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In chemistry, adsorption is a broad topic and can be further categorised as physical and chemical adsorption. Sometimes reactants may not be able to interact directly because of their placement in space. That is where chemisorption comes into the picture! Chemisorption is a specific type of adsorption that occurs when new chemical bonds are generated between the surface and the adsorbate. Chemical adsorption is not necessarily fully reversible and could require a high energy level to regenerate the adsorbent surface.
What is Chemisorption?
Chemisorption is an electrochemical process involving the exchange of electrons between molecules and active sites on the surface material. It works like a magnet with a positive and negative pole. Adsorbate molecules attach themselves to these polarised surfaces and remain at the surface only until they are ready to detach again.
Example of chemisorption – Adsorption of hydrogen, nitrogen etc., on the surface of adsorbent like ferrous catalyst at a high temperature.
This interaction is much stronger than physical adsorption. Chemisorption happens on all surfaces if there are favourable temperature and pressure conditions. It involves an exothermic bond-making process very quickly at sufficiently low temperatures. It is a particular single-layer process.
Although chemical adsorption may not be fully reversible, it can often be achieved using small amounts of energy. Carbon dioxide molecules generally bind to surfaces through affinity bonds and form monolayers in a process known as chemisorption. Because the bond energy between the gas and the surface is so low, quite a lot of energy is required to release the CO2 from its position on the surface. This capacity for holding carbon dioxide has made it possible to use chemical adsorbents like activated carbon or zeolites.
Difference between chemisorption and physisorption
Physisorption | Chemisorption |
The forces between the adsorbate molecules and the adsorbent are weak vander waals forces. | The forces between the adsorbate molecules and adsorbents are strong chemical forces similar to chemical bonds. |
Low enthalpy of adsorption of the order of 20 to 40 kJ/mol. | High enthalpy of adsorption of 80 to 240 kJ/mol order. |
Reversible in nature | Irreversible in nature. |
Physisorption normally occurs at low temperatures and decreases with increasing temperature. | Usually occurs at high temperature and increases with the increase of temperature. |
No activation energy is needed. | High activation energy is sometimes needed. |
It is not specific. | It is specific. |
The state of adsorbate is the same. | The state of adsorbate molecules may be different from that in bulk. |
Examples of Chemisorption
- Adsorption of hydrogen, nitrogen, etc., on the adsorbent surface like ferrous catalyst at a high temperature
- Corrosion and subtler effects associated with heterogeneous catalysis.
- Water filters, a common example of chemisorption
- Misty windows
- Paint – Paintings on paper, cloth, wood, metals etc
- Hydrogenation of alkenes and alkynes done by adsorption of H2 in the presence of molecules of Pd or Ni
CH≡CH+2(H2)−→−−−−−−Pd/Raney NiCH3−CH3
- Adsorption used in chromatography processes for the refining of metals
- Adsorption of yellow dye on flakes from fishery wastes
- Oxidised iron surface
- Adsorbed gases resulting in moss on rocks
- Addition of electrons on the inner surface of a battery
- Bubbles on a bar of soap after the addition of water
- Gas masks used in coal mines are based on the adsorption principle used in the adsorbent of poisonous gases, thereby purifying air for breathing.
- Silica and aluminium gels are used to adsorb moisture to reduce humidity.
- Noble gases can be separated using charcoal as an adsorbent.
- Charcoal is used for the decolonisation of sugar.
Why is chemical adsorption called “activated adsorption”?
Chemisorption begins slowly because the process involves building chemical bonds between the adsorbent and the adsorbate. Existing bonds between the adsorbate molecules must be broken to make way for new ones, but this takes an external source of activation energy that is not there. However, if a catalyst is added or some extra heat is added, activation energy may become irrelevant as bonds can form without any extra jolt. As a result, chemisorption becomes “activated” in that case, and its rate will speed up; therefore, it is called “activated adsorption”.
Characteristics of Chemisorption
- The chemisorption mechanism is said to occur when an external force pulls a particle from one surface, causing it to come together with the adsorbate of another surface.
- The two surfaces should vibrate at very similar frequencies to work well together for chemisorption to occur.
- When they are compatible, they will lock together by chemical bonds, forming covalent or ionic interactions.
- Chemisorption initially increases upon the application of heat before decreasing at higher temperatures.
- High-pressure results in higher chemisorption rates. As we gradually increase the pressure, the diffusion length decreases, resulting in lower chemical interactions and a decrease in the rate of chemisorption.
- Chemisorption also follows Le Chatelier’s Principle.
- Since it involves the formation of chemical bonds between adsorbed molecules and the adsorbent’s surface, it is highly selective.
Conclusion
Chemisorption is a chemical reaction between a gas and a solid surface, which involves adsorption of the gas on the surface. It exhibits certain characteristics which can be understood by looking at the various examples of chemisorptions. The main difference between chemisorption and physisorption is that chemisorption occurs in the gas phase, leaving the surface as an intact molecule. Physisorption is the process of physisorption that occurs in the solid phase leaving the surface with a chemisorbed molecule attached to the surface.
