Chemical Thermodynamics: JEE Main Complete Guide

Chemical thermodynamics is the Physical Chemistry chapter that most students fear and most toppers exploit. It contributes two to three questions per JEE Main session with very stable question types — enthalpy calculations using Hess's law, spontaneity using Gibbs free energy, entropy changes, and bond enthalpy problems. This guide covers the full chapter with every formula and question type systematically.

Enthalpy and Hess's Law

Enthalpy change: ΔH = H_products − H_reactants. Exothermic: ΔH < 0 (heat released). Endothermic: ΔH > 0 (heat absorbed). Standard enthalpy of formation ΔH°_f: enthalpy change when 1 mole of compound is formed from its elements in standard state. By definition, ΔH°_f of elements in their standard state = 0. Hess's law: ΔH for a reaction = sum of ΔH for any series of steps that give the same overall reaction. This is a consequence of enthalpy being a state function.

JEE Main Hess's law problems: given ΔH values for two or three reactions, find ΔH for a target reaction by appropriate addition, subtraction, and scaling. Strategy: identify which reaction equation becomes the target when the given equations are combined; reverse equations (changing sign of ΔH) and multiply as needed. For thermodynamics of gases, see our Physics Thermodynamics Guide for the conceptual parallel.

Bond Enthalpy Calculations

Bond enthalpy (bond dissociation energy): energy required to break 1 mole of a specific bond in the gaseous phase. ΔH_reaction = Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds formed). This is valid only for gaseous reactions. JEE Main gives a table of bond enthalpies and asks for ΔH of a specified gaseous reaction. Common bonds and approximate values: H−H 436, C−C 348, C=C 614, C≡C 839, O=O 498, C−H 414, N≡N 946 kJ/mol. The N≡N triple bond being exceptionally strong explains why N₂ is so inert. Take a free mock test on thermochemistry to practise Hess's law and bond enthalpy calculations.

Entropy: Definition and Trends

Entropy S is a measure of disorder or the number of microstates available to a system. Second law: entropy of the universe increases in any spontaneous process (ΔS_universe > 0). Standard entropy changes: gases have much higher entropy than liquids, which have higher entropy than solids. Entropy increases when: (1) gas is produced from solid or liquid (Δn_gas > 0); (2) temperature increases; (3) more complex molecules are involved; (4) volume increases. JEE Main tests the sign and relative magnitude of ΔS for reactions.

ΔS = ΔH/T for a reversible isothermal process. Standard entropy change for a reaction: ΔS° = ΣS°_products − ΣS°_reactants. For a reaction like N₂(g) + 3H₂(g) → 2NH₃(g): Δn_gas = 2−4 = −2 < 0, so ΔS < 0 (entropy decreases — fewer moles of gas). For combustion of a liquid fuel: Δn_gas > 0 (CO₂ + H₂O gas produced), so ΔS > 0.

Gibbs Free Energy and Spontaneity

Gibbs free energy: G = H − TS. At constant temperature and pressure: ΔG = ΔH − TΔS. Spontaneity criterion: ΔG < 0 (spontaneous), ΔG = 0 (equilibrium), ΔG > 0 (non-spontaneous). The four cases: ΔH < 0, ΔS > 0 → always spontaneous (ΔG always < 0). ΔH > 0, ΔS < 0 → never spontaneous (ΔG always > 0). ΔH < 0, ΔS < 0 → spontaneous at low T (enthalpy drives spontaneity below a crossover T). ΔH > 0, ΔS > 0 → spontaneous at high T (entropy drives spontaneity above crossover T).

Crossover temperature: T* = ΔH/ΔS. At T = T*, ΔG = 0 (equilibrium). JEE Main uses this crossover temperature to ask: "At what temperature does the reaction become spontaneous?" Answer: T = ΔH/ΔS (with correct signs and units — ΔH in J/mol, ΔS in J/(mol·K)).

ΔG° and Equilibrium Constant

ΔG° = −RT·ln K = −2.303RT·log K. At 25°C (T = 298 K, RT = 2478 J/mol): ΔG° = −5706·log K J/mol ≈ −5.7 kJ/mol × log K. Relation to cell potential: ΔG° = −nFE°_cell. So log K = nE°/0.0592 at 25°C. JEE Main connects ΔG°, E°, and K in multi-part questions — compute one from another using these bridge equations. For the electrochemistry connection, see our Nernst Equation Guide.

Exam Strategy

Chemical thermodynamics questions in JEE Main are systematic and reward formula fluency. Practise: (1) Hess's law — 10 problems until the algebraic manipulation is automatic; (2) ΔG = ΔH − TΔS spontaneity cases — know all four cases without thinking; (3) ΔG° = −RT ln K — practise computing log K from ΔG° and vice versa. Make a formula card with ΔG = ΔH − TΔS, ΔG° = −RT ln K, and ΔG° = −nFE°. These three equations connect the entire chapter. Upgrade for ₹149/month for 200+ thermochemistry problems with detailed solutions.