What is the formal definition of reactivity and how can it be quantified?

[Murtuza Tipu]

I understand the concept of a reactive species on a sort of vague and intuitive level: a reactive species is one that has lots of reactions that it can participate in. These tend to have large kinetic constants, be thermodynamically downhill and often have products that are also reactive, leading to knock-on effects. But reactivity isn't the same as energy, and it's possible to have highly reactive species with relatively low free energies and vice versa.

I'm interested in whether there's any kind of formal theory of reactivity, and/or a quantitative definition of the concept. For example, we're often told that free radicals are reactive because they have an unpaired electron and they "want" to fill their outer shell. This makes perfect sense on an intuitive level, but I'd like to know if it can be taken further. Is there some way we can write down an equation that will tell us how reactive a species will be in a given environment, or is there some way we can measure a species' reactivity empirically and use it to make predictions?

In particular, what does it really mean to say that one species is more or less reactive than another? Can we put a number to it and say that a species has a reactivity of 8.2 on some meaningful scale?

thanks.

 

[Lok]

To understand reactivity you pretty much need to look at Molecular orbital theory.

In a nutshell, the free electrons "want" to bond as by bonding they fall in a lower energy state. If the bond results in a higher energy state they will not "want" to bond and the bond will be unstable, as the direction is from higher to lower energy states.

Intuition does not help much here, you will need to understand the theory and then there are computer programs that help, as the math is an eyesore.

For a reactive species to be more or less reactive, that does not mean anything really. It is not a term for comparison as by modifying the conditions you can easily get mixed results. A reactive species could react violently to something and be mostly inert to others. A simple example is Na+H2O and Na+alkane, where Sodium's reactivity with water does not mean anything for alkanes which do react with other species.

https://www.youtube.com/watch?v=5CgLh9_f5ns&list=PL50CC04CE699FF4B3
I do make too much advertising to these courses, but they are quite good for beginners (like me).

 

[DrDu]

This is a question of reaction kinetics. High reactivity means that the activation energy for a given reaction is low.
http://en.wikipedia.org/wiki/Activation_energy