# 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.