Can anybody tell me what are the differences between physical adsorption and chemical adsorption?
Answer:
Physisorption or physical adsorption is a type of adsorption in which the adsorbate adheres to the surface only through Van der Waals force (weak intermolecular) interactions, the same that are responsible for the imperfection of real gases and the condensation of vapors. It is characterised by:
* Low temperature, always under the critical temperature of the adsorbate;
* Type of interaction: Intermolecular forces (van der Waals forces);
* Low enthalpy: ΔH < 20 KJ/mol;
* Adsorption takes place in multilayers;
* Low activation energy.
* The energy state of adsorbate is not altered.
* Reversible.
Examples
N2 on iron at 80 K
Adsorption Chromatography where solute is adsorbed on the surface of the sationary phase in HLPC columns
Chemisorption is a type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond, as opposed to the Van der Waals forces which cause physisorption. It is characterised by:
* High temperatures.
* Type of interaction: strong; covalent bond between adsorbate and surface.
* High enthalpy: 50 kJ/mol <ΔH< 800 kJ/mol
* Adsorption takes place only in a monolayer.
* High activation energy
* Increase in electron density in the adsorbent-adsorbate interface.
* Reversible only at high temperature.
Examples
The main way in which most chemists utilise the effect of chemisorption is in catalysed reactions. Like:
Self Assembled Monolayers (SAMs) are often formed by chemisorbing thiols (RS-H) onto gold surfaces forming Au-SR bonds
Another example is the Ion-exchange chromatography where mobile anions held near cations that are covalently attached to stationary phase in the HPLC column. Thus, only anion can be attached to it.
For a more technical and difficult explanation, see the IUPAC website on chemisorption and physisorption :http://www.iupac.org/reports/2001/colloi...
physical : multiple layers of adsorbate can b formed.
chemical : single layer of adsorbate can b formed.
physical : low heat reqd
chemical : high heat reqd
physical : can b reversed (reversible)
chemical : cant b reversed (irreversible)
physical : Wan der waals forces r present betn the adsorbate n adsorbent.
chemical : chemical bond is formed betn thm.
physical eg:adsorption of gases on charcoal during chem rxn
chemical eg : adsorption of oxygen on tungsten.
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