# Hertz–Knudsen equation

In surface chemistry, the **Hertz–Knudsen equation**, also known as **Knudsen-Langmuir** equation describes evaporation rates, named after Heinrich Hertz and Martin Knudsen.

## Applications

### Non-dissociative adsorption (Langmuirian adsorption)

The Hertz–Knudsen equation describes the sticking of gas molecules on a surface by expressing the time rate of change of the concentration of molecules on the surface as a function of the pressure of the gas and other parameters:[1][2]

where:

Quantity | Description |
---|---|

A | Surface area (in m^{2}) |

N | Number of gas molecules |

φ | Flux of the gas molecules (in m^{−2} s^{−1}) |

α | Sticking coefficient of the gas molecules onto the surface, 0 ≤ α ≤ 1 |

p | The gas pressure (in Pa) |

M | Molar mass (in kg mol^{−1}) |

m | Mass of a particle (in kg) |

k_{B} | Boltzmann constant |

T | Temperature (in K) |

R | Gas constant (J mol^{−1} K^{−1}) |

N_{A} | Avogadro constant (mol^{−1}) |

## See also

## References

- Kolasinski, Kurt W. (2012).
*Surface Science: Foundations of Catalysis and Nanoscience, Third Edition*. p. 203. doi:10.1002/9781119941798. - R. B. Darling, EE-527: Micro Fabrication, Virginia University (retrieved Feb. 9 2015).

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