I don't know the exact formula either, but I would guess that your could estimate it it out by figuring out the energy of reaction and dividing by the number of product molecules produced in the reaction. Then use KE=1/2 mv^2 using the mass of the molecule and solve for v.

That's not entirely accurate as there is going to be some vibrational and rotational components to the energy as well as the translational, so you answer is going to be on the high side. You'd need some knowledge of physical chem to derive equations to account for those. If they're gases, the equipartition theorem could work.

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