Do reactions whose final outcome is not even theoretically predictable by any means possible exist in Nature or does the outcome of all possible chemical reactions can (at least in theory) be predicted if we could get enough information about the conditions the reaction system is in?

My question isn't about a well-defined reaction system whose behavior is unknown to a certain degree but about the possibility in principle to discover a chemical reaction or a system of chemical reactions able to produce compounds which can't be predicted even in theory, even if we manage to acquire all the possible information about the system that can be acquired by all experimental means possible? What I want to ask is whether we can, at least in theory, know all possible products a reaction system can generate or do we "hit a wall" limiting our ability to predict certain chemical reactions over a certain limit?

I know very well the difference between a stochastic and a deterministic process, but what I'm asking isn't about stochasticity. It is about the possible predictability of all chemical reactions that could exist. As far as I know if we use the concept of stochasticity we may be able to derive possible, albeit only probable predictions about the state the system can be in. Therefore we can at least compute some probabilities different compounds can have to emerge under certain conditions. What I'm asking about is the validity of the concept of ergodicity, therefore I want to know is it possible to know the end state and all the reaction products of all possible chemical reactions, or there are some which could generate products that are impossible to predict in both theory and reality? Do reactions generating such unpredictable behavior exist in the real world or are they possible only in theory?

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    $\begingroup$ What does ergodicity have to do with predictability? I'm not clear on the distinction between predictability or stochasticity you are drawing either. Nor is the precise meaning of "predictability" as you use it. $\endgroup$
    – Curt F.
    Oct 15, 2017 at 15:22
  • $\begingroup$ @CurtF., it means even if we can only predict the probable outcomes of a reaction it still is a prediction. In the context of the question it means are there reactions whose outcomes we can't predict even to a some probability. Are there reactions we can say nothing about even if we get to know all the conditions under which the reaction is occurring? Can there be reactions we can't know the outcome of? $\endgroup$ Oct 15, 2017 at 15:33
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    $\begingroup$ If the system gets chaotic enough, it should be possible to get a system that is practically unpredictable (en.wikipedia.org/wiki/Chaos_theory). But you really need to refine your question. The way you state it here, it could be interpreted in lots of different ways, so I think it is really unfit to SE. $\endgroup$ Oct 15, 2017 at 15:36
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    $\begingroup$ Maybe it would help if you shorten your question to a single, clearly defined point. For example here, it is not clear to me if you mean with "all conditions known" that all information (e.g. atom positions, velocities, exact wavefunction etc.) is known, or only reaction conditions (e.g. temperature, concentration, reaction time) and so on. What is the point of the question? What do you want to achieve? $\endgroup$ Oct 15, 2017 at 15:40
  • $\begingroup$ I want to know are there reactions able to produce compounds we can't know of advance, even only in theory? Can there be a reaction able to yield totally surprising results, so we couldn't build a model predicting its behavior no matter how precisely we can measure the conditions it is in? $\endgroup$ Oct 15, 2017 at 15:44

3 Answers 3


The time point of the radioactive decay of a single atom cannot be predicted, even given all experimentally available information, and is not even predictable theoretically by any possible means.

Just isolate a single atom and observe its radioactive decay, and you have the purely random system you strive for. Could also take more than one atom and observe fluctuations with Geiger counter.

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    $\begingroup$ @YordanYordanov You asked if processes occur in nature and radioactive decay reactions happen naturally in our environment all the time. $\endgroup$ Oct 15, 2017 at 15:38
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    $\begingroup$ Of course: if 17F decays to 17O it has chemical consequences. Decay chains are in fact very important chemically, if you want to synthesize certain radionuclides. $\endgroup$ Oct 15, 2017 at 15:45
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    $\begingroup$ I'm a little confused where is the border between chemistry and physics here. That is why I'm asking. $\endgroup$ Oct 15, 2017 at 15:46
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    $\begingroup$ I think the radioactive decay itself is a physical process which leads to acceleration of the remaining nucleus. This acceleration will thereby cause a number of subsequent chemical changes. $\endgroup$
    – Jan
    Oct 15, 2017 at 15:59
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    $\begingroup$ The final outcome of radioactive decay is known in principle, event though there might be some uncertainty about decay chains due to paths that might not have been discovered yet. The change of the composition of radioactive material over time can also be predicted by the decay chains an nuclide life times. $\endgroup$
    – aventurin
    Oct 15, 2017 at 17:53

Life is an example of a series of chemical reactions with unpredictable results.

Forget, for a moment, the long term evolution of living creatures and consider the chemistry behind inheritance and selection (at least one sexual activity has started). The key chemical reaction that drives the DNA code for the next generation is random mixing of the genetic material of the parents to generate the DNA of the next generation. This process clearly demonstrates that the result of a chemical reaction is not entirely predictable given the components (the code is written in the same language, but the combination in children is a random combination of the parent's DNA).

Over a longer time period, we can also observe that entirely new types of life emerge via Darwinian random variation and selection. Future life is hard to predict given the existing life at a given point in time. And the underlying process is a chemical reaction involving DNA.

So maybe it isn't natural to think of this as chemistry. But it is built of chemical processes and demonstrates that, in a sufficiently complex system, chemistry can deliver results that are not easily forecastable. Perhaps simpler systems can demonstrate the same point, but this one is staring us in the face as an existence proof.

  • $\begingroup$ ,I have never thought of crossingover in such a way. It really made an impression! Thank you for your thoughtful answer. $\endgroup$ Oct 15, 2017 at 21:06
  • $\begingroup$ I only agree partially: Im principal you could calculate how the DNA evolves and recombines, or at least how probable a certain outcome is. In practice this just fails due to the huge amount of data points, but there are not infinitely many! You also could go the other way around: make the experiment and check whether you can calculate the outcome. Provided you have enough computer power and the apropiate level of theory, you can always calculate it. Because theory is supposed and designed to describe reality. $\endgroup$
    – Feodoran
    Oct 16, 2017 at 6:47

The ultimate fate of any isolated system can be predicted since it tends to thermodynamic equilibrium. However, it might be that the time to reach the end state is such long that no one ever will be able to observe it.

For closed and open systems things are more complex. But in principle it is possible to predict the "state" when the flow of matter and energy over time is known. Some of these systems may exhibit chaotic behavior, such that said "state" is actually a probability distribution over the possible states. That may be the reason why you have cited the concept of ergodicity.

  • $\begingroup$ How about kinetically controlled reactions then? $\endgroup$ Oct 15, 2017 at 19:15

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