reactivity of the halogens

sgstudent

It is stated that for the halogens reactivity decreases down the group. Buy why is this so? Since they would form elements so since they should be stable hence eg chlorine being more reactive than bromine won't the bonds be stronger making it even less reactive? So I'm quite confused about this part here. Thanks for the help!

MSchem

As you go down the halogen group the average distance of the outer p-electrons increases and so does the screening of the nuclear charge. As a result the affinity for gaining another electron is decreased.

sgstudent

But aren't the bonds between Cl and Cl stronger than Br and Br. So why is chlorine more reactive than bromine since it should be harder to break those bonds? Since the explanation of the distance of valence electrons increases works only for atoms of the molecule of the elements meaning Cl and Br. Or am I wrong here, thanks so much for the help!

aroc91

The reason that HCl is a stronger acid than HBr is that Cl- is less reactive than Br-. You have to consider the stability of the product, not just the bond energy. Equilibrium constants exist for a reason.

sgstudent

Um sorry I don't quite understand the equilibrium concept. I guessing its another factor to reactivity? Because what I'm thinking is that the Cl-Cl bond is stronger than Br-Br bond due to the reactivity of the chlorine atom. So won't it be harder to break the Cl-Cl bond and make it react? So why is Br2 less reactive than Cl2? Thanks for the help!

aroc91

Cl2 is more reactive because Cl- is more stable than Br-. Bond energy is not the only factor. You have to consider the relative stability of reactants and products. In the case of Cl2 and Br2, Cl2 is more reactive because Cl- is more stable and there's not as much pressure (where the equilibrium concept comes into play) to remain as Cl2.