Class 11 Chemistry Chapter 5 Chemical Thermodynamics – Revision Notes

This chapter distinguishes open, closed, and isolated systems before applying the first law, ΔU=q+w, to track energy changes. Enthalpy, entropy, and Gibbs free energy then combine to decide whether a reaction proceeds spontaneously, reaches equilibrium, or needs external energy to occur.

Last Updated: September 23, 2026

Systems and First Law

• Open, closed, isolated systems.
• ΔU=q+w.

Enthalpy

• H=U+PV; ΔH=q_p.
• Exothermic: ΔH<0; Endothermic: ΔH>0.
• Hess’s law: ΔH is path-independent (state function).

Entropy and Free Energy

• Entropy: measure of disorder; increases in spontaneous processes (isolated system).
• ΔG=ΔH−TΔS; ΔG<0 spontaneous, ΔG=0 equilibrium.

One-Line Summary

Chemical thermodynamics uses enthalpy, entropy, and Gibbs free energy to determine whether a chemical process will occur spontaneously under given conditions.

Frequently Asked Questions

What is the difference between a system and surroundings in thermodynamics?
The system is the specific part of the universe being studied, such as a reaction mixture, while the surroundings are everything else outside the system with which it can exchange energy or matter.

What does enthalpy change indicate about a chemical reaction?
A negative enthalpy change indicates an exothermic reaction that releases heat to the surroundings, while a positive enthalpy change indicates an endothermic reaction that absorbs heat from the surroundings.

Written by Satish

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