Hysteresis
Definition and meaning of Hysteresis in chemistry.
Hysteresis is a lag between a changing outside factor and a system's response. The path traced going forward differs from the path traced in reverse. Because of this, the system's current state depends heavily on its past history.
In more detail
This lag happens because forward and reverse processes take different physical paths. They do not perfectly retrace the exact same steps in reverse. Instead, the system gets temporarily stuck in intermediate stages called metastable states.
Tiny holes in solid materials are called mesopores. When a gas turns to liquid inside these pores, it fills them at a higher pressure. However, the liquid requires a much lower pressure to evaporate and empty out.
This difference occurs because the liquid surface curves differently when filling versus emptying. When you graph these pressure changes, the forward and reverse lines form a closed shape. The resulting shape is called a hysteresis loop.
The area inside this loop represents energy lost to the surroundings as heat. This means the overall process is not fully reversible. Chemists study these loops to figure out the exact shapes of the tiny pores. The IUPAC organization sorts these adsorption loops into types H1 through H5.
Key facts
| Field | Physical Chemistry |
|---|---|
| Common context | Gas adsorption in porous materials |
| Underlying cause | Forward and reverse processes take different paths |
| IUPAC classification | Hysteresis loop types H1 through H5 |
| Energy impact | Loop area represents dissipated energy |
| Typical pore size | 2 to 50 nanometers (mesopores) |
Consider nitrogen gas sticking to a porous glass called mesoporous silica. This silica has pore diameters between 2 and 50 nanometers. Chemists cool the system to 77 Kelvin and slowly add nitrogen gas. As pressure rises, the gas condenses and fills the tiny pores. Later, they slowly lower the pressure to remove the nitrogen. A graph of trapped gas volume versus pressure shows a clear loop. This loop appears between relative pressures of 0.4 and 0.9. The filling line sits below the emptying line on the graph. This proves that emptying the pores requires a larger pressure drop than expected.
Frequently asked questions
Why does adsorption hysteresis happen in porous materials?
Filling and emptying the pores happen through different physical mechanisms. The liquid surface forms a different curve when entering a pore compared to leaving it. This causes the filling and emptying pressures to not match up.
Is hysteresis only found in chemistry?
No. It happens in physics and engineering as well. For example, magnets show hysteresis when exposed to magnetic fields. In chemistry, you also see it during phase changes like supercooling a liquid below its freezing point.
What does the area inside a hysteresis loop mean?
The area inside the loop shows the amount of energy lost during the cycle. This energy is usually lost as heat. It proves that the forward and reverse processes are not perfectly reversible.