Adsorption of Gases: Definition & Examples

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Adsorption of gases are all around us and can affect our day-to-day life. In this lesson, we learn specifically about adsorption of gases and how it is measured.

Adsorption of Water on Glass

You are in a hurry to get to work. After pushing the snooze button a few too many times, you groan when you realize your car windows are all frosty. As you grumble about scraping the windows, you wonder what causes them to frost. Well, the reason your windows gather that thin layer of frost is because there are always water molecules in the air. These molecules are attracted to the glass and adsorb to its surface. There water molecules are frequently there, but when it gets too cold they freeze and cause your windows to frost. Adsorption is the process where molecules stick to the surface of a solid, so adsorption of gases is when gas molecules stick to the surface of a solid.

Types of Adsorption

There are two general groups of adsorption, physical (physisorption) and chemical (chemisorption). Let's look at the differences between these two types of adsorption:

Physisorption Chemisorption
Bond type Weak, long range Strong, short range
Surface type Any surface Needs specific surface
Equilibrium Increasing temperature decreases Increasing temperature activates
Layers Multilayer Single layer

Water molecules absorbing to your glass is an example of physisorption. An example of chemisorption is hydrogen adsorbing onto a copper or nitrogen metal plate. Industries try to prevent chemisorption of hydrogen gas onto iron when making steel because this weakens it.

Measuring and Predicting Adsorption

We can determine how much gas will adsorb onto a surface using an isotherm. An isotherm is either a curve showing how adsorption reaches equilibrium or an equation telling us how much gas can absorb onto the surface.

When gas first starts adsorbing to the solid, there is a lot more adsorption than desorption. But if there is a lot of gas adsorbed to the surface, then we will start seeing a lot more desorption than adsorption. Eventually, we reach an equilibrium where the amount of observed gas on the solid appears to stay the same even though there is still adsorption and desorption at equal rates.

There are several different isotherms that can be used. The most common ones are the Freundlich and Langmuir isotherms. The Freundlich isotherm is an empirical formula, as it was developed based on observations. It works best at low pressures. The Langmuir isotherm is a rational basis formula, and it was developed based on theoretical calculations.

Looking at these isotherms will tell us a lot about what increases or decreases the rate of adsorption.

Freundlich Isotherm

The Freundlich isotherm uses the adsorption capacity (dependent upon the solid being adsorbed onto) and the intensity of adsorption (dependent upon the conditions for adsorption such as temperature). The equation is:

Freundlich equation
Freundlich equation

  • qe is the ratio of absorbed gas
  • KF is the adsorption capacity, which is dependent upon the maximum capacity of the solid
  • Ce is the concentration of the gas being adsorbed
  • 1/n is the intensity of adsorption; the value of 'n' will change the shape of the curve

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