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Name: Nelly
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 3/22/2004


Question:
Hi, I know that diffusion is a passive transport, i.e. does not require energy expenditure. But certainly there must be energy that drives the movement of molecules from high concentration to low concentration. So I am wondering what is the energy that causes diffusion to occur? and how does concentration gradient provides a driving force for movement?


Replies:
Nelly,

At all temperatures above absolute zero (-273C), atoms and molecules are is a state of incessant vibrational, rotational, and translational motion. Those motions (the extent to which are proportional to the absolute temperature) are what enables diffusion to occur. The second law of thermodynamics describes the direction of diffusion processes -- that is, from high concentration (a state of order) to low concentration (a state of increased disorder). Overall, uninfluenced natural processes tend to proceed from order to chaos.

Regards,
ProfHoff 831


There need not be any energy driving diffusion. It is the change in entropy that is the driving force. In simple terms here is what happens. Consider only the translation of the particles. Each particle is free to occupy any translational state consistent with the energy of the system (which depends only upon the temperature). So every particle 'bounces' from one allowed translational state to another with equal random probability. The NUMBER of such states (one can show) is proportional to the volume occupied by the particles, so we can say the "density" of such translational states is proportional to the volume. The dependence of the density of such states is so vastly increases with volume that a particle "likes" the added space so much that it will diffuse to occupy those newly accessible states. No energy is required, Now it turns out that the increase in entropy S2 - S1 = R* ln(V2/V1). It is the random shuffling of the particles between all translational states (consistent with the temperature) that "drives" the particles to diffuse from regions of higher concentration to lower concentration. A mathematically rigorous derivation is somewhat involved (see e.g. "The Principles of Statistical Mechanics" by Richard Tolman) but it is a driving force nonetheless. We are so used to thinking that going from higher energy to lower energy is the only driving force that the more subtle, but equally strong driving force, increasing the density of allowed states, is frequently overlooked.

Vince Calder



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