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# 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 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.

## Related Documents

• 1st Law of Thermodynamics - The First Law of Thermodynamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing.
• 2nd Law of Thermodynamics - Entropy and disorder.
• Air - Thermophysical Properties - Thermal properties of air at different temperatures - density, viscosity, critical temperature and pressure, triple point, enthalpi and entropi, thermal conductivity and diffusivity and more.
• Equilibrium - The mechanical, thermal, electrostatic, phase or chemical states of equilibrium.
• Standardized Enthalpies and Entropies - Standardized enthalpies and entropies for some common substances.
• Steam Entropy - Basic steam thermodynamics - entropy diagram.
• Thermodynamic Terms - Functions and Relations - Common thermodynamic terms and functions - potential energy, kinetic energy, thermal or internal energy, chemical energy, nuclear energy and more.
• Water - Thermophysical Properties - Thermal properties of water at different temperatures like density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more.
• Zeroth Law of Temperature - The direction of heat flow.

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