Physisorption and Chemisorption — Definition

Definition

Imagine you have a sponge. If you dip it in water, the water goes *inside* the sponge, filling up its pores. That's absorption. Now, imagine you have a piece of chalk, and you rub it on a blackboard. The chalk particles stick *on the surface* of the blackboard, they don't go inside. This sticking on the surface is what we call adsorption.

In chemistry, adsorption is a fascinating process where molecules of a gas or liquid (called the 'adsorbate') get attracted and held onto the surface of a solid or liquid (called the 'adsorbent'). It's like dust settling on a table – the dust is the adsorbate, and the table is the adsorbent. This phenomenon is crucial in many everyday applications, from water purification to industrial catalysis.

Now, there are two main ways this 'sticking' can happen, and they are very different:

1
Physisorption (Physical Adsorption): — Think of a weak magnet attracting a tiny paperclip. The attraction is there, but it's not very strong. You can easily pull the paperclip away. Physisorption works similarly. The adsorbate molecules are held onto the adsorbent surface by weak intermolecular forces, primarily Van der Waals forces (like the forces that hold gas molecules together in a liquid state). These forces are not chemical bonds. Because the forces are weak, physisorption is usually reversible – you can easily remove the adsorbed layer by heating or reducing pressure. It can also form multiple layers of adsorbate molecules on the surface, just like you can stack many paperclips on a magnet if the attraction is weak enough and there's space. This process releases a small amount of heat, known as the enthalpy of adsorption, typically in the range of $20-40 \text{ kJ/mol}$.

1
Chemisorption (Chemical Adsorption): — Now, imagine you're gluing two pieces of wood together. Once the glue dries, they form a strong, permanent bond. You can't easily separate them without breaking the wood itself. Chemisorption is like this. The adsorbate molecules form actual chemical bonds (covalent or ionic) with the atoms on the adsorbent surface. This is a much stronger interaction than in physisorption. Because chemical bonds are formed, chemisorption is usually irreversible. It's also highly specific, meaning only certain gases will adsorb onto certain surfaces if they can form a chemical bond. Unlike physisorption, chemisorption typically forms only a single layer (monolayer) of adsorbate molecules on the surface, as each surface site can only form one chemical bond. This process releases a much larger amount of heat, typically $80-240 \text{ kJ/mol}$, because new chemical bonds are being formed. It often requires an activation energy, meaning a certain amount of energy input is needed to initiate the chemical bonding process.