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I'm very familiar with the concept that chemical reactions occur because the reactants a state of stability via the reaction. It's a very satisfying and fulfilling statement. But my question has always been - "How do the molecules know what to do? Do they have a certain amount of intelligence? Are they living?" I'm really sorry that the question isn't properly framed because this topic is just a mess inside my mind.

It's like the reactants already know what to do in order to attain stability.

The question arises because we are taught chemistry in a way relatable with moral values and actions humans perform under a given circumstance. So who is to be frowned upon - the way we are taught, or the lack of understanding within the students?

Thanks for going through the mess :)

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  • $\begingroup$ Does a ball need "intelligence" to fall down if dropped? $\endgroup$
    – Mithoron
    Commented Feb 21, 2018 at 15:24
  • $\begingroup$ Physics and Chemistry are two different scenarios. We've seen a ball fall numerous times in our lifetime and it comes with no surprise that gravity is the culprit behind it. Newton must have also thought "Why only downwards, why sideways or upwards?" He might have also speculated the presence of "intelligence" in a falling object. Only after years of hard work did he come up with a possible explanation that we call the "Gravitational Theory". Much to our surprise, it is being criticised by modern physicists. My question was out of sheer curiosity too. I had no viable explanation for the above. $\endgroup$
    – YourAverageEuler
    Commented Feb 22, 2018 at 4:26

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It boils down to thermodynamics. No, molecules do not possess intelligence. But they respect the laws of thermodynamics, which in a general look state:

  1. Energy of a closed system is constant.
  2. The entropy of the universe is always increasing.
  3. You can't get to absolute zero temperature! (Ok, this one is less important for this analysis)

The second law is the key: in order to augment the entropy of the universe a certain transformation can do one of two things: increase its own entropy or the entropy of its surroundings. The latter can be achieved by releasing heat, which is something exothermic reactions do. The sum of these effects is described by the system's Gibbs free energy:

$$G = H - TS$$

For a chemical reaction occurring at a constant temperature and pressure, $G$ has to decrease and becomes minimized at equilibrium. We then say the system is (thermodynamically) "stable". So when you hear someone say "chemical reactions minimize their energy", it would be better spoken as "... minimize their free energy".

Now you might wonder why the second law is true. Why does entropy has to increase? Why does the free energy has to be minimized? It certainly doesn't want to.

The truth lies in the molecular interpretation of entropy: it measures the amount of micro-states at which system can be found. Put together a bunch of blue balls with another bunch of red balls in a box, shake it all and look at how they end up. They surely won't be found in separate sides of the box, they will mix! The scenario where they are separated corresponds to a very low-entropy state: you can think of the red ones to one side or vice-versa: not that many possibilities of arrangements. The mixed state though has a much higher entropy: think of how many arrangements are possible by exchanging the positions of red and blue balls!

Since molecules in a closed system are very very much like tiny balls being shaken inside a box - not by a mechanical source but by their own thermal energy -, their behavior with respect to entropy is the same. Again, it's not that they want to increase the entropy of universe, there just is a higher probability that they do rather than don't. Given that we are usually observing systems of sextillion molecules or more, statistics kicks in and tell us probabilities mean more or less certainty: we are pretty much certain that chemical reactions will keep on trying to maximize the entropy of the universe.

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  • $\begingroup$ Ok, so what you're trying to explain is that in the massive ocean of atoms, the random microstates are more possible that the non random ones? $\endgroup$
    – YourAverageEuler
    Commented Feb 21, 2018 at 6:04
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    $\begingroup$ Yes. That's the second law of thermodynamics. Now to truly understand how the entropy of a chemical system can be increased, you have to check the shape, size and interaction energies of the molecules, while also keeping track of the overall homogeneity in the case of a mixture. They all contribute to the total amount of possible micro-states. $\endgroup$
    – Vinícius Godim
    Commented Feb 21, 2018 at 12:16
  • $\begingroup$ @Godim What about Formation of NaCl from NaOH and HCl? What about the entropy change here? $\endgroup$
    – YourAverageEuler
    Commented Feb 22, 2018 at 6:23
  • $\begingroup$ $\ce{NaCl}$ is not a chemical species in solution made of $\ce{NaOH}$ and $\ce{HCl}$. The ions stay dissolved as $\ce{Na+}$ and $\ce{Cl-}$. If you start from a solution of each reactant, the only true reaction taking place is water self-ionization $\ce{H2O <=> OH- + H3O+}$. With neutralization, the only proceeds to the left and presumably induces some entropy loss, but the total entropy is still increased by the other ions remaining in solution. $\endgroup$
    – Vinícius Godim
    Commented Feb 22, 2018 at 17:04

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