Chemical Thermodynamics

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Chemical thermodynamics quantifies the energy that flows into, out of, and around a reaction, and — crucially — predicts whether that reaction will happen at all. This topic builds from the basic vocabulary of system, surroundings and internal energy, through enthalpy and Hess's law for heats of reaction, to entropy and the second law governing direction, culminating in Gibbs free energy, the single quantity JEE uses to decide spontaneity.

Chemistry Syllabus Structure Physical Chemistry ~33% weightage

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Thermodynamic System

1Open SystemExchanges matter & energy

2Closed SystemExchanges only energy

3Isolated SystemExchanges neither

4Internal Energy (U)State function, ΔU = q + w

1Thermodynamic Terms & Internal Energy

Before any calculation, thermodynamics forces you to draw a boundary: what counts as the system, and what counts as everything else.

Learning Objectives

Define system, surroundings and boundary, and classify systems as open, closed or isolated.
State the first law of thermodynamics and its sign convention for heat and work.
Distinguish state functions (U, H, S, G) from path functions (q, w).
Calculate PV work done during expansion or compression against a constant external pressure.

Key Terms

SystemThe specific part of the universe under study, separated from its surroundings by a boundary.

Internal Energy (U)The total energy (kinetic + potential) contained within a system; a state function.

First Law of ThermodynamicsEnergy can neither be created nor destroyed: ΔU = q + w (IUPAC convention).

State FunctionA property depending only on the current state, not the path taken to reach it — e.g. U, H, S, G, unlike q or w.

Thermodynamics tracks energy as it moves between a defined **system** and everything outside it — the **surroundings** — across a **boundary**. Every calculation starts by fixing exactly what that boundary encloses.

Open, closed and isolated systems

Open system — exchanges both matter and energy with its surroundings, e.g. a reaction in an open beaker.
Closed system — exchanges only energy, not matter, e.g. a sealed reaction flask.
Isolated system — exchanges neither matter nor energy, e.g. an ideal insulated thermos flask.

The first law of thermodynamics

ΔU = q + w (IUPAC convention: heat absorbed by the system, q > 0; work done on the system, w > 0)

This is simply energy conservation: a system's change in internal energy equals the heat it absorbs plus the work done on it. For expansion against constant external pressure, w = -PextΔV.

Common MistakeSign convention trips up almost everyone at first — always confirm whether a question uses IUPAC's ΔU = q + w or the older ΔU = q - w before assigning signs to heat and work.

State functions vs. path functions

U, H, S and G depend only on a system's initial and final states — take any path and ΔU comes out the same. Heat (q) and work (w) individually depend on the path taken, even though their sum, ΔU, does not.