A reaction in nature is, as we know, generally an interaction between the electron clouds of different atoms and nuclei. Why does this takes time?

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    $\begingroup$ The (large) thermal excitations that allow for productive interactions of this kind are rare and infrequent. $\endgroup$ Jul 26 '18 at 18:58
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    $\begingroup$ Besides, it takes time for atoms to meet. $\endgroup$ Jul 26 '18 at 19:12

Firstly, some reactions have a very low probability, secondly, the probability depends on temperature (activation energy!), and thirdly, diffusion is not exacly a fast process, and takes time even in a gas phase. Once the reaction progresses, the remaining educt particles have an increasingly hard time finding each other even if you thoroughly mixed everything at the beginning.

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    $\begingroup$ Well diffusion in a gas is faster than a liquid which is faster than a solid. But even in gas diffusion takes time. $\endgroup$
    – MaxW
    Jul 26 '18 at 20:30

Not all reactions are bimolecular $A+B\to $ products but if the reaction is bimolecular diffusion comes into play in these reactions; i.e molecule have to meet before they can react. For a bond dissociation or cis-trans isomerisation a collision is not necessary.

The fastest a bimolecular reactions can occur at is when there is a low energy barrier between reactants and products compared to the thermal energy available at, say, room temperature and then reaction occurs at first encounter. In solution this clearly depends on the viscosity, but in water or other low viscosity solvents the rate constant is typically between $10^{10}$ and $10^{11}\mathrm{dm^3mol^{-1}s^{-1}}$. In viscous solvents such as glycerol the diffusion rate constant is about ten thousand times smaller. In some electron transfer reactions, the reactant and product (electron donor and acceptor) have been synthesised to be held adjacent to one another and the reaction started by a short pulse of light, then reaction rates can now be even faster as they are not limited by diffusion, typically reaction occurs in $\approx 10^{-13}$ s.

If there is a big energy barrier between reactants and products there is no lower limit to the rate constant at, say, room temperature; eg. diamonds in air, because here the barrier to reaction is so high that according the the Boltzmann distribution of energies virtually no molecules will gain enough energy to react.

In an isomerisation or dissociation reaction no collision is necessary since a photon can provide the energy to cause reaction. In this case the fastest the bond can break in, say, a diatomic molecule is half a vibrational period or typically $10^{-14}$ s. In a more complex molecule energy has to find its way around the molecule to the bond that has to be broken and this can take a bit longer or a lot longer, it just depends on the molecule....


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