JEE · Chemistry
Decide energy change, heat, work, spontaneity and equilibrium tendency by defining the system, path, sign convention and constraints before applying any thermodynamic relation.
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In short
Chemical thermodynamics predicts energy changes and the direction of spontaneous change from macroscopic states. A valid solution must define the system, state the sign convention, distinguish state functions from path functions, and apply each relation only under its pressure, temperature, reversibility and composition conditions.
The official JEE documents define content scope. They do not publish chapter weightage, so none is asserted here.
Verified against the current JEE Main 2026 and JEE Advanced 2026 syllabus documents on 8 September 2026. This is a wording and scope mapping, not a claim about question difficulty or frequency.
Sources: JEE Main 2026 syllabus and JEE Advanced 2026 syllabus, both linked in the sources section below.
Prerequisite
Mole Concept
Balancing equations and molar quantities are used throughout enthalpy and Hess-law calculations.
Prerequisite
States of Matter
Gas behaviour and phase concepts underlie pressure-volume work and phase-change enthalpies.
Parent
JEE Chemistry
Subject hub for the Chemistry chapter set.
Parent
JEE syllabus
Exam-level official Chemistry scope.
This is a readiness check, not a weightage or scoring-priority list.
Define the system, surroundings and whether matter or energy can cross the boundary.
Every thermodynamic problem starts by defining the system, the surroundings, and whether matter or energy can cross the boundary between them.
Internal energy, enthalpy, entropy and Gibbs energy are state functions; they depend only on the current state.
Specify composition, temperature, pressure and phase. Internal energy, enthalpy, entropy and Gibbs energy are state functions, meaning their value depends only on the current state, not on how it was reached.
Heat and work describe energy transfer along a process, not stored contents of a state.
Heat and work are path functions: they describe energy transfer during a process and depend on the path taken, unlike state functions.
Under the Chemistry sign convention, energy entering as heat or work is positive for the system.
Under the Chemistry sign convention used on this page, heat or work entering the system is taken as positive, and the internal energy change equals heat plus work.
At constant pressure with only pressure-volume work, the heat exchanged equals the enthalpy change.
At constant pressure, with only pressure-volume work occurring, the heat exchanged by the system equals its enthalpy change.
At constant temperature and pressure, a negative Gibbs energy change indicates a spontaneous forward change for the defined process.
At constant temperature and pressure, a negative Gibbs energy change indicates spontaneity for the defined process; other constraints require the entropy-of-universe criterion instead.
At equilibrium, the reaction Gibbs change is zero, while the standard Gibbs energy change need not be zero.
At equilibrium under constant temperature and pressure, the Gibbs energy change for the reaction's progress is zero, while the standard Gibbs energy change need not be zero.
Match the question signal to the correct relation and its operating constraint before any calculation.
For a process under constant temperature and pressure, deltaG = deltaH - T deltaS. This sign table assumes deltaH and deltaS are approximately constant over the temperature range; near phase changes or over a broad range, that approximation requires review.
Internal energy changes by heat and work transferred to the system.
First law of thermodynamics in the Chemistry sign convention.
Use when — q and w are positive when energy enters the system, under the Chemistry sign convention.
Common trap — Importing the Physics work-by-system sign convention without noting the switch.
Expansion does work on the surroundings, so system work is negative under constant external pressure.
Pressure-volume work for a one-step process against constant external pressure.
Use when — The process occurs against a constant external pressure.
Common trap — Replacing P_ext with the gas's own pressure in an irreversible step.
Work depends on the path through the external pressure.
Reversible pressure-volume work along a quasistatic path.
Use when — The path is quasistatic and reversible.
Common trap — Calling every slow-looking process reversible without checking the quasistatic condition.
Enthalpy combines internal energy with the pressure-volume term.
Definition of enthalpy for a macroscopic thermodynamic state.
Use when — A macroscopic thermodynamic state is defined.
Common trap — Calling enthalpy the same as heat under all conditions.
Under these limits, constant-pressure heat measures enthalpy change.
Heat exchanged at constant pressure equals the enthalpy change.
Use when — The system is closed, pressure is constant, and only pressure-volume work occurs.
Common trap — Applying it when electrical or other non-pressure-volume work also acts.
Gas expansion changes the pressure-volume contribution between internal energy and enthalpy.
Relation between reaction enthalpy and internal-energy change for gas-phase stoichiometric change.
Use when — Ideal gases at one temperature, with deltaN_g counting only gaseous species.
Common trap — Counting liquids and solids in deltaN_g.
Build the reaction enthalpy from standard formation steps.
Standard reaction enthalpy computed from standard formation enthalpies.
Use when — The same standard-state convention applies and the reaction is balanced.
Common trap — Omitting stoichiometric coefficients when summing formation enthalpies.
Enthalpy and entropy compete through temperature.
Gibbs energy change at fixed temperature for the compared states.
Use when — Temperature is consistent, and the criterion is used at constant temperature and pressure.
Common trap — Using Celsius, or ignoring the units of entropy relative to enthalpy.
Composition changes the current driving force away from the standard value.
Gibbs energy change away from standard composition.
Use when — Activities are defined and the temperature matches the standard-state reference.
Common trap — Using the equilibrium constant in place of the current reaction quotient.
Standard tendency determines the equilibrium position, but the reaction's own Gibbs change is zero at equilibrium, not the standard one.
Relation between standard Gibbs energy change and the equilibrium constant.
Use when — Equilibrium is established at temperature T with a properly defined dimensionless K.
Common trap — Saying deltaG(standard) becomes zero once an equilibrium mixture forms.
Answer: The reaction becomes spontaneous above the temperature where T deltaS exceeds deltaH, under the constant-deltaH-and-deltaS approximation.
Use deltaG = deltaH - T deltaS at constant temperature and pressure.
With deltaH positive and deltaS positive, the -T deltaS term becomes more negative as T increases.
Above a threshold temperature, the entropy term outweighs the positive enthalpy term and deltaG becomes negative, provided deltaH and deltaS are approximately constant over the range considered.
Mixing work done on the system with work done by the system.
Execution errorWhy it happens
Chemistry and Physics commonly use opposite sign conventions for work, and switching mid-calculation produces the wrong sign for deltaU.
How it is corrected
Fix one sign convention at the start of the problem and apply it consistently.
Calling heat and work state functions.
Knowledge gapWhy it happens
Heat and work depend on the path taken, unlike internal energy, enthalpy, entropy and Gibbs energy.
How it is corrected
Classify each quantity as a state function or a path function before using it.
Using q_p = deltaH without the constant-pressure and pressure-volume-work-only restrictions.
Decision / selection errorWhy it happens
This equality holds only when pressure is constant and no non-pressure-volume work occurs.
How it is corrected
Check both conditions before equating measured heat with enthalpy change.
Counting condensed species (liquids or solids) in deltaN_g.
Execution errorWhy it happens
Only gaseous stoichiometric moles contribute to the pressure-volume work term linking deltaH and deltaU.
How it is corrected
Identify the physical state of every species before computing deltaN_g.
Using Celsius in the T deltaS term.
Execution errorWhy it happens
The Gibbs and entropy relations require an absolute temperature scale.
How it is corrected
Convert temperature to kelvin before substituting into any T deltaS or deltaG expression.
Saying a spontaneous reaction must be fast.
Decision / selection errorWhy it happens
Thermodynamics addresses favourability of a change, not the rate at which it proceeds; rate is governed by kinetics.
How it is corrected
Keep spontaneity and rate as separate questions, and route rate questions to Chemical Kinetics.
Saying deltaG(standard) becomes zero when an equilibrium mixture forms.
Knowledge gapWhy it happens
At equilibrium, the reaction's own Gibbs change is zero, but the standard Gibbs energy change is a fixed reference value related to K, not to the current mixture.
How it is corrected
Keep deltaG (current) and deltaG(standard) as distinct quantities linked by deltaG = deltaG(standard) + RT ln Q.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
Work done on the system is positive, so expansion against positive external pressure gives negative w.
No. Thermodynamics addresses favourability; kinetics addresses rate.
Not generally. At equilibrium the reaction Gibbs change is zero, while deltaG(standard) = -RT ln K.
Scope claims are verified against the current NTA JEE Main and JEE Advanced syllabus documents. Sign convention, laws, enthalpy, entropy and Gibbs-energy treatment follow NCERT Thermodynamics. The gas constant value follows NIST CODATA. Official-paper archives are linked for provenance only; no counts, weightage or frequency are asserted.
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