Reaction Rate Order for High Temperature Gas-Solid Reactions

[uby]

Hello chemists!

In the classical treatment of reaction kinetics, the overall reaction rate is usually written as a constant times the concentration of species raised to some power (called the order of reaction for that species). It is impressed in high school level students that this reaction order is experimentally determined and cannot be divined by reaction stoichiometry, the reason being that the rate-controlling elementary step does not always involve species in the same stoichiometries (nor even the same species showing up in the overall reaction!).

However, I am wondering if this begins to break down as one considers high temperature gas-solid reactions. I would like to believe the following is true: At high temperatures, the kinetic energy of gas molecules and thermal energy of atoms at the surface of the solid are quite high and it is can become reasonable to assume at high enough temperatures that no activation barriers exist for any chemical reaction (i.e. - that a complete local equilibrium of all species can occur and there exist no kinetic barriers due to chemical reaction). Thus, there may be no "slow" step at all, and the overall reaction stoichiometric coefficients may be used to predict the order of the reaction.

Does anyone know this to be the case? Or, and especially if you know this to be false, if there are any theories out there discussing order of reaction at high temperatures?

Thanks!

 

[Borek]

As far as I know kinetics at high temperatures doesn't differ from kinetics at low temperatures - that is, numbers are different, reaction pathways are different, but everything else holds.

 

[DDTea]

It's possible to theorize reaction pathways/kinetics and thermodynamics does put a strong constraint on them (look at a book called Thermochemical Kinetics by Sidney Benson). However, when you have to decide between several logical pathways for how a product could form, experimentation is really the best way to determine it.

As far as the "slow" step goes, it's simply slower than all the other steps in the reaction. Experimentally, it's only possible to observe the rate limiting step. Whatever happens afterward does not affect the reaction rate appreciably.

That's an interesting question, though: if all reactant molecules had enough kinetic energy AND that only one reaction pathway were possible (otherwise, they could react in any way!), would it be possible to observe a rate-limiting step?