Third Law of Thermodynamics

The entropy of a substance approaches zero as the absolute temperature approaches zero Kelvin. This tool helps you understand and calculate entropy relationships based on the Third Law of Thermodynamics.

PropertySymbolDescriptionUnit
EntropySMeasure of disorder or randomness in a substanceJ/K
Absolute TemperatureTTemperature measured in Kelvin (K)K
Pure Substance—Single element or compound with fixed composition—
Thermodynamic Equilibrium—State where no spontaneous changes occur—
Absolute Zero0 KReference point where entropy approaches zeroK

Mathematical Expression

The Third Law of Thermodynamics can be expressed as:

lim (T→0) S = 0

Where:

  • S = entropy (J/K)
  • T = absolute temperature (K)

Key Principles

  1. At absolute zero (0 K), there is no thermal energy or heat
  2. Atoms in a pure crystalline substance are perfectly aligned with no motion
  3. There is no entropy of mixing since the substance is pure
  4. Absolute zero serves as the reference point for entropy determination

Application

Absolute entropy of a substance can be calculated from measured thermodynamic properties by integrating differential equations of state from absolute zero. For a gas, this requires integrating through solid, liquid, and gaseous phases.