Chemical Thermodynamics – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Fundamentals of Thermodynamics

• System and Surroundings: The part under study is the system; everything else is the surroundings.
• Extensive Properties: Depend on the amount of matter (e.g., volume, energy).
• Intensive Properties: Independent of the amount of matter (e.g., temperature, pressure).
• State Functions: Properties like internal energy (U), enthalpy (H), and entropy (S) that depend only on the state.
• Types of Processes: Isothermal, adiabatic, isobaric, isochoric, reversible, irreversible.

2. First Law of Thermodynamics

• Concept: Energy can neither be created nor destroyed. ΔU = q + w
• Work (w): Work done by/on the system (e.g., expansion work).
• Heat (q): Energy exchanged due to temperature difference.
• Internal Energy (U): Total energy of the system.
• Enthalpy (H): H = U + PV, useful in constant pressure processes.
• Heat Capacity (C): Amount of heat required to raise temperature.
• Molar Heat Capacity: Heat required to raise the temperature of one mole by 1°C (or 1 K).

3. Hess’s Law of Constant Heat Summation

The total enthalpy change of a reaction is the same, irrespective of the number of steps taken. It allows calculation of enthalpies of complex reactions using simpler known reactions.

4. Types of Enthalpies

• Bond Dissociation Enthalpy: Energy required to break one mole of bonds in gaseous molecules.
• Enthalpy of Combustion: Heat released when one mole of a substance burns completely in oxygen.
• Enthalpy of Formation: Heat change when one mole of compound forms from its elements in their standard states.
• Enthalpy of Atomization: Energy to convert one mole of a substance into its atoms in gaseous state.
• Enthalpy of Sublimation: Heat required to convert solid directly into gas.
• Enthalpy of Phase Transition: Includes fusion and vaporization enthalpies.
• Enthalpy of Hydration: Heat released when 1 mole of ions is hydrated.
• Enthalpy of Ionization: Energy to remove electrons from atoms or ions.
• Enthalpy of Solution: Heat change when 1 mole of solute dissolves in solvent.

5. Second Law of Thermodynamics

• Spontaneity: Process that occurs naturally without external work. Driven by increase in entropy or decrease in free energy.
• Entropy (ΔS): Measure of randomness or disorder; increases in spontaneous processes.
• ΔS_universe: For spontaneous processes, ΔS_univ > 0.
• Gibbs Free Energy (ΔG): ΔG = ΔH - TΔS. Process is spontaneous if ΔG < 0.