How does a chemical "know" which other species are present and where equilibrium is?

Question

Please forgive me if this is a vague question. I have always wondered how a chemical reaction "knows" where its equilibrium should be. For example, using a basic example of Le Chatelier's principle, for some theoretical reaction $$\ce{A + B <-> C + D}$$ If I were to add extra A and B to the tube, the reaction would proceed to the right until a new equilibrium was established. How do the chemicals "know" that I have added extra A and B?

Am I thinking about this incorrectly? Is it not actually a matter of the chemicals "sensing/knowing" which other species are present, but rather their own individual properties and how likely they are to exist in one form or another?

Related to this quandary, I learned in my metabolism course that NADH and NADPH are used to regulate different reactions because the NAD+/NADH and NADP+/NADPH ratios need to be different, and that merely adding the phosphate allows these separate species to have different equilibria. Can NAD+ really distinguish NADH from NADPH?

Answer 1

What we got here is a dynamic equilibrium, not a static one. For a static one the molecules would need to know when to stop reacting or when to react in a certain way, in a dynamic equilibrium you don't need to know.

Here's a ELI5 explanation of what's happening:

Let's say you've got a garden with an appletree and hundred of apples are on the ground. You don't want them there so you throw them to your neighbor. he's an old man and doesn't want them either, so he throws them back. You are much faster than him and there are hundreds of apples around you, so you don't even need to move to throw them and you can throw them very fast. He on the other hand has only a few apples laying around and has to move quite far, so he's throwing them back much slower. But what happens after time?

Well, there will be a point where there are more apples on the old mans side than on yours, so you have to run a lot to get to those apples while the old man has to move only some steps to reach a new apple. Eventually it will level out and both of you will throw apples at the same rate. That's a dynamic equilibrium. Because there are more apples on the old mans side he can throw them back as fast as you can throw them to him, even if you are faster.

Now here the chemical explanation:

Reaction rates are dependent on the concentration. High concentration equals fast reaction, low concentration equals slow reaction. At the beginning you got only starting material, it will react fast to the product. The product will react back but there is extremly low concentration so it's slow. But after some time you will reach a point where the concentration of the starting material is so low that the reaction is so slow, that it matches the speed of the back reaction which got faster because you got more and more product. At that time there will equal amounts react in both directions, making it look like the reaction is standing still.

For example: If we got the reaction $\ce{A + B \rightleftharpoons C + D}$ and the reaction rate of the forward reaction is $r_1=k_1[A][B]$ and the backwards rate is $r_2=k_2[C][D]$ then we will reach an equilibrium $r_1=r_2$ and that's the case if $k_1[A][B]=k_2[C][D]$.