# Why does a reaction take time? [closed]

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?

## closed as too broad by Nilay Ghosh, A.K., Tyberius, Jannis Andreska, M.A.R.Jul 28 '18 at 20:26

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• The (large) thermal excitations that allow for productive interactions of this kind are rare and infrequent. – a-cyclohexane-molecule Jul 26 '18 at 18:58
• Besides, it takes time for atoms to meet. – Ivan Neretin Jul 26 '18 at 19:12

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.
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....