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Third Law of Thermodynamics

The entropy of a substance is zero if the absolute temperature is zero

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The First Law of Thermodynamics forms the

  • basis for quantitative analysis of chemical reactions

The Second Law of Thermodynamics is used to

  • identify the directions of chemical reactions

The Third Law of Thermodynamics states that

  • the entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero (Kelvin), or conversely
  • the temperature (Kelvin) of any pure substance in thermodynamic equilibrium approaches zero when the entropy approaches zero

The Third Law of Thermodynamics can mathematically be expressed as

lim ST→0 = 0                               (1)

where

S = entropy (J/K)

T = absolute temperature (K)

At temperature absolute zero there is no thermal energy or heat. At temperature zero Kelvin the atoms in a pure crystalline substance are aligned perfectly and do not move. There is no entropy of mixing since the substance is pure.

The absolute zero temperature is the reference point for determination entropy. 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.

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