Fick’s Law of Diffusion relates diffusion flux to the diffusion coefficient, change in concentration and change in position. The diffusive flux of a substance goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the change in concentration and the change in position, or put together the concentration gradient.
Fick’s Law of Diffusion is commonly written as:
J = -D(delta[C]/delta[x])
J is the diffusion flux
D is the diffusion coefficient
C is the concentration
x is the position
The negative sign in the above equation indicates that the diffusion is moving down the concentration gradient, for a net positive diffusion flux.
For example, if the concentration is 10^5 units/cm^3, which drops by 20% over a distance of 2 cm. Then the gradient (delta[C]/delta[x]) is -10^5. Assume the diffusion coefficient is 10^5. Then the diffusion flux equals:
J = -(10^5)(-10^5) = 10^10 units cm^-2 s^-1
Fick’s Law of Diffusion is very important for understanding the motion of gases with respect to external factors such as the concentration gradient, and the diffusion coefficient which is an intrinsic value unique to a particular gas. However, this law is not limited to just gases in Physical Chemistry, as it can be applied to liquids diffusing across biological membranes in Biology. Thus, understanding this law is crucial for understanding the overall process of diffusion.
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