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Chemical vs. Physical Adsorption

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  • 0:04 Adsorption Definition
  • 0:27 Physical Adsorption
  • 1:30 Chemical Adsorption
  • 3:38 Lesson Summary
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Lesson Transcript
Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we discuss the meanings and differences between chemical adsorption (chemisorption) and physical adsorption (physisorption). These are important processes that occur all around us every day.

Adsorption Definition

Adsorption is the sticking of a particle to a surface. The sticking could be permanent, but can usually be reversed, either with chemicals or simply by heating the surface. The difficulty the particles have leaving the surface, or desorption, depends on whether the particle is physically adsorbed or chemically adsorbed. Let's discuss the difference between physical and chemical adsorption.

Physical Adsorption

Physical adsorption, or physisorption, is a type of adsorption that occurs for most particles in contact with a solid or liquid surface. Condensation of water molecules sticking to a drinking glass is an example. An extremely cold glass will build up many hundreds or thousands of layers of water molecules because each new layer of water molecules can stick to the previous layer. It's not necessary for incoming water molecules to ''find a spot'' on the glass surface in order to stick.

Phisisorption of Water Molecule to a Drinking Glass
physisorption of water molecules to a drinking glass

The top of this glass is open to air and slightly warmer. As a result, fewer water molecules (which are not drawn to scale) can stick to the top part of the glass. If you warm the glass back up, you'll notice the condensation dripping off because the physisorption gets weaker as atoms in the glass jiggle around at higher temperature. As a result, water molecules occasionally get an extra jolt of energy - just enough to free them from their sticky neighbors and melt off the glass. In other words, the molecules desorb from the surface. Desorption is the opposite of adsorption.

Chemical Adsorption

Chemical adsorption, or chemisorption, is characterized by chemical interactions that can range from very weak to at least 100 times stronger than physisorption. In chemical adsorption (as the name implies) there is some chemistry involved. In other words, chemical bond formation is required. Some molecules can chemisorb to a given surface, but others cannot.

Because chemisorption is a chemical process, there is often an activation energy associated with it. Molecules at a given temperature have different amounts of energies as they tumble around our universe. In fact, what we mean by ''temperature'' is the average kinetic energy, or energy of motion, of a sample of molecules. Only those molecules in a sample with at least the activation energy can chemisorb to a surface, just as only those above a certain height can ride dangerous roller coasters at an amusement park.

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