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Engineering ToolBox > Third Law of Thermodynamics

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

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 S_T→0 = 0 (1)

where

S = entropy (J/K)

T = absolute temperature (K)

At a temperature of absolute zero there is no thermal energy or heat. At a temperature of 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 temperature of absolute zero is the reference point for determination entropy. The absolute entropy of a substance can be calculated from measured thermodynamic properties by integrating the differential equations of state from absolute zero. For a gas this requires integrating through solid, liquid and gaseous phases.