definitivelly not. Can you imagine the consequances of this? Energy activation can be very low sometime. Are you sure with your modeling parameters.

Two things:

-the activation barrier can actually be zero. The classical textbook example for this would be a radical-radical recombination reaction, but note that such examples are pretty rare.

-often a barrier in a calculation may seem zero because the method may underestimate van der Waals interactions which would form a preliminary complex from which an extremely low barrier would have to be overcome.

Morin jean-charles Thank you for your response. The calculated activation energies are not zero, they are -0.5 and -2.5 Kj/Mol. Do you think they are meaningful?

Have you performed an IRC starting from your transition state geometry? That might help solving the puzzle. If your activation barrier is negative, your initial state might be incorrect.

Jürgen Weippert Dear Dr Weippert; many thanks for your useful answers. Our reaction is a cementation reaction. The Al powder added to a solution which was contained Lead which causes to Pb precipitation and Al dissolution. The reaction finished at around 10 min.

-Do you think this kind of reaction can be radical-radical recombination reaction?

-Could you please explain me (or send me a Reference) about IRC analysis steps?

If Ea=0, then k=constant for that particular reaction which implies it is independent from the reactant concentrations. This statement cannot be valid all the time as one of the reactant is always limiting.

Therefore, it should either be positive or negative.

HTH

Oh, so you have received those values not by a calculation, but from an experiment? Then forget about that IRC stuff, that's for TST simulations. Instead, maybe check out this discussion:

https://www.researchgate.net/post/Negative_activation_energy_Is_it_possible

Jürgen Weippert Mnay thanks Dr Weippert, yes I received these AEs from laboratory cementation experiments. Thank you for yous link, I will follow it.

Sina... Have you tried you experiment at various temperatures? Most reactions are controlled by the Gibbs free energy equation the combines the entropy and enthalpy with temperature. Even if your reactants may give more than one possible product, the free energy will be the determining factor. Also, the thermodynamic nature of your reaction is an important factor, being endo or exothermic.... Good luck

Dear Prof. Mahmood M. Barbooti Thank you for your useful information.

yes, I did the experiments at different temperatures. The reaction efficiency (metals precipitation) increase by increasing the temperature up to around 60C, but it decrease by increasing the temperature from 60 to 90 C. The slope of Arrhenius plot was around zero, in temperature range of 30 to 90 C.

Sina.... Good morning... The decrease of reaction rate with temperature increase suggests endothermic nature of your reaction. However, by all means activation energy cannot be zero. In your case you need to apply Arrhenius relation for the reaction rate versus 1/T (T= reaction temperature in kelvin). The slope will give you The activation energy will be calculated from the slope of the plot. Best regards.

Dear Prof. Mahmood M. Barbooti Many thanks for your useful information. Regards

You are welcome dear Miss Sina.. If you have any specific things related to your research, my Email: [email protected] .... Regards

I think no possible reaction could have zero activation energy but you can minimizing it by using suitable catalyst maybe Nano-Fluid Catalyst