Thermodynamics

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Thermodynamics is the branch of chemistry that deals with the relationships between heat, work, energy, and matter. It explains how energy flows during chemical and physical processes. In NEET, this topic accounts for 3-5 questions every year, and understanding the core principles can score you 12-20 marks directly.

Key Definitions:

• System: The part of the universe being studied
• Surroundings: Everything outside the system
• Boundary: The separating surface between system and surroundings
• Open system: Exchanges both mass and energy with surroundings
• Closed system: Exchanges only energy (not mass) with surroundings
• Isolated system: Exchanges neither mass nor energy

First Law of Thermodynamics (Law of Conservation of Energy): $$\Delta U = q + w$$

Where $\Delta U$ = change in internal energy, $q$ = heat exchanged, $w$ = work done on/by the system.

Sign conventions:

• $q > 0$: Heat absorbed by system (endothermic)
• $q < 0$: Heat evolved by system (exothermic)
• $w > 0$: Work done on the system
• $w < 0$: Work done by the system (expansion work)

For expansion work at constant pressure: $w = -P_{\text{ext}} \Delta V$

Enthalpy (H): $$H = U + PV$$ $$\Delta H = \Delta U + \Delta n_g \cdot RT$$ (for gas-phase reactions)

⚡ Exam Tip: For NEET, the relationship $\Delta H = \Delta U + \Delta n_g RT$ is crucial. $\Delta n_g$ = moles of gaseous products - moles of gaseous reactants. If $\Delta n_g > 0$, then $\Delta H > \Delta U$. For the reaction N₂O₄(g) → 2NO₂(g), $\Delta n_g = 2 - 1 = +1$, so $\Delta H = \Delta U + RT$.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding and problem-solving practice.

Types of Processes:

Process	Condition	Key Feature
Isothermal	$\Delta T = 0$	$\Delta U = 0$, $q = -w$
Adiabatic	$q = 0$	$\Delta U = w$
Isobaric	$\Delta P = 0$	$P$ constant throughout
Isochoric	$\Delta V = 0$	$w = 0$, $\Delta U = q$
Cyclic	$\Delta U = 0$	$q = -w$
Reversible	Quasi-equilibrium	System in equilibrium at each step
Irreversible	Non-equilibrium	Faster, real processes

Thermodynamic Equations for Ideal Gases:

For isothermal reversible process: $$w_{\text{rev}} = -2.303 , nRT \log_{10}\frac{V_2}{V_1} = -2.303 , nRT \log_{10}\frac{P_1}{P_2}$$

For adiabatic reversible process: $$PV^\gamma = \text{constant}$$ where $\gamma = \frac{C_P}{C_V}$ $$TV^{\gamma-1} = \text{constant}$$ $$T^\gamma P^{1-\gamma} = \text{constant}$$

Heat Capacity: $$C = \frac{q}{\Delta T}$$

• Molar heat capacity at constant volume: $C_V = \left(\frac{\partial U}{\partial T}\right)_V$
• Molar heat capacity at constant pressure: $C_P = \left(\frac{\partial H}{\partial T}\right)_P$
• Relationship: $C_P - C_V = R$ (for ideal gases)
• For monatomic gas: $C_V = \frac{3}{2}R$, $C_P = \frac{5}{2}R$
• For diatomic gas (linear): $C_V = \frac{5}{2}R$, $C_P = \frac{7}{2}R$

Standard Enthalpies of Reaction: $$\Delta H^\circ_{\text{rxn}} = \sum \Delta H^\circ_f(\text{products}) - \sum \Delta H^\circ_f(\text{reactants})$$

Hess’s Law: $\Delta H$ is independent of the path taken — the enthalpy change equals the sum of enthalpy changes for individual steps.

Born-Haber Cycle: $\Delta H^\circ_f$ (ionic solid) = $\Delta H_{\text{atom}} + \Delta H_{\text{ionisation}} + \Delta H_{\text{electron affinity}} + \Delta H_{\text{lattice}}$

⚡ NEET-Specific Tip: Born-Haber cycle questions are frequently asked. For NaCl: sublimation of Na(s) = +107 kJ/mol, dissociation of $\frac{1}{2}$Cl₂ = +121 kJ/mol, ionisation of Na(g) = +496 kJ/mol, electron affinity of Cl(g) = -349 kJ/mol, lattice enthalpy = +788 kJ/mol. Sum = +107 + 121 + 496 - 349 + 788 = +411 kJ/mol (experimental $\Delta H^\circ_f$ = -411 kJ/mol for NaCl).

Common Student Mistakes:

• Confusing sign conventions for $q$ and $w$
• Forgetting that only expansion work counts for $w = -P\Delta V$ under constant pressure
• Using wrong units (R = 8.314 J mol⁻¹ K⁻¹ = 2 cal mol⁻¹ K⁻¹)

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory with derivations and exam pattern analysis.

Second Law of Thermodynamics:

Heat cannot flow spontaneously from a colder body to a hotter body (Clausius statement).

No device operating in a cycle can convert heat completely into work without some other effect (Kelvin-Planck statement — perpetuum mobile of the second kind is impossible).

Entropy (S): $$dS = \frac{dq_{\text{rev}}}{T}$$

Entropy is a measure of disorder or randomness. For a reversible process: $$\Delta S = \frac{q_{\text{rev}}}{T} = \frac{\Delta H}{T}$$ (for phase transitions)

Entropy Changes:

• For ideal gas: $\Delta S = nC_V \ln\frac{T_2}{T_1} + nR ln\frac{V_2}{V_1}$
• Phase change: $\Delta S = \frac{\Delta H_{\text{transition}}}{T_{\text{transition}}}$
• Mixing of ideal gases: $\Delta S_{\text{mix}} = -nR \sum x_i \ln x_i$

Third Law of Thermodynamics: The entropy of a perfect crystal at absolute zero (0 K) is zero: $S_0 = 0$.

This allows calculation of absolute entropies, not just changes.

Gibbs Free Energy (G): $$G = H - TS$$ $$\Delta G = \Delta H - T\Delta S$$

At constant $T$ and $P$: $\Delta G < 0$ (spontaneous), $\Delta G = 0$ (equilibrium), $\Delta G > 0$ (non-spontaneous)

Temperature and spontaneity: For $\Delta H > 0$ and $\Delta S > 0$: spontaneous at high $T$ (when $T > \frac{\Delta H}{\Delta S}$) For $\Delta H > 0$ and $\Delta S < 0$: never spontaneous For $\Delta H < 0$ and $\Delta S > 0$: always spontaneous For $\Delta H < 0$ and $\Delta S < 0$: spontaneous at low $T$ (when $T < \frac{\Delta H}{\Delta S}$)

Relationship between $\Delta G$ and Equilibrium Constant: $$\Delta G = \Delta G^\circ + 2.303 RT \log_{10} Q$$ At equilibrium: $\Delta G = 0$, so $\Delta G^\circ = -2.303 RT \log_{10} K$ $$\Delta G^\circ = -RT \ln K$$

Spontaneity Criteria Derivation: From $\Delta G = \Delta H - T\Delta S$ and the fact that for any spontaneous process, total entropy increases ($\Delta S_{\text{total}} > 0$): $$\Delta S_{\text{total}} = \Delta S_{\text{system}} + \Delta S_{\text{surroundings}} = \frac{-\Delta H}{T} + \Delta S > 0$$ $$\Delta H < T\Delta S \Rightarrow \Delta G < 0$$

NEET Previous Year Patterns (2019-2024):

• 2019: Calculation of entropy change for isothermal expansion of ideal gas (3 marks)
• 2020: Relationship between $\Delta G$ and $K$ for reaction at 298 K (3 marks)
• 2021: Born-Haber cycle for MgO (3 marks)
• 2022: Prediction of spontaneity based on $\Delta H$ and $\Delta S$ signs (2 marks)
• 2023: Work done in isothermal reversible expansion (4 marks)
• 2024: Comparison of heat capacities $C_P$ vs $C_V$ for real vs ideal gases (2 marks)

⚡ Advanced Tip: For NEET, remember that for the reaction quotient $Q$: when $Q < K$, the reaction proceeds forward; when $Q > K$, it proceeds backward. Also note: $\Delta G^\circ = -2.303 RT \log_{10} K$ means if $K > 1$, then $\Delta G^\circ < 0$ (spontaneous under standard conditions). For every 10-fold increase in $K$ at 298 K, $\Delta G^\circ$ becomes more negative by about 11.4 kJ mol⁻¹ (since $2.303 \times 8.314 \times 298 / 1000 \approx 5.7$ kJ per order of magnitude).

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📊 NEET UG Exam Essentials

Detail	Value
Questions	200 (180 mandatory + 10 optional)
Time	3h 20min
Marks	720
Section	Physics (50), Chemistry (50), Biology (100)
Negative	−1 for wrong answer
Qualifying	50th percentile (general category)

🎯 High-Yield Topics for NEET UG

• Human Physiology — 18 marks
• Genetics & Evolution — 16 marks
• Ecology & Environment — 12 marks
• Organic Chemistry (Reactions) — 15 marks
• Electrodynamics (Physics) — 18 marks
• Chemical Equilibrium — 10 marks

📝 Previous Year Question Patterns

• Q: “A particle moves in a circle…” [2024 Physics — 2 marks]
• Q: “Identify the incorrect statement about DNA…” [2024 Biology — 4 marks]
• Q: “The major product ofFriedel-Crafts acylation is…” [2024 Chemistry — 3 marks]

💡 Pro Tips

• NCERT Biology is the single most important resource — 80%+ questions are from NCERT lines
• Focus on Human Physiology, Genetics, and Ecology — together they make ~40% of Biology
• In Physics, master Electrostatics + Current Electricity + Magnetism (combined ~20%)
• Organic Chemistry: learn named reactions with mechanisms — they repeat across years

🔗 Official Resources

• NTA Official
• NEET Syllabus PDF

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