# 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 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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