Thermodynamics & Chemical Energetics

Topics covered: Thermodynamic systems, laws of thermodynamics, work, heat, internal energy, enthalpy, entropy, Gibbs free energy and spontaneity.

1) Thermodynamic System & Surroundings

• System: Part of the universe under observation.
• Surroundings: Rest of the universe excluding the system.

Types of Systems:

• Open system: Exchanges matter and energy
• Closed system: Exchanges energy only
• Isolated system: Exchanges neither matter nor energy

2) State Functions & Properties

State Function: Depends only on initial and final state, not on path.

• Examples: Internal energy (U), Enthalpy (H), Entropy (S), Free energy (G)

Extensive Properties:

• Depend on amount of matter (mass, volume, energy)

Intensive Properties:

• Independent of amount (temperature, pressure, density)

3) Thermodynamic Processes

• Isothermal: ΔT = 0
• Adiabatic: q = 0
• Isobaric: ΔP = 0
• Isochoric: ΔV = 0
• Cyclic: System returns to initial state (Δstate = 0)
• Reversible: Infinitely slow, equilibrium maintained
• Irreversible: Rapid, equilibrium not maintained

4) Heat, Work & Internal Energy

• Heat (q): Energy transfer due to temperature difference
• Heat absorbed: q > 0
• Heat released: q < 0

• Work (w):
• Work done by system: w < 0
• Work done on system: w > 0

Internal Energy (U): Total energy contained in a system

5) Zeroth Law of Thermodynamics

If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

6) First Law of Thermodynamics

Energy of the universe is conserved.

Special Cases:

• Isochoric: w = 0 → q = ΔU
• Isobaric: q = ΔU + PΔV
• Isothermal (ideal gas): ΔU = 0
• Adiabatic: q = 0

7) Enthalpy (H)

• At constant pressure: ΔH = q_p
• State function and extensive property

8) Heat Capacity

• Specific heat: Q = m s ΔT
• Molar heat capacity: C_m = C × M

9) Enthalpy Changes

• Enthalpy of reaction: ΔH = ΣH(products) − ΣH(reactants)
• Enthalpy of formation: Formation of 1 mole of compound
• Enthalpy of combustion: Complete combustion of 1 mole
• Enthalpy of fusion (ΔH_fus): Solid → liquid
• Enthalpy of vaporisation (ΔH_vap): Liquid → gas
• Enthalpy of sublimation (ΔH_sub): Solid → gas

10) Hess’s Law

The enthalpy change of a reaction is independent of the path followed.

11) Entropy (S)

Measure of randomness or disorder of a system.

• State function
• Extensive property
• Unit: J mol⁻¹ K⁻¹

12) Second Law of Thermodynamics

Total entropy of the universe increases for a spontaneous process.

• ΔS_total > 0 → spontaneous
• ΔS_total = 0 → equilibrium

13) Gibbs Free Energy (G)

Criteria of spontaneity (constant T & P):

• ΔG < 0 → spontaneous
• ΔG > 0 → non-spontaneous
• ΔG = 0 → equilibrium

14) Third Law of Thermodynamics

Entropy of a perfectly crystalline substance is zero at absolute zero (0 K).