Science Class 11 - Thermodynamics Notes

Thermodynamics

Welcome to the chapter on Thermodynamics for Class 11. In this chapter, you will learn about the fundamental concepts of energy, heat, work, and the laws that govern the flow of energy in physical and chemical processes. By the end of this chapter, you will be able to apply thermodynamic principles to solve problems in chemistry and understand real-life phenomena.

Key Concepts

• System and Surroundings: The part of the universe under study is the system; everything else is the surroundings.
• Types of Systems: Open, closed, and isolated systems.
• State Functions: Properties that depend only on the state of the system (e.g., pressure, temperature, volume, enthalpy).
• Internal Energy (U): The total energy contained within the system.
• Heat (q) and Work (w): Ways in which energy can be transferred between system and surroundings.

The First Law of Thermodynamics

The first law states that energy can neither be created nor destroyed, only converted from one form to another.

Mathematical form: ΔU = q + w

• ΔU: Change in internal energy
• q: Heat absorbed by the system (+ve if absorbed, -ve if released)
• w: Work done on the system (+ve if done on, -ve if done by the system)

Enthalpy (H)

Enthalpy is the heat content of a system at constant pressure.

Formula: H = U + PV

Change in enthalpy: ΔH = ΔU + PΔV

• ΔH is positive for endothermic reactions (heat absorbed).
• ΔH is negative for exothermic reactions (heat released).

The Second Law of Thermodynamics

The second law states that the total entropy of an isolated system can never decrease over time. Entropy (S) is a measure of disorder or randomness.

• Spontaneous processes increase the entropy of the universe.
• ΔS > 0 for spontaneous processes.

Gibbs Free Energy (G)

Gibbs free energy helps predict the spontaneity of a process at constant temperature and pressure.

Formula: ΔG = ΔH - TΔS

• ΔG < 0: Process is spontaneous
• ΔG > 0: Process is non-spontaneous
• ΔG = 0: System is at equilibrium

Applications of Thermodynamics

• Understanding chemical reactions and predicting their direction.
• Calculating energy changes in physical and chemical processes.
• Explaining phenomena like melting, boiling, and dissolving.

Practice Questions

1. Define system and surroundings with examples.
2. State the first law of thermodynamics and write its mathematical form.
3. What is the difference between endothermic and exothermic reactions?
4. Explain entropy in your own words.
5. If ΔH = -100 kJ, ΔS = 200 J/K, and T = 298 K, calculate ΔG.

Challenge Yourself

• Give an example of a spontaneous and a non-spontaneous process.
• Explain why ice melts at room temperature using thermodynamic terms.

Did You Know?

• The laws of thermodynamics apply to everything in the universe, from engines to living cells!
• The word "thermodynamics" comes from Greek words meaning "heat" and "power".

Glossary

• System: The part of the universe chosen for study.
• Surroundings: Everything outside the system.
• Enthalpy (H): Heat content at constant pressure.
• Entropy (S): Measure of disorder or randomness.
• Gibbs Free Energy (G): Energy available to do work at constant temperature and pressure.

Answers to Practice Questions

1. A system is the part of the universe we study (e.g., water in a beaker). Surroundings are everything else (e.g., air around the beaker).
2. Energy cannot be created or destroyed, only changed from one form to another. ΔU = q + w
3. Endothermic: absorbs heat (ΔH > 0); Exothermic: releases heat (ΔH < 0).
4. Entropy is the measure of disorder or randomness in a system.
5. ΔG = ΔH - TΔS = (-100,000 J) - (298 × 200) = -100,000 - 59,600 = -159,600 J = -159.6 kJ

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Understanding thermodynamics helps you explain and predict changes in the world around you!