JEE Main Chemistry · Thermodynamics PYQ
JEE Main Chemical Thermodynamics PYQ (2002–2025)
Chemical Thermodynamics is a P1 Physical Chemistry chapter — ~5% JEE Main weightage with 1–2 questions per session. Alongside Equilibrium, Electrochemistry and Kinetics, it forms the Physical Chemistry core that drives ~20% of the Chemistry section.
Chemical Thermodynamics PYQs from 2002 to 2025, tagged by sub-topic (first law, enthalpy, entropy, Gibbs free energy, spontaneity). Every solution shows the formula substitution and sign-convention reasoning.
Chemical Thermodynamics at a Glance
Key Sub-Topics & What's Tested
First Law of Thermodynamics
ΔU = q + w, sign conventions for work and heat, internal energy as state function, expansion work.
Enthalpy & Hess's Law
Enthalpy H = U + PV, ΔH at constant pressure, Hess's law for reaction enthalpy, enthalpy cycles.
Standard Enthalpy Changes
ΔH°_f (formation), ΔH°_c (combustion), ΔH°_hydration, ΔH°_atomisation, ΔH°_ionisation, ΔH°_solution, ΔH°_neutralisation.
Bond Enthalpy & Lattice Energy
Calculating ΔH from bond enthalpies, Born-Haber cycle for lattice energy, Madelung constant conceptual treatment.
Entropy
ΔS = q_rev/T, entropy change in phase transitions, statistical interpretation, spontaneity criterion for isolated systems.
Gibbs Free Energy
ΔG = ΔH - TΔS, spontaneity at constant T and P, ΔG° and equilibrium constant (ΔG° = -RT ln K).
Thermochemistry Sign Conventions
Exothermic: ΔH < 0. Endothermic: ΔH > 0. Spontaneous: ΔG < 0. Non-spontaneous: ΔG > 0. Equilibrium: ΔG = 0.
Second & Third Laws of Thermodynamics
Entropy of universe increases in spontaneous processes, entropy of perfect crystal at 0 K is zero.
Question Type Distribution
| Question Type | Share (approx) | Example Pattern |
|---|---|---|
| Enthalpy Calculation (Hess's Law) | 25% | Given three reactions with ΔH, compute ΔH of target reaction using Hess's law. |
| Bond Enthalpy Application | 15% | Calculate ΔH_reaction from bond enthalpies of reactants and products. |
| Entropy Change | 15% | Compute ΔS for reversible isothermal expansion of ideal gas. |
| Gibbs Free Energy / Spontaneity | 20% | Given ΔH and ΔS, find temperature above which reaction is spontaneous. |
| Work and Heat Calculation | 15% | Find q and w for isothermal, adiabatic, isobaric or isochoric processes. |
| Standard Enthalpy Types | 10% | Match given enthalpy changes (formation, neutralisation, combustion) to their definitions. |
How to Solve Chemical Thermodynamics PYQs
- 1Memorise sign conventions rigorously. Work done ON system is +w (new IUPAC). Work done BY system is -w. Heat absorbed BY system is +q. Wrong sign is one of the most common ways PYQ answers go wrong.
- 2For Hess's law, set up cycle diagram. Draw reactants → target product. Combine given reactions to reach target with correct coefficients. Algebraic manipulation fails without the diagram.
- 3ΔG = ΔH - TΔS: the master equation. Almost every spontaneity/equilibrium PYQ reduces to this. Know when ΔG is negative (spontaneous), positive (non-spontaneous), zero (equilibrium).
- 4Standard states at 25°C, 1 atm. ΔH°, ΔS°, ΔG° refer to standard conditions. Non-standard requires adjustment.
- 5Bond enthalpy approach: gas-phase bonds only. Bond enthalpies are for gas-phase molecules. For liquid or solid reactants, first account for vaporisation/sublimation.
Common Mistakes That Cost Marks
- Using old sign convention (w = -PΔV) vs new (+PΔV). IUPAC 2005+ uses ΔU = q + w with w positive when work done on system. Check which convention the problem uses.
- Missing -ve sign in ΔG = -RT ln K. For spontaneous (K > 1), ln K > 0, so ΔG° < 0. Sign error swaps spontaneity direction.
- Confusing ΔU and ΔH. ΔH = ΔU + Δ(PV). For solids/liquids, ΔH ≈ ΔU. For gases, ΔH = ΔU + Δn_gas·RT.
- Using K = Kp when Kc is needed. ΔG° = -RT ln K uses equilibrium constant — be clear on Kp vs Kc based on reaction.
- Ignoring temperature in spontaneity questions. ΔG = ΔH - TΔS depends on T. At low T, ΔH dominates. At high T, TΔS dominates. Know the temperature boundary.
Related JEE Main Practice
Frequently asked questions
How many Chemical Thermodynamics PYQs should I solve?
Target 60–80 PYQs across 2010–2025. Given 5% weightage + high formula density, focused practice yields strong mock returns.
Is Chemical Thermodynamics the same as Physics Thermodynamics?
Underlying laws are identical. Chemistry focuses on enthalpy, entropy, Gibbs free energy of reactions. Physics focuses on heat engines, kinetic theory of gases. Both are P1 in their subjects.
Do I need to know the Born-Haber cycle in detail?
For JEE Main, conceptual understanding and direct application suffice. Born-Haber cycle questions typically give lattice energy components and ask for one unknown.
Which formula is most important for JEE Main Thermodynamics?
ΔG = ΔH - TΔS. It connects spontaneity, equilibrium and temperature dependence. At least 30% of PYQs directly apply this formula.
What's the most-tested Hess's law template?
Combustion of C and H₂ combined with formation of water to compute ΔH_formation of CH₄. Appears in 3–4 variants per 10-year window.
Do I need Carnot efficiency in Chemical Thermodynamics?
No — that's Physics Thermodynamics. Chemistry stays with enthalpy, entropy and Gibbs free energy of reactions. No heat engines.
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