- #1
TVI_1405
- 17
- 0
I have been wondering if there is an explanation to why diffusion always goes down the concentration gradient? If it is a random molecular movement than why do we always end up with a uniform distribution of molecules?
AlephZero said:Diffusion goes at the same rate in both directions, but the amount of material that moves from any point is proportional to the amount of material that was there to start with.
Supposed there is 50% concentration at A and 20% at B, and in a given time one tenth the material diffuses the distance between A and B.
The material diffusing from A reduces the concentration at A by 5% (one tenth of 50%) and increases the contentration at B by 5%.
The material diffusing from B reduces B by 2% and increases A by 2%.
So overall, the concentration at A reduces by 3% and at B increases 3%.
If all the material is the identical, you can't measure the fact that some of it moved UP the concentration gradient, and the only interesting numbers are the overall 3% change.
sophiecentaur said:It is a bit hard to swallow the idea that 'statistics' can cause a 'force', though. In the end, you just need to accept that sort of thing and see the statements fro what they really mean.
Edgardo said:As mfb and the others have already mentioned, it has something to do with entropy. Entropy basically means that you count the number of possibilities that a certain state can occur.
Imagine that you sprayed perfume in your room. And imagine further that you divided your room in 1m3 cubes. Then consider the two states:
State 1: All perfume molecules are in a 1m3 cube, e.g. in the corner of your room.
State 2: All perfume molecules are "evenly" distributed, i.e. every 1m3 cube has the same number of perfume molecules.
It turns out that there are much more possibilities to "realize" state 2.
I recommend reading the websites below. They explain the concept of macrostates and microstates.
An Introduction to Entropy by Gary Felder
Lecture 5 : Statistical Interpretation of Entropy
Entropy and a box of air
Checkerboard example on page 3 of the PDF
This explains how to calculate the number of possibilities using the binomial coefficient.
mfb said:The mixed state has the highest entropy.
Put 100 blue balls in a container, add 100 red balls on top of them, mix it. Remove the upper 100 balls. What do you expect?
While there is the option to have 100 red balls, or 100 blue balls, most options to draw "100 out of 200" are close to 50 blue and 50 red balls.
HallsofIvy said:You can also think of it in terms of "random motion". There are many possible motions that will mix "red balls" with "blue balls", very few that will result in separating them. And that is simply because there are far more possible arrangements for the balls mixed together than for the same balls separated by color.
For a very simple example, if you have n1 blue balls and n2 red balls, arranged in a line, there are a total of (n1+ n2)! possible arrangements. Of those only 2(n1!n2!) have "all red" on one side and "all blue" on the other. For large n1 and n2 the first will be far larger than the latter.
2000! is on the order of 10374 while 2(100!100!) is of the order 103216, not particularly different, but the difference increases as the numbers increase.
Both. Sometimes, they are close to each other, but this is really rare (depending on the number of molecules). However, if you wait long enough, you can be sure (with probability 1) that it will happen sometimes.TVI_1405 said:(would they ever come up close to one another or would they mostly stay apart)...
microstates macrostate
[ ABCD | - ] (4,0)
microstates macrostate
[ AB | CD ] (2,2)
[ AC | BD ]
[ AD | BC ]
[ BC | AD ]
[ BD | AC ]
[ CD | AB ]
Diffusion is the movement of particles from an area of high concentration to an area of low concentration. This process occurs due to the natural tendency of particles to move from areas of higher energy to areas of lower energy. Therefore, diffusion always goes down the concentration gradient because it is the most energetically favorable direction for the particles to move.
The concentration gradient plays a vital role in the process of diffusion. It is the driving force that causes particles to move from an area of high concentration to an area of low concentration. The steeper the concentration gradient, the faster the rate of diffusion. Without a concentration gradient, there would be no net movement of particles and diffusion would not occur.
In rare cases, diffusion can occur in the opposite direction of the concentration gradient. This is known as reverse diffusion or active transport. It requires the input of energy to move particles from an area of low concentration to an area of high concentration. This process is essential for maintaining the balance of certain substances in cells and is often carried out by specialized proteins called pumps.
Temperature has a significant impact on the rate of diffusion. As temperature increases, the kinetic energy of particles also increases, causing them to move faster and spread out more quickly. This results in a higher rate of diffusion. On the other hand, lower temperatures decrease the kinetic energy of particles, slowing down diffusion.
There are a few factors that can affect the direction of diffusion, such as pressure, electrical charge, and gravity. In some cases, these factors can overcome the concentration gradient and cause diffusion to occur in a different direction. For example, in the human body, pressure differences can drive the movement of fluids and gases in the opposite direction of the concentration gradient.