I was studying about heterogenous catalysis, and my Professor said something along the line, "When the adsorption energy is comparable to (or lower than) heat of evaporation, then it is physisorption. When it is larger than that, it tends to be chemisorption"

By this logic, that meant the energy for chemisorption is different for each chemical species. Is this statement true? Or am I understanding it wrong?

If anyone have a reference on this, please let me know. I've been searching for quite a while.

  • $\begingroup$ Why it should be the same? $\endgroup$
    – Poutnik
    Commented Jul 8, 2023 at 4:38
  • 1
    $\begingroup$ Just keep in mind that human language and human generated labels do not affect the natural and fundamental phenomena in anyway. If we say electron has a positive charge and proton has a negative charge, will it change the electron or proton in anyway? Chemisorption and physisorption are just labels and there is no sharp boundary line to mathematical define boundary of chemisorption and physisorption. At best, accept these labels qualitatively. $\endgroup$
    – ACR
    Commented Jul 8, 2023 at 6:57

1 Answer 1


Take it as a conventional rule of thumb.

If desorption enthalpy is not bigger than evaporation enthalpy, we can assume the cohesion forces of adsorbed molecules between each other and with the surface are of the same character as cohesion forces between molecules of the respective bulk liquid. We can then take it as physisorption.

If desorption enthalpy is bigger than evaporation enthalpy, we can assume the cohesion forces of adsorbed molecules with the surface are, at least partially, of other character and there is some exothermic chemical bonding. We can take it then as chemisorption.

But by nature of interactions, they are rather both physical and chemical, so it is rather arbitrary clasification.

If the clasification was defined by some arbitrary border value between physisorption and chemisorption, if would be very formal and both ways of interactions would be present in both groups in much bigger extent than in the above way.


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