JEE Chemistry — Thermodynamics and Thermochemistry Complete Chapter Guide

Chapter Overview & Weightage

Thermodynamics is one of those chapters where JEE rewards students who understand the logic behind the rules, not just memorization. A student who knows why ΔG must be negative for spontaneity will never confuse the sign conventions — but one who just memorized it will get trapped by the examiner every time.

Thermodynamics + Thermochemistry together contribute 6-8% of JEE Main Chemistry — roughly 2-3 questions per paper. In JEE Advanced, expect conceptual, multi-step problems rather than formula substitution. This is also a high-weightage topic for BITSAT and state engineering entrances.

Year	JEE Main Questions	Marks	Key Topics Tested
2024	2	8	Gibbs energy, spontaneity
2023	3	12	Hess’s law, entropy, bond energy
2022	2	8	First law, work done by gas
2021	2	8	Gibbs free energy, ΔH calculation
2020	3	12	Entropy changes, spontaneity criteria
2019	2	8	Thermochemical equations, Hess’s law

The trend is clear: spontaneity (ΔG, ΔS) and Hess’s law calculations dominate. Pure First Law numericals are less frequent now — examiners have shifted to conceptual questions.

Key Concepts You Must Know

Prioritized by how frequently they appear in PYQs:

Tier 1 — Almost guaranteed every year:

Gibbs free energy: ΔG = ΔH − TΔS and spontaneity criteria
Hess’s law applications (formation, combustion, bond energies)
Entropy and the Second Law — predicting sign of ΔS from physical changes
Standard enthalpy of formation vs. combustion vs. reaction

Tier 2 — High probability:

First Law: ΔU = q + w (IUPAC sign convention — w = −PΔV for expansion)
Work done in isothermal reversible expansion: $w = -nRT\ln(V_f/V_i)$
Kirchhoff’s equation: how ΔH changes with temperature
Resonance energy and lattice energy via Born-Haber cycle

Tier 3 — Occasional appearance:

Carnot cycle efficiency: $\eta = 1 - T_c/T_h$
Trouton’s rule (ΔS of vaporization ≈ 88 J/mol·K for most liquids)
Helmholtz free energy (A = U − TS) — rare in JEE Main, possible in Advanced

Important Formulas

\Delta U = q + w

Work done on the system is positive (IUPAC). For expansion against constant pressure:

w = -P_{ext}\Delta V

When to use: Any question involving heat, work, and internal energy change together. Remember: for an ideal gas at constant temperature, ΔU = 0, so q = −w.

\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})

Also usable with bond energies:

\Delta H_{rxn} = \sum BE(\text{bonds broken}) - \sum BE(\text{bonds formed})

When to use: Any ΔH calculation. Hess’s law lets you add/subtract thermochemical equations — treat them like algebraic equations (reverse = change sign, multiply = multiply ΔH).

\Delta G = \Delta H - T\Delta S

\Delta G^\circ = -RT\ln K

\Delta G^\circ = -nFE^\circ_{cell}

When to use: Spontaneity questions. If ΔG < 0, reaction is spontaneous. The relationship with K and E°cell appears often in combined chapters (electrochemistry + thermodynamics).

\Delta S = nC_v\ln\frac{T_2}{T_1} + nR\ln\frac{V_2}{V_1}

\Delta S = nC_p\ln\frac{T_2}{T_1} - nR\ln\frac{P_2}{P_1}

When to use: When a gas undergoes simultaneous temperature and volume/pressure change. For isothermal processes, the temperature terms vanish.

\Delta H_{T_2} = \Delta H_{T_1} + \Delta C_p(T_2 - T_1)

where $\Delta C_p = \sum C_p(\text{products}) - \sum C_p(\text{reactants})$

When to use: When ΔH at one temperature is given and you need it at another. Less frequent in JEE Main but fair game in Advanced.

Solved Previous Year Questions

PYQ 1 — JEE Main 2023 (April Session)

Question: For the reaction: $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$ , ΔH = −92 kJ/mol. Given bond energies: N≡N = 945 kJ/mol, H−H = 436 kJ/mol, N−H = 391 kJ/mol. Calculate ΔH using bond energies and compare.

Solution:

Bonds broken (reactants):

1 × N≡N = 945 kJ
3 × H−H = 3 × 436 = 1308 kJ
Total broken = 2253 kJ

Bonds formed (products):

2 × NH₃ means 2 × 3 = 6 N−H bonds
6 × N−H = 6 × 391 = 2346 kJ

\Delta H = 2253 - 2346 = -93 \text{ kJ/mol}

This matches the given value (−92 kJ/mol), with the small discrepancy due to average bond energies.

Bond energy method gives approximate ΔH. The rule is: break bonds = energy IN (endothermic), form bonds = energy OUT (exothermic). Many students get the sign wrong by confusing which is positive and which is negative. Just remember: breaking requires energy (you pay), forming releases energy (you get paid).

PYQ 2 — JEE Main 2024 Shift 1

Question: At 298 K, the standard Gibbs free energy of formation of NO(g) is +86.6 kJ/mol and of NO₂(g) is +51.3 kJ/mol. Calculate ΔG° for the reaction:

\text{NO}(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{NO}_2(g)

Is the reaction spontaneous at 298 K?

Solution:

\Delta G^\circ = \Delta G_f^\circ(\text{NO}_2) - [\Delta G_f^\circ(\text{NO}) + \frac{1}{2}\Delta G_f^\circ(\text{O}_2)]

Note: ΔG°f of O₂(g) = 0 (elemental standard state).

\Delta G^\circ = 51.3 - [86.6 + 0] = -35.3 \text{ kJ/mol}

Since ΔG° < 0, the reaction is spontaneous at 298 K.

The most common error here: students forget that ΔG°f of any element in its standard state is zero. O₂, N₂, H₂, graphite C — all zero. Only compounds have non-zero formation enthalpies/free energies.

PYQ 3 — JEE Advanced 2022 (Paper 1)

Question: For a process to be spontaneous at all temperatures, which condition must hold?

(A) ΔH < 0, ΔS < 0
(B) ΔH < 0, ΔS > 0
(C) ΔH > 0, ΔS > 0
(D) ΔH > 0, ΔS < 0

Answer: (B)

Why this is the correct logic: ΔG = ΔH − TΔS must be negative at all temperatures. If ΔH < 0 and ΔS > 0, then ΔG = (negative) − T(positive) = negative − positive = always negative, regardless of T.

Let’s check the other cases:

(A): ΔG = (−) − T(−) = (−) + T(+) → becomes positive at high T. Not spontaneous at all temperatures.
(C): ΔG = (+) − T(+) → might be negative at high T, but not at low T.
(D): ΔG = (+) − T(−) = (+) + T(+) = always positive. Never spontaneous.

The four combinations of ΔH and ΔS signs is a JEE Advanced favourite. Make a 2×2 table in your head: only ΔH < 0 and ΔS > 0 gives spontaneity at all temperatures. Only ΔH > 0 and ΔS < 0 gives non-spontaneity at all temperatures.

Difficulty Distribution

Based on analysis of 2018–2024 JEE Main papers:

Difficulty	Percentage	Typical Question Type
Easy	30%	Direct ΔH calculation using formation values, sign of ΔS from physical change
Medium	50%	Hess’s law with 2-3 step combination, Gibbs energy spontaneity, bond energy calculation
Hard	20%	Multi-concept (ΔG + equilibrium + electrochemistry), Kirchhoff’s equation, entropy calculations

JEE Advanced skews harder — expect 50% medium and 50% hard, with questions that mix thermodynamics with chemical equilibrium or electrochemistry.

Expert Strategy

Week 1 — Build the framework first

Before touching numericals, understand the physical meaning of each state function. ΔU is the total energy change; ΔH accounts for PV work done against atmosphere; ΔG tells you what’s left after accounting for both enthalpy and the system’s tendency toward disorder.

The scoring approach for JEE Main

Hess’s law and bond energy questions are high-confidence marks — they’re formula-based and predictable. Spend 60% of your revision time here. Spontaneity (ΔG sign analysis) is conceptual but quick once you’ve built the 2×2 mental model.

When revising, solve PYQs year-wise rather than topic-wise. Thermodynamics questions often overlap with chemical equilibrium (via ΔG° = −RT ln K) and electrochemistry (via ΔG° = −nFE°). Recognizing these connections at exam time saves you from treating each question as isolated.

For JEE Advanced specifically

The Advanced paper tests whether you understand the direction of reasoning, not just computation. A question might give you K at two temperatures and ask you to infer ΔH. This requires knowing van’t Hoff equation + the relationship between K and ΔG° + Gibbs-Helmholtz. Practice these multi-step inferences deliberately.

The 48-hour revision strategy

Day 1: Revise all formulas → solve 15 Hess’s law PYQs → solve 10 Gibbs energy PYQs.
Day 2: Attempt 2 full mock sections (25 minutes each) → review errors → identify which sign convention mistakes you’re making.

Common Traps

Sign convention trap (catches ~40% of students): IUPAC convention says work done on the system is positive. So when a gas is compressed, w is positive. When a gas expands, w is negative. Many students use the old chemistry convention (w = +PΔV for expansion) and get the opposite sign.

ΔH vs ΔU confusion: For reactions involving gases, ΔH = ΔU + ΔnᵍRT, where Δnᵍ is the change in moles of gas. Examiners routinely give ΔH at constant pressure and ask for ΔU (or vice versa). Always check: are gases involved? If yes, apply the correction.

Hess’s law reversal errors: When you reverse a thermochemical equation, change the sign of ΔH. When you multiply by a factor, multiply ΔH by the same factor. The trap: students reverse an equation and forget to change the sign, then wonder why the final answer is off by a factor of 2 or has the wrong sign.

Entropy sign prediction: Dissolving a solid in water — is ΔS positive or negative? Usually positive (more disorder), but for some salts that form ordered hydration shells, ΔS can be negative. Examiners test this with ion hydration. The rule: small, highly charged ions (like Al³⁺, Mg²⁺) order water molecules around them so strongly that solution entropy decreases.

Gibbs energy at equilibrium: At equilibrium, ΔG = 0, not ΔG° = 0. Standard Gibbs energy ΔG° corresponds to reactants and products in their standard states (1 atm, 1 M) — which is rarely the equilibrium condition. ΔG = 0 only when the system has actually reached equilibrium. This distinction appears in ~15% of JEE Advanced thermodynamics questions.

One last pattern worth knowing: JEE Main 2022 and 2023 both had questions where the correct option required checking all four ΔH/ΔS combinations for temperature-dependent spontaneity. This is not a coincidence — it’s now a standard question type. Treat it as guaranteed marks once you’ve internalized the 2×2 grid.