Dear colleagues,
I have a reaction involving calcium phosphate at pH 6.80 in a complex system that dissociated at low temperatures (I use ionic calcium concentration), which overall is in a metastable state (not in equilibria). In details, I have calculated the order or reaction (first-order) and all the K (rate constant) using ionic calcium concentration as my C values, at different temperatures from 4 to 35C. It is known that calcium phosphate has inverse solubility and thus the rate constants are higher when temperature decreases. Therefore, is it possible to have negative activation energy (Ea) for such reaction? I read that is possible for reactions involving halogens and ozone.
Thanks
Regards,
Giovanni
Giovanni Barone, Activation energy only makes sense for elementary reactions/processes. The value you determined is not the activation energy.
Mirosław Grzesik already said the most important thing here. For more details on "negative activation energies", see this discussion:
https://www.researchgate.net/post/What_is_the_phenomenon_of_negative_activation_energy2
There are known cases where the rate of the process increases as the temperature decreases. This is the case in the freezing of meat, where the rate of enzymatic reactions in the liquid to be frozen therein may temporarily increase due to an increase in the concentration of enzymes and, generally speaking, substrates. The rate of physicosorption also generally decreases with increasing temperature. There are more such cases, but they should not be associated with negative activation energy. These are complex processes/reactions for which the activation energy makes no sense.
Nitroxides decay by formation of an intermediate diamagnetic dimer which means that an equilibrium constant is a part of the total kinetic expression. This leads to an apparent negative activation energy. Keith Ingold observed this in the 1970's.
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