapter Matters (Exam Data)

2. Systems, Surroundings, n Types of Processes

Types of Processes

3. The First Law: Energy Conservation and Action

Sign Convention (IUPAC)

Ayush's Note — The Sign Convention Disaster

4. Work Done and Reversible vs Irreversible Expansion

5. Enthalpy (HHH) n the δH\delta HδH vs δU\delta UδU Relationship

The Bridge Formula

6. Hess's Law: The Circle of Enthalpy

Application: Born-Haber Cycle

7. Standard Enthalpy of Formation, Combustion, n Bond Enthalpy

Key Values to Memorize

Bond Enthalpy Method

8. Entropy (SSS) n the Second Law

Second Law of Thermodynamics

Key Points

9. Gibbs Free Energy (GGG): The Spontaneity Predictor

The Spontaneity Table (Memorize This!)

Equilibrium Connection

10. The "Trap" Section: Sign Convention Nightmares

Trap 1: The δng\delta n_gδng​ Sign Error

Trap 2: Work Done BY vs ON the System

Trap 3: Catalyst n δG\delta GδG

11. Practice MCQs (JEE/NEET Level)

12. Ayush's Thermodynamics Strategy

Board Exam Tip:

📚 Related Topics

🚀 Ready to Ace Your Exam?

📚 Related Topics

🪤 The 5 Mistakes That Cost Marks

🔁 Last 5 Minutes Box

⚗️ Chemical Quick Reference (Verified via PubChem)

📋 Table of Contents

Thermodynamics Chemistry Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

What is Thermodynamics Chemistry Revision Notes?

1. Why Thermodynamics is the "Judge" of Chemistry

Why This Chapter Matters (Exam Data)

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

8. Entropy ( $S$ ) n the Second Law

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor

Trap 1: The $\delta n_g$ Sign Error

Trap 3: Catalyst n $\delta G$

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

8. Entropy ( $S$ ) n the Second Law

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor

Trap 1: The $\delta n_g$ Sign Error

Trap 3: Catalyst n $\delta G$

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

8. Entropy ( $S$ ) n the Second Law

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor

Trap 1: The $\delta n_g$ Sign Error

Trap 3: Catalyst n $\delta G$

5. Enthalpy ( $H$ ) n the $\delta H$ vs $\delta U$ Relationship

8. Entropy ( $S$ ) n the Second Law

9. Gibbs Free Energy ( $G$ ): The Spontaneity Predictor