# 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

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]

${\displaystyle {\frac {1}{A}}{\frac {\mathrm {d} N}{\mathrm {d} t}}\equiv \varphi ={\frac {\alpha p}{\sqrt {2\pi mk_{\text{B}}T}}}={\frac {\alpha pN_{A}}{\sqrt {2\pi MRT}}},}$

where:

Quantity Description
ASurface area (in m2)
NNumber of gas molecules
φFlux of the gas molecules (in m−2 s−1)
αSticking coefficient of the gas molecules onto the surface, 0 α 1
pThe gas pressure (in Pa)
MMolar mass (in kg mol−1)
mMass of a particle (in kg)
kBBoltzmann constant
TTemperature (in K)
RGas constant (J mol−1 K−1)