Introduction to Thermodynamics

Thermodynamics is the branch of chemistry that deals with energy changes in chemical reactions, particularly heat and work. Understanding thermodynamics is crucial for NEET, as it explains why reactions occur, energy transfer, and spontaneity of processes.

• Focuses on energy transformations in chemical systems.

• Conceptual clarity is more important than numeric calculations.

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System, Surroundings, and Universe

• System: The part of the universe under study (reactants/products).

• Surroundings: Everything outside the system.

• Universe: System + surroundings.

NEET Tip: Always distinguish between system and surroundings; it helps in understanding heat flow and work.

Types of Systems

• Open System: Exchange of both matter and energy with surroundings (e.g., boiling water).

• Closed System: Exchange of energy only, not matter (e.g., steam in a sealed container).

• Isolated System: No exchange of energy or matter (e.g., thermos flask).

First Law of Thermodynamics (Law of Energy Conservation)

Energy can neither be created nor destroyed; it can only change forms.

• Expressed conceptually as:
  ΔU=q+w\Delta U = q + wΔU=q+w

  • ΔU: Change in internal energy

  • q: Heat added to the system

  • w: Work done on the system

• Significance: Helps predict heat changes and work done in chemical processes.

NEET Tip: Exothermic reactions release heat (q < 0), endothermic absorb heat (q > 0).

Enthalpy (H)

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

• Exothermic reaction: ΔH < 0 (heat released)

• Endothermic reaction: ΔH > 0 (heat absorbed)

Important Concepts:

• Enthalpy of formation (ΔHf): Heat change when 1 mole of compound forms from elements.

• Enthalpy of reaction (ΔHr): Heat change during chemical reaction.

• Enthalpy of combustion (ΔHc): Heat released when 1 mole of substance burns in oxygen.

NEET Focus: Conceptual understanding of heat flow in reactions.

Hess’s Law of Constant Heat Summation

The total enthalpy change for a reaction is independent of the pathway taken, depends only on initial and final states.

• Helps determine ΔH for reactions that are difficult to measure directly.

NEET Tip: Understand Hess’s law conceptually rather than memorizing equations.

Second Law of Thermodynamics

In any spontaneous process, the entropy of the universe increases.

• Entropy (S): Measure of disorder or randomness in a system.

• Spontaneous reactions: ΔS_universe > 0

• Non-spontaneous reactions: ΔS_universe < 0

Conceptual Understanding:

• ΔS increases in melting, vaporization, or dissolution.

• ΔS decreases in freezing or condensation.

NEET Tip: Link entropy with spontaneity of chemical reactions.

Gibbs Free Energy (G)

• Gibbs free energy predicts spontaneity of reactions at constant temperature and pressure.

• Conceptually:
  ΔG=ΔH−TΔSΔG = ΔH - TΔSΔG=ΔH−TΔS

• Interpretation:

  • ΔG < 0 → spontaneous

  • ΔG > 0 → non-spontaneous

  • ΔG = 0 → system at equilibrium

NEET Focus: ΔG combines enthalpy and entropy to predict reaction feasibility.

Spontaneity of Reactions

• Determined by ΔG and ΔS.

• Exothermic reactions (ΔH < 0) and increased entropy (ΔS > 0) → highly spontaneous.

• Endothermic reactions (ΔH > 0) can be spontaneous if ΔS is sufficiently large.

Example Concept:

• Melting of ice at 0°C: ΔH > 0, ΔS > 0 → ΔG = 0 (at equilibrium)

Applications of Thermodynamics

1. Predicting Reaction Feasibility:

• ΔG and ΔS help determine which reactions occur spontaneously.

2. Industrial Processes:

• Haber process (ammonia synthesis) uses ΔH and ΔS to optimize temperature and pressure.

3. Biological Systems:

• ATP hydrolysis releases energy, driving biochemical reactions.

4. Environmental Chemistry:

• Thermodynamic principles explain combustion, phase changes, and energy efficiency.

Quick Concept Summary Table

Conclusion

Thermodynamics is a key chapter for NEET Chemistry, explaining energy changes, heat flow, entropy, spontaneity, and reaction feasibility. Conceptual clarity is critical because NEET emphasizes reasoning-based questions and application-oriented scenarios rather than lengthy calculations. Understanding first law, second law, enthalpy, entropy, and Gibbs free energy allows students to predict chemical behavior efficiently. StudentBro.in provides structured, conceptual notes to help NEET aspirants master Thermodynamics confidently.