Chemistry Chemical Thermodynamics Syllabus for NEET 2026

Chemical reactions do not happen randomly. Some release heat instantly, some absorb energy slowly, while others stop even when reactants are present. Chemical Thermodynamics helps you understand why these changes happen and how energy controls every chemical process. 

Chemical Thermodynamics is important in NEET because it combines formulas, sign conventions, graphs, and numerical applications based on heat and work interactions. Many questions involve identifying whether a reaction is spontaneous, calculating enthalpy changes, or applying the laws of thermodynamics correctly. Regular practice of formulas, derivations, and numerical problems with Physics Wallah resources helps you build better conceptual clarity and improve accuracy in NEET Chemistry.

Basic Thermodynamic Terms and System Boundaries

This foundational topic introduces the physical framework of thermodynamic analysis, outlining how matter and energy transfer between defined portions of the universe.

System:
The specific portion of the universe under thermodynamic study or observation.

Surroundings:
Everything else in the universe outside the system boundaries.

Boundary:
The real or imaginary wall separating the system from its surroundings.

Classification of Systems

Open System:
Can exchange both energy and matter with the surroundings.
Example: Hot coffee in an open beaker.

Closed System:
Can exchange energy (heat/work) but not matter with the surroundings.
Example: Reactants in a sealed cylinder.

Isolated System:
Can exchange neither energy nor matter with its surroundings.
Example: Liquid in a perfectly insulated thermos flask.

State Functions vs. Path Functions

This section establishes how physical properties are measured during changes of state, classifying them based on whether they depend on the history of the process or simply the final outcome.

State Functions

Properties whose values depend only on the initial and final thermodynamic states of the system, completely independent of the path taken.

Examples: Internal Energy (U), Enthalpy (H), Entropy (S), Gibbs Free Energy (G), Pressure (P), Volume (V), and Temperature (T).

Path Functions

Properties whose values depend explicitly on the mechanism or path/process chosen during a transition.

Examples: Heat (q) and Work (w).

Extensive Properties

Dependent on the quantity or mass of matter present in the system.

Examples: Mass, Volume, Enthalpy, Heat Capacity.

Intensive Properties

Completely independent of the quantity or mass of matter present in the system.

Examples: Temperature, Pressure, Density, Molar Heat Capacity, Refractive Index.

Internal Energy and The First Law of Thermodynamics

Internal Energy and the First Law of Thermodynamics treats the mathematical formulation of the law of conservation of energy applied to molecular systems, showing how heat and mechanical work alter internal energy states.

Internal Energy (U)

The total sum of all possible microscopic forms of kinetic and potential energies residing within the molecules of a system.

First Law of Thermodynamics (FLOT)

Energy can neither be created nor destroyed; the total energy of an isolated universe remains constant.

ΔU = q + w

IUPAC Sign Convention for NEET

• Heat absorbed by the system = +q

• Heat released by the system = −q

• Work done on the system (Compression) = +w

• Work done by the system (Expansion) = −w

Thermodynamic Processes and Work Calculations

This section provides specific equations for mechanical work (w) and heat transfer (q) operating under distinct chemical and physical constraints.

Isothermal Process (ΔT = 0)

Temperature remains perfectly constant.

For an ideal gas expanding isothermally:
ΔU = 0 and ΔH = 0

Therefore:
q = −w

Reversible Isothermal Work (wrev)

wrev = −2.303 nRT log₁₀(V₂/V₁)

wrev = −2.303 nRT log₁₀(P₁/P₂)

Irreversible Isothermal Work (wirr)

wirr = −Pex(V₂ − V₁)

wirr = −PexΔV

Adiabatic Process (q = 0)

No heat exchange occurs through the system boundaries.

ΔU = wadiabatic = nCv(T₂ − T₁)

Poisson Relation:

PVᵞ = Constant

Where:
γ = Cp/Cv

Isochoric Process (ΔV = 0)

Volume remains constant, therefore:

w = 0

Hence:

ΔU = qv

Isobaric Process (ΔP = 0)

Pressure remains constant.

w = −PΔV

ΔH = qp = nCpΔT

Free Expansion

Expansion of gas into a vacuum, where Pex = 0.

Therefore:

w = 0

For an ideal gas under adiabatic free expansion:

w = 0, q = 0, ΔU = 0, ΔT = 0, ΔH = 0

Enthalpy (ΔH) and Thermochemistry Relations

This section tracks heat transformations accompanying chemical bonds, detailing how modifications in gaseous volumes establish the difference between ΔH and ΔU.

Enthalpy (H)

State function is defined as the total heat content of a system at constant pressure.

H = U + PV

Fundamental NEET Formula

ΔH = ΔU + Δ(PV)

ΔH = ΔU + ΔngRT

Where:

Δng = (moles of gaseous products) − (moles of gaseous reactants)

• If Δng = 0 → ΔH = ΔU

• If Δng > 0 → ΔH > ΔU

• If Δng < 0 → ΔH < ΔU

Hess’s Law of Constant Heat Summation

The net enthalpy change for a chemical reaction remains identical whether the reaction takes place in one step or several steps.

Entropy (ΔS) and The Second Law of Thermodynamics

This metric introduces molecular disorder to explain why chemical reactions favour a natural directional flow without external interference.

Entropy (S)

State function acting as a measure of randomness or disorder within a molecular system.

Second Law of Thermodynamics

The entropy of an isolated universe continuously increases during any spontaneous process.

ΔSuniverse = ΔSsystem + ΔSsurroundings > 0

Mathematical Calculations

For a process:

ΔS = qrev/T

For isothermal expansion of an ideal gas:

ΔS = 2.303 nR log₁₀(V₂/V₁)

Entropy of Fusion

ΔfusS° = ΔfusH°/Tf

Entropy of Vaporization

ΔvapS° = ΔvapH°/Tb

Gibbs Free Energy (ΔG) and Spontaneity Criteria

This analytical parameter consolidates enthalpy and entropy into a single metric to determine the thermodynamic feasibility of processes.

Gibbs Free Energy (G)

An extensive thermodynamic state property defining the maximum non-expansion work available from a system.

G = H − TS

Gibbs-Helmholtz Equation

ΔG = ΔH − TΔS

(At constant temperature and pressure)

Spontaneity Conditions Checklist for NEET

Equilibrium Condition

When ΔG = 0, the forward and reverse reactions are perfectly balanced.

ΔG° = −2.303 RT log₁₀K

Chemical Thermodynamics: Study Resources By PW

Physics Wallah offers a range of study and revision resources for chapter-wise NEET preparation. These resources help improve conceptual understanding, formula revision, and numerical-solving ability.