Diffusion
Definition and meaning of Diffusion in chemistry.
Diffusion is the net macroscopic movement of particles, such as atoms, ions, or molecules, from a physical region of higher concentration to a region of lower concentration. This spontaneous transport process is driven by the random thermal motion of individual particles and results in a gradual, uniform mixing of materials until thermodynamic equilibrium is reached.
In more detail
The overall rate of diffusion in a system is quantitatively described by Fick's laws of diffusion, which directly relate the magnitude of the diffusive flux to the steepness of the concentration gradient. In gases, particles move rapidly and experience frequent elastic collisions, leading to high macroscopic diffusion rates governed by Graham's law, which states that the rate of diffusion is inversely proportional to the square root of the gas's molar mass. Diffusion in liquids is significantly slower due to the closely packed nature of the molecules and the continuously shifting intermolecular forces that resist free movement. Solid-state diffusion is the slowest form and typically occurs via vacancy or interstitial mechanisms within the rigid crystal lattice, strictly requiring high temperatures to proceed at any measurable rate. The entire process is highly temperature-dependent, as elevated temperatures increase the average kinetic energy of the particles, thereby increasing their molecular velocity and the overall diffusion coefficient.
Key facts
| Field | Physical Chemistry |
|---|---|
| Primary driving force | Concentration gradient (chemical potential gradient) |
| Underlying microscopic mechanism | Random thermal motion (Brownian motion) |
| Governing mathematical laws | Fick's laws of diffusion and Graham's law for gases |
| Temperature dependence | Diffusion rate increases exponentially with higher temperature |
When a single drop of concentrated food coloring is added to a glass of still water, the dye molecules slowly diffuse outward throughout the liquid volume until the entire glass possesses a perfectly uniform color.
Frequently asked questions
What is the defining difference between diffusion and effusion?
Diffusion is the gradual mixing of particles of different substances in a shared volume due to random motion. Effusion is the specific process where a gas escapes through a microscopic pinhole into a vacuum, governed entirely by the frequency of molecules striking the hole.
How does particle size specifically affect the rate of diffusion in a medium?
Smaller, less massive particles travel faster at any given temperature because they have a higher average velocity for the same kinetic energy. This results in a proportionally higher rate of diffusion when compared to larger, heavier particles in the same medium.