Thermodynamics and Thermochemistry Notes for JEE Preparation

Thermodynamics and Thermochemistry are foundational topics in physical chemistry that form a crucial part of the JEE syllabus. This chapter explores the principles governing energy changes in chemical and physical processes, focusing on concepts like heat, work, internal energy, enthalpy, and entropy. These notes are designed to provide a comprehensive understanding of the subject, covering theoretical aspects, key formulas, and problem-solving techniques essential for JEE Main and Advanced.

Basic Concepts of Thermodynamics

Thermodynamics deals with the study of energy transformations and the relationships between heat, work, and other forms of energy in a system. It is critical to understand the fundamental terms and definitions before diving into complex calculations.

• System, Surroundings, and Boundary: A system is the part of the universe under study, while the surroundings include everything outside it. The boundary separates the system from its surroundings and can be real or imaginary.
• Types of Systems: Systems are classified as open (exchange of matter and energy), closed (exchange of energy only), or isolated (no exchange).
• State Functions and Path Functions: Properties like internal energy (U), enthalpy (H), and entropy (S) depend only on the state of the system, not the path taken, whereas work (W) and heat (Q) are path-dependent.

Laws of Thermodynamics

The laws of thermodynamics form the backbone of this chapter and govern all energy-related processes.

• Zeroth Law: Establishes the concept of temperature; if two systems are in thermal equilibrium with a third, they are in equilibrium with each other.
• First Law: Also known as the law of energy conservation, it states that the total energy of an isolated system is constant. Mathematically, ΔU = Q + W, where ΔU is the change in internal energy, Q is heat added, and W is work done on the system.
• Second Law: Introduces entropy (S) as a measure of disorder, stating that the entropy of an isolated system always increases in a spontaneous process. It also defines the direction of heat flow.
• Third Law: States that the entropy of a perfect crystal at absolute zero (0 K) is zero, providing a reference point for entropy calculations.

Thermodynamic Processes

Understanding different processes is key to solving numerical problems in thermodynamics.

• Isothermal Process: Temperature remains constant (ΔT = 0), so ΔU = 0 for an ideal gas. Work done is W = nRT ln(V₂/V₁).
• Adiabatic Process: No heat exchange (Q = 0), and the relation between pressure and volume is given by PVγ = constant, where γ is the ratio of specific heats.
• Isobaric Process: Pressure remains constant, and work done is W = PΔV.
• Isochoric Process: Volume remains constant (ΔV = 0), so W = 0, and ΔU = Q.

Internal Energy and Enthalpy

These state functions are central to thermodynamics and thermochemistry calculations.

• Internal Energy (U): The total energy within a system, including kinetic and potential energies of particles. For an ideal gas, U depends only on temperature.
• Enthalpy (H): Defined as H = U + PV, it accounts for energy at constant pressure. The change in enthalpy (ΔH) is critical in thermochemical reactions.

Thermochemistry

Thermochemistry focuses on heat changes associated with chemical reactions, a topic heavily tested in JEE.

• Heat of Reaction: The heat absorbed or released during a reaction. It can be exothermic (ΔH < 0) or endothermic (ΔH > 0).
• Standard Enthalpy Changes: Includes enthalpy of formation (ΔH_f°), combustion (ΔH_c°), and neutralization. Hess’s Law is often used to calculate these indirectly.
• Bond Enthalpy: The energy required to break a specific bond, useful for estimating ΔH in gaseous reactions.
• Calorimetry: Experimental measurement of heat changes using devices like bomb calorimeters or coffee-cup calorimeters.

Entropy and Gibbs Free Energy

These concepts determine the spontaneity of processes, a higher-level topic in JEE.

• Entropy (S): Measures randomness or disorder. For a reversible process, ΔS = Q_rev/T. Total entropy change (system + surroundings) decides spontaneity.
• Gibbs Free Energy (G): Defined as G = H - TS, it predicts reaction spontaneity. If ΔG < 0, the process is spontaneous; if ΔG = 0, the system is at equilibrium.

Key Formulas and Applications

Memorizing and applying formulas correctly is essential for JEE success.

• Work Done: W = -P_ext ΔV (for expansion, W is negative).
• First Law Application: ΔU = nC_vΔT for all processes involving ideal gases.
• Enthalpy Change: ΔH = ΔU + Δn_gRT, where Δn_g is the change in moles of gas.
• Entropy Change: ΔS = nC_p ln(T₂/T₁) - nR ln(P₂/P₁) for ideal gases.

Tips for JEE Preparation

• Focus on numerical problems involving adiabatic and isothermal processes.
• Master Hess’s Law and bond enthalpy calculations for thermochemistry.
• Practice graph-based questions (e.g., P-V diagrams).
• Understand the physical significance of ΔG and its relation to equilibrium constants.