A precise difinition of reactivity of an element?

Question

We say some elements more reactive than other. Is the reactivity of an element is independent of the reagent or substant it is interacting? I reasonably suppose it is not. So the question arises is there any standard substance which is used to test the reactivity of a given element or species (ions, molecules etc) or any arbitrarily attributed condition .. Or simply the term reativity is not precise (a rough one...)?

Answer 1

You are absolutely right. "Reactivity" is a rather vague expression, and if you want to make it more exact you have to specify the reaction involved.

For example, chlorine is highly reactive towards hydrocarbons, but only mildly reactive towards water. Sodium is highly reactive towards water, but unreactive towards most hydrocarbons. It does not make sense to ask which of the two is more reactive.

You can still compare like with like; it makes sense to say that chlorine is more reactive than iodine.

You also need to distinguish between kinetic and thermodynamic reactivity. For example, if you look at redox potentials you will see that lithium is more reducing than sodium, so it is more reactive thermodynamically, but it reacts more slowly, making it more reactive if you use rate as your criterion.

Nice question. You can learn a lot by looking hard at fuzzy definitions.

Answer 2

I would say that the reactivity of a matter is more dependent on its relative energy state. The higher its energy, the more unstable it is, i.e. it is more prone to reactive.

Usually, scientists measure the enthalpy of the reaction to determine how reactive a substance is.